FHGTDC-HEMTCAL PROCESSES . * , ,5; _ :f _ GFE’GLYCHLORINATED BTPHENYLS. m SOLUT'ON , . THGTOPHTTDUGTS AND KTHETTcs 4-3: ;;;.:-f;.j ; . Dissertation for the GegTee Hf Ph D MTCNIGAN STATE UNNERSTTY LUIS” OCTAVIO RUZO . V L 1374 y , ‘ LIBRARY Michigan State University This is to certify that the thesis entitled Photochemical Processes of Polychlorinated Biphenyls in Solution: Photoproducts and Kinetics ' presented by Luis Octavio Ruzo has been accepted towards fulfillment of the requirements for Ph . D . degree in Chemistry a \_ [k A %\ R L" Y\ , Major professor Date 5/15/74 0-7639 JBRARY BINDERS ABSTRACT PHOTOCHEMICAL PROCESSES OF POLYCHLORINATED BIPHENYLS IN SOLUTION: PHOTOPRODUCTS AND KINETICS by Luis Octavio Ruzo A comparison of the photoreactivities of several symmetrical and unsymmetrical tri- and tetrachlorinated biphenyls has revealed differ- ences in the photoproducts obtained and in the kinetic parameters of the excited state. (l) The photoproducts of eleven polychlorinated biphenyls (PCB) were identified from reactions in cyclohexane and methanol solutions at 300 1 l0 nm. The reactivities of chlorines in the orthg_position were found to be greater in all cases than those in meta_or pg[g_positions Cleavage of halogen to yield dechlorinated PCB and methoxy-substitution in the aromatic rings were the major reaction pathways. Possible mechanisms for both processes are advanced and discussed. (2) Quantum yields and reaction rate constants were determined for all compounds studied. A correlation between the degree of chlor— ination, the position of the substituents and the kinetic parameters was observed. PCB containing 2,4 substitution were more reactive than those with 2,3; 2,5; and 2,6 substituted positions. The lowest quantum yields obtained were for PCB with 3,4 and 3,5 chlorination. (3) The triplet excited state was found to be the reactive inter- mediate. Its lifetime was measured by quenching methods (Stern-Volmer plots). The reaction and decay rate constants (kr and kd) of the triplet were determined. The values were found to correlate well with existing information regarding the geometry, electron distribution and stability of aromatic triplets. ii PHOTOCHEMICAL PROCESSES OF POLYCHLORINATED BIPHENYLS IN SOLUTION: PHOTOPRODUCTS AND KINETICS By Luis Octavio Ruzo A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1974 With the drawing of this love and the voice of this calling, we shall not cease from exploration and the end of all our exploring will be to arrive where we started, and know the place for the first time. T. 8. Eliot iv ACKNOWLEDGEMENTS I am very happy to have worked with Drs. Matthew Zabik and Robert Schuetz. Both have been extremely helpful and encouraging throughout my years at Michigan State University. I am especially grateful not only for the efficient manner in which knowledge and experience was trans- mitted, but also for the degree of freedom I was given in research at all times. Drs. George Niles and Richard Leavitt have earned my sincere gratitude and friendship. There has been many a time when there appeared to be no simple way out of a particular difficulty, thanks to their advice and willingness to discuss problems a solution gradually became evident. I wish to thank the Department of Chemistry and the Pesticide Research Center for providing excellent facilities for conducting research. In this matter Dr. Zabik's ability in acquiring instrumentation and keeping it running cannot be overrated. While writing these lines it quickly becomes evident that there's no way to acknowledge everyone's help. The faculty members in my committee, Dr. Peter Wagner and my fellow graduate students have all contributed in some way to the completion of this project. Totally outside Chemistry, my mother and father have constantly encouraged me and provided their unconditional support. Marlene, who knows better than anyone else how much all this entailed, has been and is now my driving force. TABLE OF CONTENTS INTRODUCTION ............................ A. Photochemical Background .................... 1. Environmental Importance of Polychlorinated Biphenyls . . . 2. Photoproducts ....................... 3. Excited State of Polychlorinated Biphenyls ......... 4. General Mechanistic Scheme and Kinetic Expressions ..... B. Research Objectives ...................... RESULTS .............................. A. Gas Chromatography ....................... B. Ultraviolet Spectroscopy .................... C. Photoproducts ......................... 2, 2' ,4 4' -Tetrachlorobiphenyl (I) ............. 2, 2', 5, 5' -Tetrachlorobiphenyl (II) ............. 2,2',3,3'-Tetrachlorobiphenyl (III) ............ ,6 ,6'-Tetrachlorobiphenyl (IV) ............. 5'-Tetrachlorobiphenyl EV) ............. ,4'-Tetrachlorobiphenyl -Trichlorobiphenyl (VII) ............... -Trichlorobiphenyl (VIII) ............... S- -Tetrachlorobiphenyl (IX) .............. , , 6- -Tetrachlorobiphenyl (X) .............. ll. 3,4, 2- -Trichlorobiphenyl (XI) ................ D. Reaction Rate Constants and Quantum Yields ........... l. Rate Constant Determination ................ 2. Quantum Yield Determinations ................ E. Quenching Studies. Lifetime of Triplet ............ F. Intersystem Crossing Quantum Yields .............. ,2' .3':5 ,3' 4 ’4’ ,4 ,3 3 u—l oscoowosmwa—n o o o o o o c o o NNNNwwN 6- ,5- 4 5, DISCUSSION ............................. Ultraviolet Spectroscopy and the Excited State of Biphenyl. . . Reaction Mechanism ....................... Photochemical Mechanism .................... Environmental Significance ................... Summary ............................ Further Experiments ...................... 'T'II'HUOW) EXPERIMENTAL ............................ PART 1. Materials and Procedures ................. A. Preparation and Purification of Materials ........... I POIYChlorinated Biphenyls ................. 2. Solvents .......................... 3. Quenchers ......................... 4 Sensitizers ........................ 5 Internal Standards ..................... vi Page I l l 2 6 10 T3 Page B. Photolysis Procedures ..................... 62 1. Preparation of Samples ................... 62 2. Degassing ......................... 62 3. Irradiation ........................ 63 C. Analysis of Photolysate .................... 66 1. Instruments ........................ 66 2. Product Identification ................... 66 3. Standardization ...................... 66 D. Actinometry .......................... 68 l. Quantum Yields of Reaction (¢r) .............. 68 2. Intersystem Crossing Quantum Yields (¢isc) ......... 68 E. Ultraviolet SpectroscOpy .................... 69 PART II. Kinetic Data ....................... 70 A. Reaction Rate Constants for Polychlorinated Biphenyls in Cyclohexane Solution ...................... 70 B. Quantum Yield of Reaction for Polychlorinated Biphenyls in Cyclohexane Solution ...................... 73 C. Quenching ........................... 76 D. Determination of Intersystem Crossing Quantum Yields ...... 80 LIST OF REFERENCES ......................... 8l vii LIST OF TABLES Table Page l. Gas Chromatographic Retention Times of Polychlorinated Biphenyls in Cyclohexane .................. l5 2. Molar Extinction Coefficients of PCB in Cyclohexane Solution .......................... l9 3. Photoproducts of 2,2',4,4'-Tetrachlorobiphenyl (I) in Cyclohexane and Methanol Solutions ............. 3l 4. Photoproducts of 2,2',5,5'-Tetrachlorobiphenyl (II) in Cyclohexane and Methanol Solutions ............. 3l 5. Photoproducts of 2,2',3,3'-Tetrachlorobiphenyl (III) in Cyclohexane and Methanol Solutions ............. 32 6. Photoproducts of 2,2',6,6'-Tetrachlorobiphenyl (IV) in Cyclohexane and Methanol Solutions ............. 32 7. Photoproducts of 3,3',5,5'-Tetrachlorobiphenyl (V) in Cyclohexane and Methanol Solutions ............. 33 8. Photoproducts of 3,3',4,4'-Tetrachlorobiphenyl (VI) in Cyclohexane and Methanol Solutions ............. 33 9. Photoproducts of Unsymmetrical PCB (VII-XI) in Cyclohexane Solution. . ........................ 35 10. Reaction Rate Constants (k) and Quantum Yields (¢r) for PCB in Cyclohexane ..................... 38 ll. One Point Reaction Rate Constants (k) for Unsymmetrical Substituted PCB in Cyclohexane Solution .......... 38 l2. Lifetimes of the Triplet Excited States of Polychlorinated Biphenyls ......................... 43 13. Intersystem Crossing Quantum Yields of Selected Polychlorobiphenyls .................... 48 14. Summary of Triplet State Reactivities of Polychlorinated Biphenyls ......................... 48 l5. Reaction Rate Constants for Polychlorinated Biphenyls (I-VI) in Cyclohexane Solution .................. 7l l5a. 2,2',4,4'-Tetrachlorobiphenyl ............... 7l l5b. 2,2',5,5'-Tetrachlorobiphenyl ............... 7T l5c. 2,2',3,3'-Tetrachlorobiphenyl ............... 7] viii Table Page lSd. 2,2',6,6'-Tetrachlorobiphenyl ............... 72 lSe. 3,3',4,4'-Tetrachlorobiphenyl ............... 72 15f. 3,3',5,5‘-Tetrachlorobiphenyl ............... 72 T6. Quantum Yields of Reaction for I-VI ............ 73 l6a. 2,2',4,4'-Tetrachlorobiphenyl ............... 73 16b. 2,2',5,5'-Tetrachlorobiphenyl ............... 73 16c. 2,2',3,3'-Tetrachlorobiphenyl ............... 74 l6d. 2,2',6,6'-Tetrachlorobiphenyl ............... 74 l6e. 3,3',5,5'-Tetrachlorobiphenyl ............... 74 l6f. 3,3',4,4'-Tetrachlorobiphenyl ............... 75 17. 2,2',4,4'-Tetrachlorobiphenyl (2.9 x 10'"3 M) in Methanol. Quenching with l,3-Cyclohexadiene ............. 76 18. 2,2',5,5'-Tetrachlorobiphenyl (2.02 x 10'3 M) in Methanol. Quenching with l,3-Cyclohexadiene ............. 76 19. 2,2',3,3'-Tetrachlorobiphenyl (3.26 x 10'3 M) in Methanol. Quenching with l,3-Cyclohexadiene ............. 77 20. 2,2',6,6'-Tetrachlorobiphenyl (2.55 x 10"3 M) in Methanol. Quenching with l,3-Cyclohexadiene ............. 77 21. 3,3',4,4'-Tetrachlorobiphenyl (3.28 x 10'3 M) in Methanol. Quenching with l,3-Cyclohexadiene ............. 78 22. 3,3',5,5'-Tetrachlorobiphenyl (1.49 x 10'3 M) in Methanol. Quenching with l,3-Cyclohexadiene ............. 78 -3 23. 2,2',4,4'-Tetrachlorobiphenyl (6.l9 x l0 M) in Methanol. Quenching with l,3-Cyclohexadiene ............. 79 24. 2,2',6,6'-Tetrachlorobiphenyl (2.25 x 10"3 M) in Methanol. Quenching with l,3-Cyclohexadiene ............. 79 25. Triplet Sensitized gj§7trans isomerization of Piperilene. . 80 26. Triplet Sensitized Phosphorescence of Biacetyl ....... 8O ix LIST OF FIGURES Figure Page l. Structures of Symmetrically Substituted Tetrachloro- biphenyls ......................... l6 2. Structures of Unsymmetrical Tri-and Tetrachlorobiphenyls. . l7 3. Ultraviolet absorption spectra of 2,2',4,4'-(I) , 2,2‘,5,5'- (II), and 2,2',6,6'-tetrachlorobiphenyl (III). . 20 4. Ultraviolet absorption spectra of 3,3',5,5'- (V), 3,3',4,4'-tetrachlorobiphenyl (VI) and Biphenyl ...... 2l 5. Ultraviolet absorption spectra of 2,4,6- (VII), 2,4,5- (VIII) trichlorobiphenyls and 2,3,4,5- (IX), 2,3,5,6- (X) tetrachlorobiphenyls .................... 22 6. Vpc-ms separation of the products of 2,2'~4,4'-Tetrachloro- biphenyl (I) in Methanol .................. 24 7. Ms scan of 4,4'-Dichlorobiphenyl .............. 25 8. Ms scan of 2,4,4'-Trichlorobiphenyl ............ 26 9. Ms scan of 2,2',4,4'~Tetrachlorobiphenyl (I) ........ 27 l0. Ms scan of 2,4,4'-Trichloro-2'-methoxybiphenyl ....... 28 ll. Vpc traces of 2,2',4,4'- (I), 2,2',5,5'- (II), 2,2',3,3‘- (III). 2.Z'.6.6'- (Iv). 3,3',5,5'- (V) and 3,3',4,4'- Tetrachlorobiphenyls .................... 30 l2. Vpc Traces of 2,4,6- (VII), 2,4,5- (VIII) and 3,4,21 (XI) Trichlorobiphenyls and of 2,3,4,5- (IX) and 2,3,5,6- (X) Tetrachlorobiphenyls .................... 36 l3. Reaction Rate Constants in Cyclohexane of 3,3',4,4'- (VI) A, and 3,3',5,5'- (V) Tetrachlorobiphenyls .......... 39 T4. Reaction Rate Constants in Cyclohexane of 2,2',5,5'- (II) A, 2,2',3,3'- (III) and 2,2',6,6'- (IV) Tetrachloro- biphenyls ......................... 40 l5. Reaction Rate Constants in Cyclohexane of 2,2',4,4'-Tetra- chlorobiphenyl (I) ..................... 4l l6. Stern-Volmer plots of 3,3',5,5'- (V) and 2,2',6,6'- (IV) Tetrachlorobiphenyl using l,3-Cyclohexadiene in Methanol. . 44 l7. Stern-Volmer plots of 2,2',4,4'- (I) and 2,2',5,5'- (II) Tetrachlorobiphenyl using l,3-Cyclohexadiene in Methanol. . 45 Figure Page l8. Stern-Volmer plots of 3,3',4,4'- (VI) and 2,2',3,3'- (III) Tetrachlorobiphenyl using l,3-Cyclohexadiene in Methanol. . 46 l9. Stern—Volmer plots of 2,2',6,6'- (IV) and 2,2',4,4'- (I) Tetrachlorobiphenyl using l,3-Cyclohexadiene in Methanol. . 47 20. Energy output distribution of RUL 3000 UV lamps ...... 64 2l. Energy output of RUL 3000 lamps .............. 65 xi INTRODUCTION A. Photochemical Background l. Environmental Importance of Polychlorinated Biphenyls Within the last few years a sudden interest has developed in a hitherto relatively unknown class of industrial organochlorine com- pounds: the polychlorinated biphenyls (PCB). Traces of these compounds have been reported in environmental 1 samples since l966. In the following years PCB contamination was found to be almost universal,2'7 including human milk,8 human adipose 9 TO tissue and brain tissue of infants. Little is known about the toxic effects of PCB after long-term exposures, as observed in the endemic poisoning resulting from PCB contaminated rice oil.n Polychlorobiphenyls are prepared commercially from the iron catalyzed chlorination of biphenyl.12 The product is a complex mixture containing at least sixty different PCB isomers with varying chlorine content. The characteristic properties of PCB which make them desirable for industrial use are their high dielectric constant and their thermo- chemical stability up to 900°C. Their stability coupled with the ease with which they are taken up by living organisms and accumulated at higher levels of the food web, has resulted in environmental problems. At present they are used as dielectric fluids in capacitors and transformers; as industrial fluids in hydraulic systems, gas turbines 2 and vacuum pumps; as plasticisers in adhesives; in textiles, surface coatings, sealants and copy paper; as heat transfer agents, fire retardants and fruit preservers. With such a variety of uses it is not surprising that industrial leaks are largely responsible for the presence of PCB in the ecosystem. Metabolism and Toxicology. There is evidence that PCB are meta- bolized slowly in some organisms to their hydroxy derivatives.13 However, little is known about the efficiency of uptake and excretion and it is considered probable that a steady-state situation exists in some areas. Even though all the available commercial PCB mixtures (Arochlors) are toxic to some extent, those containing four or less chlorine sub- stitutents have the highest toxicity, as reported in a study of their interaction with tissue culture cells.14 2. Photoproducts At the beginning of our research (l970) no reports existed in the literature concerning the photochemical behavior of PCB. Several chloroaromatics had been studied and in every case either chlorine cleavage with subsequent free radical hydrogen abstractionw"17 or 18-20 nucleophilic displacement of chlorine occurred. The photochemistry of iodobiphenyls at 254 nm in benzene and hydro- 2] Kharasch found that iodine carbon solvents was reported in l968. cleavage resulted in the formation of a biphenylyl radical which could then either attack the solvent to yield terphenyl or abstract hydrogen and from biphenyl. 'W-fi C6H61 @@@ In the presence of oxygen the recombination reaction becomes negligible and p-hydroxybiphenyl is obtained. Further proof of the intermediacy of free radicals is obtained when the reaction is run in the presence of nitric oxide (NO) a known radical scavenger which can inhibit the back reaction, thus the quantum yield of iodobiphenyl reaction increases. The photolysis of iodobenzene in fluorocarbon solvents at 77°K has been studied by ESR. The results also suggest the formation of free phenyl radicals upon photolysis.22 In l97l the first study of PCB photochemistry appeared in the literature.23 Dechlorination of 2,2',4,4',6,6'-hexachlorobiphenyl at wavelengths greater than 300 nm in hexane yielded more than ten pro- ducts containing two to five chlorines per molecule. The individual isomers were not identified beyong establishing their parent peaks (M) from the mass spectra. Our studies of the photolysis of 3,3',4,4'-tetrachlorobiphenyl showed stepwise dechlorination to be the only reaction undergone by this compound in hexane.24 C' c: 1 MM, go o “23% o C. Cl Ci Cl Cl At about the same time 2,2',5,5'-tetrachlorobiphenyl was photolysed in dioxane-water and hydroxy substitution was obtained in addition to small amounts of condensation and reductive dechlorination25 products. 2,2',5,5'- 3,3',4,4'- and 2,2',6,6'- etrachlorobiphenyls were found to decompose at about the same rate in non-degassed solutions. However, no accurate kinetic values were obtained and no photoproducts were identified beyond their molecular weight. The photolysis of 2,2',4,4',6,6'-hexachlorobiphenyl in oxygen saturated methanol solution has been found to yield oxygenated, )26 methoxylated and dechlorinated products (A,B,C in small amounts. x- B 0 c1 X c1 c1 _ , 2 X i A X—l ts) c1 __ Clx_' x I C These compounds arise from cleavage of orth9_chlorines and subsequent radical reactions with oxygen. None of them are observed in degassed solutions. All these results point to the intermediacy of biphenyl free radicals, either abstracting hydrogen from the solvent, or combining with triplet oxygen to give peroxides and subsequently ethers. The methoxylated or hydroxylated products observed may be formed in a manner similar to that of several haloaromatic systems studied.]8’27 hv 254 __ '- ROH X R = CH3\ H X: C|,Br,I The ionic intermediate can be stabilized by electron withdrawing groups gr;hg_or page to X. The reaction usually has a low quantum yield (<0.l) and the preferred course is elimination of X before nucleophilic attack takes place. Haloterphenyls have also been found to dechlorinate photochemically with subsequent formation of terphenyls and cyclization products.28 3. The Excited State of Polychlorinated Biphenyls In l933 Adams showed that those derivatives of biphenyl which had substituents above a certain limiting size in the grth9_position could be resolved into steroisomers.29 This phenomenon was explained as a result of restricted rotation of the two benzene rings by interference from the substituent groups. The ultraviolet absorption spectra of several 2,2'-disubstituted biphenyls showed unexpected differences with their benzene analogs.30 The band maxima in the benzene derivatives corresponded to similar ones in the biphenyl compounds which are displaced toward lower energies. Absorption was nearly additive for the two rings. With 4,4'-dichloro biphenyl the red shift was greater and the band intensity showed a l03 7 fold increase compared to that of 2,2'-dichlorobiphenyl. At the time (l936) no explanation was offered for this phenomenon. In l938 the geometry of biphenyl derivatives was studied on the 31 basis of their dipole moments. The carbons 4-l-l'-4' were found to be colinear in a large majority of the compounds studied. X-ray studies confirmed these findings.32 The earliest observations with regard to the actual excited state 33 The triplet of biphenyls were given by Lewis and Kasha in l944. energy (Et) of several haloaromatics was calculated from their phosphorescence spectra. Et values were lowest for 4,4'-disubstituted biphenyls (<66 kcal) and highest for the orthg_substituted PCB (>70 kcal). The solution spectrum of biphenyl shows a single structureless band with Amax around 250 nm and Emax of 18,000. Presumably this band represents the 208 nm transition of benzene, which is shifted by conjugation with the chromophoric phenyl substituent. The forbidden benzene transition at 256 nm is probably hidden below the main biphenyl band. The additional weak band displayed at 270-290 nm by some halo- biphenyls is probably the result of interactions between the substitu- 34,35 ent's free electrons in the n system. The electron distribution in the excited state can be represented by l or 2. CT CT c1—Q:C)=CI+ Cf-HCI 8 Thus, if the excited state of PCB is a dipolar or diradical species with c0planar rings and a Cl-Cl' linkage of essentially complete double bond character then the short wave shift in the conjugation band of an grthg_chlorinated PCB would arise from an increase in the transition energy resulting from the raising of the triplet energy. The excited triplet is thus shown to be more sensitive to steric effects than the ground state. On this basis, a correlation may exist between the steric requirements of the triplet and the photochemical lability of the grth9_ chlorines, relative to that of those in the meta_or para_positions. Studies of deuterium isotope effects have been carried out.36 A molecule such as biphenyl d10 would be expected to be closer to planarity than normal biphenyl h1O owing to the shorter length of the C-D bond compared to the C-H bond. The extinction coefficient of biphenyl d10 reflects the increase in resonance between the rings, in hexane solution a ratio of eH/eD = 0.96 has been obtained.37 In l967, Wagner reported the actual triplet energy of biphenyl.38 He found that there was a 10 kcal difference between the highest energy phosphorescence band at 65.5 kcal and the lowest energy absorption band at 75.5 kcal. This case of non-overlapping emission and absorption spectra is a direct result of the Franck-Condon principle.39 When the lowest vibrational levels of ground and excited states possess signifi- cantly different geometries, true spectroscopic O-O transitions are forbidden, and the apparently divergent O-O bands observed represent transitions from one vibrationally relaxed state to a vibrationally excited mode of another state. In the case of biphenyl this implies a large difference in geometry between the ground state and the excited 9 triplet. The band at 65.5 kcal in the emission spectrum arises from a transition of the triplet to a vibrationally "hot" ground state. The absorption band at 75.5 kcal would correspond to a transition from the relaxed ground state to a non-equilibrium or "phantom" triplet. The true O-O band must lie somewhere between the two values. Quenching experiments relative to benzophenone would indicate an Et value of 69.5 kcal. The most stable conformation of ground state biphenyl depends strongly on the medium. The dihedral angle between the rings is 40-50° 4O 4] in the gas phase, 20-25° in solution, and 0° in the crystalline state.42 Since QIEDQ substituents increase this angle,better quenching could be expected if the excited state was non-planar, and worse quenching if planar. Wagner found that the quenching efficienCy of substituted biphenyls was less than expected when quenching a series of ketones. Other evidence for the planarity of the excited state was provided by Hirota.43 The S-T* absorption spectra of several compounds in the crystal form was measured in the presence of a “doping" agent, a com- pound with lower Et which could trap the excitation. The spectrum of biphenyl thus obtained had a 0-0 band at 65.5 kcal in exact agreement with the highest energy phosphorescence band. That band must arise from a transition between planar conformations of both ground and excited states. Such a transition is a true O-O band in the crystal where the dihedral angle is 0°, but not in solution where the conforma- tion of the ground state is twisted. 4. General Mechanistic Scheme and Kinetic Expressions One advantage liquid-phase photochemistry has over the gas phase is that several kinetic simplifications can be made. Absorption of light by a PCB can produce several excited singlets with many corres- ponding vibrational levels. In solution, decay to the lowest excited 0"2 44 Thus, it may be singlet occurs almost instantaneously (l sec). assumed that all singlet processes happen from the lowest singlet (5]). Experiments using mono- and dichlorobiphenyls show that inter- system crossing quantum yields (¢isc) increase with increasing number of chlorine substituents.45 Though small, chlorine has a measurable heavy atom effect (HAE). With iodine and bromine HAE values are 46,47 greater. Such effects on S-T transitions are quite general and are probably the result of spin orbit coupling which causes mixing of singlet and triplet states.48 The singlet then acquires a small amount of triplet character and the triplet some singlet character, so the transition loses some of its forbidden character. HA effects are responsible for large phosphorescence/fluorescence ratios obtained with 49 haloaromatics. It has also been pointed out that HAE involving n-n* excited state are quite pronounced since normally negligible amounts of spin orbit coupling are present in symmetrical aromatics.50 For kinetic calculations in solution the rate of phosphorescence can usually be ignored since it is much slower than the rates of other triplet processes. The following mechanistic scheme can be written for the photochemical processes of PCB (A) in solvent (SH), with quencher Q. 10 ll SCHEME 1 Proge§§_ Rate A +hv-I—-a~+A* LSE+A* I (ifa. =1) (1) o l 3 a 1sc A;-> A0 Kd [4;] <2) A;-—+ A- + c1- Kr [A3] (3) A- + SH ——+ A - H + s. (4) S- + S- —~+ S - S (5) A- + A- ——+ A - A (5) Cl- + SH -—+ H - Cl + S~ (7) Cl- + A. -—+ A0 (8) A- + s- -—+ A - s (9) A; + o —-» A0 + 4‘; Kq [01111;] (10) Under steady—state conditions dA*3/dt = O and the only directly measurable parameter is the quantum yield of reaction. Using the def- inition of Wagner,51 the quantum yield for a particular photoprocess i is given by: ¢i = ¢ES ¢R Pi (1‘) where ¢ES represents the probability that absorption of light will lead to the required EXCltEd states; the probability that the excited state in will undergo the primary photoreaction necessary for process i; Pi is 12 the probability that any metastable ground state intermediate will lead to stable product, completing process i rather than forming by-products or reverting to starting material. Assuming ¢isc = l for PCB containing three or more chlorines: 4- = P. ———!‘-—— (12) so that ¢i becomes the quantum yield of reaction. When a triplet quencher is used equation 12 becomes: K r ¢. = P. (13) 1 1 Kr + Kd + Kq(Q) Dividing equation 12 by equation 13 the familiar Stern-Volmer relationship is obtained: L=1+ K Tm) (14) 40 q Where ¢Q represents the quantum yield of reaction in the presence of quencher (Q). The lifetime of the triplet (r) is the reciprocal of the sum of the rates of all the reactions undergone by the triplet. t = (K + K )’1 (15) r d A plot of the relative quantum yield versus quencher concentration is linear with a slope equal to the product qu and an intercept of l, if only one excited state is reacting and being quenched. 8. Research Objectives After we observed that 3,3',4,4'-tetrachlorobiphenyl exhibited unusual characteristics in its ultraviolet spectrum and that it under- went photodecomposition, a set of experiments was designed to elucidate the structure of the resulting photoproducts and the kinetics of PCB photoreactions. (1) With possible free radical and ionic mechanisms for chloro- aromatics, product formation from PCB at wavelengths found in the higher energy end of solar radiation, in alkane and alcohol solvents, can be expected to differ. The nature of the products would be of environmental and chemical interest. (2) To determine the simple reaction rate constant of PCB upon photolysis in the range 290-310 nm was of interest in order to evaluate their period of permanence in the ecosystem. PCB do not degrade to an appreciable extent, either thermally or metabolically, thus photo- reactions remain as the only viable degradation pathway under field conditions. (3) To determine the effect of the different chlorine substitution patterns in either one or both rings. Chlorination at the ortho, meta or para positions could have different effects on the photochemical processes available to PCB. Excited state lifetimes and reactivities should vary within a series of structurally related PCB. Thus, T, ¢r’ Kd, Kr’ and ¢isc needed evaluat1on. (4) To determine the possibility of trans-annular effects, leading to abnormalities in the photochemical behavior of PCB. 13 RESULTS A. Gas Chromatography Using COL l (apparatus described in exp. sec.) with an oven temp- erature of 170°C and a nitrogen flow of 8 ml/min, several polychlorinated biphenyls were analyzed and their vpc retention times obtained (Table l). The values listed were used to identify the photoproducts of I-XI, whose structures are shown in Figures 1 and 2. 14 15 Table 1. Gas Chromatographic Retention Times of Polychlorinated Biphenyls in Cyclohexanea PCB Rt PCB Rt Designation Monochloro Trichloro 2- 0.60 2,4,6- 1.55 (VII) 3- 0.82 2,2',6- 1.65 4- 0.85 2,2',5- 1.85 Dichloro 2,4,5- 2.20 (VIII) 2,6- 0.92 2,3',5- 2.45 2,2'- 1.00 2,4,'- 2.60 2,5- 1.15 3,4,2'- 2.75 (XI) 2,4- 1.25 3,4,5- 3.60 2,3— 1.40 3,3',4- 4.30 3.5- 1.50 Tetrachloro 3,3'- 1.75 2,2',4,4'- 3.70 (I) 3.4- 1.80 2,2',5,5'— 3.65 (11) 4,4'- 1.95 2,2',3,3'- 4.80 (III) 2,2',6,6'- 2.30 (IV) 3,3',5,5'- 5.70 (V) 3,3',4,4'- 8.90 (VI) Methoxylated PCB 2,3,4,5- 4.80 (IX) 2,4,4'-Trichloro-2'- 2,3,5,6- 3.35 (X) methoxy 5.05 4,4'-Dichloro-2,2' dimethoxy 6.30 aRt relative to cyclohexane. . .. so“ .. 111 c G) | 9 IV :9 Figure 1. Structures of Symmetrically Substituted Tetrachlorobiphenyls 17 VII (9 Q :1 VIII Q Q 1 IX XI Figure 2. Structures of Unsymmetrical Tri- and Tetrachlorobiphenyls B. Ultraviolet Spectroscopy The uv spectra of I-XI were obtained in cyclohexane solution. Scanning was carried out in the 320-220 nm region. Typically PCB solutions were 0.1 - 1.0 x 10'4M except in cases where higher concen- trations had to be used in order to obtain the molar absorptivity (e) in the 280-300 nm region. The values of a were calculated at 290 nm and at the maximum wavelength of the conjugation band (240-270 nm). In cases where this band disappeared the extinction coefficient was calculated at 255 nm. Since PCB absorption in the uv conformed to Beer's Law, 5 was calculated using equation 17: log IO/I = A = ebM (17) Those PCB containing para_and no orth9_chlorines exhibited a band in the 240-270 nm region of greater intensity than that of biphenyl itself. PCB containing g§§h9_chlorines showed decreased intensity relative to that of unsubstituted biphenyl. When only meta chlorines were present the intensity of absorption was similar to biphenyl. Uv spectra are given in Figures 3, 4 and 5. Extinction coefficients are listed in Table 2. 18 19 Table 2. Molar Extinction Coefficients of PCB in Cyclohexane Solution PCB e(290 nm) e(240-270 nm) Amax I 320 ‘ 10,000 253 II 300 1,200 (255) III 170 1,500 (255) IV 50 300 (255) V 950 18,000 252 VI 6,000 22,000 262 VII 85 4,800 (255) VIII 1,200 11,800 260 IX 1,100 19,200 260 X 390 2,800 (255) XI 730 13,100 252 Bipheny1 900 15,000 249 20 Acxv _»=a;aLaOLo,;uLta-_~.e.m eeeAHHHV _»ce;aLAOLopeeeLaee-.m.e..~.~ .AHHV ..m.m..~.~ .AHV -.e.e..~.~ cc eteeeam eoLCQLOmae ee_ow>ete_= .m acumen “say :ewzmsm>V .Aeegaenotopeeeteee-.e.e..m.m A>v-.m.m..m.m to eteeeam eoeeatomaa eeeoa>etepz .e etemwa 25 Egg; .you ORR“ .onu .ona" .o-u gig==_ .5, “Au o:— mozangomn< 22 mPAea;QCQOLo_eeeteee Axv -a.m.m.m .AXHV -m.e.m.m use sexeeeaca020_;awte AHHH>V -m.e.~ .AHH>V -m.e.~ Lo eteeeem compatomaa eepoc>ete_= .m seamen Asev :ewzmum><3 DON 28 Can 008 ’ D P 1 iii} - I 3 Gen mucmncomg C. Phot0products In all photolyses described the photoproducts obtained were identical in both degassed and non-degassed solutions. Hydrogen chloride gas was observed by the introduction of indicator paper into the reaction vessel and by its characteristic odor. Dark controls were employed in each case and no reaction was observed in these over the photolysis time period. All reactions were carried out by irradi- ation between 290-310 nm. In all cases mass balances based on reacted PCB were obtained. Photoproducts and their mass spectra parent peaks (M) are shown in Tables 3-9. Simplified vpc traces of photolyzed PCB in cyclohexane are given in Figures 11 and 12. The mass spectra of the photoproducts of 2,2',4,4'-tetrachlorobiphenyl (I) are shown in Figures 6-10. Photoproducts arising from I-XI exhibited fragmentation patterns dependent only on the number of chlorines present. 1. 2,2',4,4'-Tetrachlorobipheny1 (I). Irradiation of a 0.01M cyclo- hexane solution for 25 hours yielded three products with vpc Rt at 0.8, 1.9 and 2.6 minutes. By comparison with PCB standards available (Table 1) and their mass spectra these were identified as 4-chlorobi- phenyl ( .m mczmwm cow on :3” 25 Fxcmgawnocopcowou_¢.e mo :aom m: .n mgzmwu om Z99% of the incident light was absorbed. Equation 19 gives the rate constant for a 0 order reaction (k): (pea) - (pea)t = k t (18) where t is the time of irradiation and (PCB) is the substrate concentra- tion at O and t times. Calculating the amount reacted at several time periods of irradiation of degassed PCB solutions, a plot of substrate reacted vs time elapsed with a slope equal to k can be obtained. Plots for I-VI are shown in Figures 13-15. The k values are given in Table 10. For compounds VII-XI one point rate constants were determined, (Table 11). The k values obtained in cyclohexane were found to be slightly lower than those in methanol solution. The largest k values were obtained for PCB containing ortho-para chlorination (I, VII-IX, XI). Compounds with ortho-meta chlorination showed intermediate k values (II-IV,X), and those with mete-pgrg_ substitution,(V-VI) the lowest. 2. Quantum Yield Determinations. Absolute quantum yields of reaction (¢r) where determined for I-XI. Degassed cyclohexane solutions of PCB containing internal standard were irradiated in parallel with actinometer solutions at 3001:10 nm. After irradiation each tube was analyzed for PCB reacted by vpc. Values for ¢r are given in Tables 10 and 11. The quantum yields obtained correlated will with the rate constants. The extremely low values of ¢r found for V and VI by necessity must 37 38 contain a relatively large error and were calculated basically for comparison purposes. Table 10. Reaction Rate Constants(k) and Quantum Yields (¢r) for PCB in Cyclohexane PCB k, 10‘9M sec“ ¢r I 74.5 0.100 II 7.4 0.010 111 5.0 0.007 IV 3.8 0.005 v 1.2 0.002 V1 2.8 0.005 Table 11. One Point Reaction Rate Constants(k) for Unsymmetrically Substituted PCB in Cyclohexane Solution 9 1 PCB k, 10' M sec" ¢r VII 14.0 0.02 VIII 42.4 0.05 IX 29.3 0.04 X 6.0 < 0.01 XI 12.0 0.02 39 322389528288 0 3 new. .m.m van 4 AH>V u.¢.¢..m.m we mcmxmsopuxu cw mpcmpmcou mung cowpomwm .m_ weaned .0 . uoumo— EL b m. 2 . a w€_01 'P’PP‘” (83d) 3 40 mleeeazoazoehoe o :5 38.9.3 2; AVAHHHV -.m.m._N.N .a AHHV -_m.m-.~.~ Lo aeaxo;o_ozu e2 mecaemeou apex eoepoaam 8“ no. 6:5 m. o. m \ C o\o \ o\ b \ b .e_ assmca "'2 o .0.— wz_01 'wamoa) 41 AHV excegaLQOLopeoeeeee-.e.e._~.N mo mcmxosopozu cw ucmumcou mpmm :owuommm .m_ mcsm_a 8. no. as: m m T m o o\ 10... nu D . hum m D. o .0.» m. m . . Z W Q 1!: E. Quenching Studies. Lifetime of Triplet. In order to determine the lifetime of the excited state of I-VI quenching of the dechlorination reaction was attempted with isoprene (Et = 60 kcal/mole) and benzyl (52 kcal/mole). Both compounds proved inefficient, either because of high Et value or strong absorption in the region of irradiation. 1.3-Cyclohexadiene (Et = 50 kcal/mole) was found to be appropriate for the systems under study. Degassed solutions containing PCB and internal standard with varying quencher concentrations were irradiated in parallel at 300: 10 nm to 20% PCB conversion. Vpc analysis of PCB reacted permitted the calculation of relative quantum yields, ¢°/¢. The Stern-Volmer plots obtained were linear with unity intercepts (Figures 14-17). The slopes are equal to qu. The diffusion rate constants in cyclohexane and methanol are known,52 however, some change is expected in their value 53 at 30°C, according to the Debye equation which shows kq to be temperature dependent. - 1 3 kq - 4 (2 + d]/d2 + dZ/dl) 8RT/3 x 10 n (19) where d1 = d2, R = 8.31 x 107 ergs/mole deg K and n is the viscosity in gr sec'1 cm']. In cyclohexane at 303°K k = 0.81 x 1010 1 10 M sec-1. M sec’ and in methanol 1.3 x 10 Using these values the excited state lifetimes were calculated (Table 12). The T values were the same in both solvents within experimental error. 42 43 Table 12. Lifetimes of the Triplet Excited States of Polychlorinated Biphenyls PCB solvent qu T, 10'8 sec I methanol 101.4 0.78 I cyclohexane 68.8 0.85 II methanol 87.1 0.67 III methanol 100.1 0.77 IV methanol 91.0 0.70 IV cyclohexane 76.9 0.95 V methanol 248.3 1.91 V methanol 286.0 2.20 F. Intersystem Crossing Quantum Yields (oisc) The sensitized isomerization of §i§_+-trgn§_piperylene was used to determine isc for several PCB. Reproducible results were only obtained for 3,3',4,4'-tetrachlorobiphenyl (VI) indicating a value of unity by comparison with benzophenone. The sensitized phosphorecence of biacetyl gave better results allowing the calculation of isc for I, III, V and VI. Benzene solutions of PCB and biacetyl were degassed and their phosphorescence emission at 512 nm compared with that of benzene solu- tions of ben20phenone/biacetyl. In all cases ¢isc= 1 (Table 16). At 25°C PCB I-VI did not fluoresce or phosphoresce. 44 I I 1 t 1 £5 '1() 155 (Q), 10'3M Figure 16. Stern-Volmer plots of 3,3',5,5'- (V) O and 2,2',6,6'- (IV) . Tetrachlorobiphenyls using l,3-Cyclohexadiene in Methanol 45 I V I 1 :5 TC) 1:5 (Q). 10'3 M Figure 17. Stern-Volmer plots of 2,2',4,4'- (I). and 2,2',5,5‘- (II)() Tetrachlorobiphenyls using l,3-Cyclohexadiene in Methanol 46 .1 2 . .11 4D 0 .I o o’ P 1 1 5 10 (Q). 10‘3 M Figure 18. Stern-Volmer plots of 3,3',4,4'- (VI)() and 2,2',3,3'- (III). Tetrachlorobiphenyls using 1,3-Cyclohexadiene in Methanol 115 47 ‘ V V 0 5 10 15 (a), 10'3111 Figure 19. Stern-Volmer plots of 2,2'-6,6'- (IV) C and 2,2',4,4'- (1)0 Tetrachlorobiphenyls using l,3-Cyclohexadiene in Cyclohexane G. 48 Table 13. Intersystem Crossing Quantum Yields of Selected Polychlorobiphenyls PCB ¢isc I 1.00 i 0.01 III 0.97 i 0.03 V 0.99 i 0.02 VI 0.99 i 0.00 Reaction (kr) and Decay (kd) Rate Constants of the PCB Triplet. Using equations 20 and 21 and the values obtained for the quantum yield of reaction and the triplet lifetime, the rate constants of decay and reaction of I-VI were calculated (Table 17). -1 r = kd + kr (20) - -l ¢r - ¢isc kr (kd + kr) (2]) Table 14. Summary of Triplet State Reactivities of Polychlorinated Biphenyls PCB ¢r T, 10'8 sec I/T, 107 sec‘] kr’ 107 sec'1 kd, 107 sec-1 I 0.100 0.78 12.82 1.282 11.54 II 0.010 0.67 14.92 0.149 14.77 III 0.007 0.77 12.99 0.091 12.90 IV 0.006 0.70 14.28 0.086 14.20 V 0.002 1.91 5.23 0.010 5.22 VI 0.005 2.20 4.54 0.023 4.52 DISCUSSION A. Ultraviolet Spectroscopy and the Excited State of Biphenyl In general, when halogen atoms are associated to an aromatic system there is a shift of the n-n* absorption region towards the red. This is attributed to interactions between the lone-pair orbitals associated with the halogen atoms, and the aromatic n orbitals (reso- nance effect). The electronic transitions of lowest energy are still n-n*, but the upper orbital now has both C-C and C-X antibonding character, and the photodissociation of either bond is possible. The small bathochromic shifts obtained in the uv spectra of PCB in methanol point to the expected n-n* transition. 4,4'-Dichlorobi- phenyl shows Amax at 259 nm in cyclohexane and 261 nm in methanol. The absorption resulting from this transition tails out to 310 nm for I-XI, its intensity depending on the degree and position of the chlorine substituents. The extinction coefficient of the 249 nm band of biphenyl absorption (6 = 18000) indicated the probability of a particular trans- ient species forming. As shown in figures 3-5 that band's 5 decreases considerably in the presence of orthg_chlorines and increases with parg_ chlorination. This effect must be the result of successful inhibition 32’33 and of an of planarity of the triplet observed in other systems, increase in the double bond character of the Cl-Cl' bond by electron donation from the pgrg_position. From this data it could be predicted that in any photoreaction of PCB the driving force would be enhanced by the destabilizing effect of the grthg.chlorines and the stabilizing effect of the para_chlorines on the excited state. 49 8. Reaction Mechanism With the exception of most fluoroaromatics the primary process in haloaromatics involves cleavage of the C—X bond to yield radicals. In some of the early experiments the presence of free halogen atoms was observed from transient spectra due to formation of C10, BrO, I0, following the flash photolysis of the halogenated compound.16b The formation of dechlorinated PCB in hydrogen donating solvents such as cyclohexane and methanol points to the presence of a free radical species capable of hydrogen abstraction. Cleavage of the C-Cl bond (83 kcal/mole) in the triplet yields a phenyl radical in a primary rate determining step. Formation of HCl was detected during irradiation in keeping with such a mechanism. As shown in the results section it is preferentially the grthg_ chlorines that cleave. Some meta_cleavage was also observed (III), but could be considered negligible compared to orthg_dechlorination. In the absence of orth9_substituents meta_dechlorination occurs preferentially (VI, VIII, IX). These results point to the importance of steric and resonance effects in determining the course of the reaction. Dechlorination in I-VI was in a stepwise manner, chlorine cleaving first on one ring, then in the same position in the other ring. In none of the cases studied was one ring dechlorinated preferentially over the other. The methoxylated products obtained in methanol solution most likely arise from nucleophilic attack of the solvent with intermediacy of an ionic species stabilized by charge delocalization over the biphenyl system. 50 SCHEME 2 'SH +CI° CI Cl CI CI HCI HCI 52 Since no methoxylation occurs at the pa:a_p0sition it must be con- cluded that bond weakening of the orth9_chlorine plays an important part in this process. Attack of the nucleophile can be visualized as occurring on an already partially ionized C-Cl bond, where carbon has developed a partial positive and chlorine a partial negative charge. This weakening of the bond would not occur at the para_carbon-chlorine position since it does not destabilize the excited state. The mechanistic pathways for reductive dechlorination and photo- nucleophilic substitution are outlined in Scheme 2 for 2,2',4,4'- tetrachlorobiphenyl at 300 nm. C. Photochemical Mechanism 1. The following mechanistic scheme can be drawn from our results: 0 hv 1 * 100% 3 * (P-Cl) _——+ (P-Cl) _____+ (P-Cl) (22) I isc a 3(p-01)*'-§-+ (P-Cl) (23) d 3 * (P-Cl) ”‘E-+ products (24) r where o(P-Cl), 1(P-Cl)* and 3(P-Cl)* represent the PCB in its ground, excited singlet and excited reactive triplet states. The quanta absorbed in the initial excitation step are represented as Ia‘ Normal kinetic analysis yields expressions 25-29. -d °(P-C1) _ 3 * dt - Ia - kd (P-Cl) (25) 53 0 = kd 3(P-c1)* + k -d 3(P-c1)* dt r 3(P-c1)* - 1a (26) 3(P-c1)* = Ia/(kd + kr) (27) -d °(P-C1 -1 , dt 4 - (Ia) - 4,. (28) 4 =1- —-—-E—kd (29) r kd + r Expression 29 indicates that the quantum yield of reaction is independent of concentration and is determined only by kr and kd. Further manipulation of (29) leads to expression (30): _ -1 ¢r " ¢isc kr (kd + kr) (30) Equation (30) allowed direct calculation of the reaction rate constant for the excited state. This is the same expression normally employed for calculation of kinetic parameters in photoprocesses of triplet ketones when isc approaches 100%.54 1 and the absence of The linearity of the Stern-Volmer plots5 fluorescence from tetrachlorobiphenyls indicate the existence of only one reactive excited state, a triplet. Whether it is the lowest (T0) or one of the higher energy triplets has not been determined in this investigation. 2. Reactivity)of the Excited State. Evidence for reactivity of the triplet can be obtained directly from the quantum yield and lifetime measured. Since the lifetimes have been found to be the same in polar 54 and non-polar solvents, it can be assumed that the excited species is not polar and a diradical might provide a more accurate representation of its electron distribution. The lifetimes obtained for I-VI (Table 12) show little variation among the ortho chlorinated compounds. However, 3,3',5,5'- and 3,3',4,4'— tetrachlorobiphenyls show values greater by a factor of three than those of I-IV. Apparently two gr;hg_chlorines are sufficient to decrease the amount of inter-ring conjugation to an appreciable extent. The stabilizing effect of para_chlorination only slightly affects the lifetime. Quantum yields give a more dramatic demonstration of the substit- uent effects as shown in tables 10 and 11. The value of ¢r for 2,2'4,4‘- tetrachlorobiphenyl (0.100) is greater by a factor of 10-50 than those obtained for II-VI. The unsymmetrical PCB VII-IX and XI also show larger values. The lowest values obtained are for those compounds containing no orthg_chlorines (V and VI). It is clear that the reactiv- ity of PCB could at least partially be explained on the basis of the Aggthg? effect, however, the large difference between the 2,4 and 2,3 substituted PCB could not be explained without calculation of the kd and kr values. The ratios of decay/reaction for six PCB triplets are listed below. PCB kd/kr I 9 II 100 III 120 IV 150 V 520 VI 200 55 In all cases decay is preferred to reaction, however this tendency grows more pronounced in the order 2,4 - 2,3 - 3,4 - 3,5 substitution. In compounds having 2,4 substitution the reaction becomes a competitive path to reversion to the ground state. Transannular effects are shown to be present in the reaction of 3,4,2'-trichlorobipheny1 (XI) to yield 3,4-dichlorobiphenyl (¢r = 0.02). This quantum yield is the smallest of those obtained for 2,4 PCB, but still greater than those of the rest, the highest of which is 0.01 (II). It can be concluded that the driving force for the reaction is greatest when both the orthg_and pgra_chlorines are on the same ring and that the rates increase when the effects arise from both rings. 0. Environmental Significance Since PCB are thermally and biologically non-degradable, their presence in the environment, either by misuse or accidental leakage, constitutes a long-term problem. Photochemical degradation is shown to be a major if not the only degradation pathway, since solar radiation of the wavelength required is present. The prevalent substitution patterns of tetrachlorobiphenyls found in Arochlor 1242 and 1248 (42 and 48% chlorine respectively) are 2,5; 2,3; 2,4; and 3,410. Of these the only ones that can be degraded with relative ease are the 2,4 substituted isomers. The photochemical introduction of biological "handles", which make metabolic degradation possible, is shown to take place in methanol and in water. Methoxy and hydroxy groups are introduced rather slowly, this detracts from their environmental significance. Furthermore, since PCB introduction in the aquatic environment usually takes place in the 56 presence of other hydrophobic substances (0115), contact with the potential nucleophile, water, would be minimized. E. Summary The involvement of steric and electronic effects in the biphenyl system is well supported. Triplet reactivities showed a marked sensi- tivity toward the degree and position of chlorine substituents. Greater quantum yield and rate constants of reaction are characteristic of 2,4 substitution while the reverse is true for 3,4 or 3,5 patterns. The triplet lifetimes were found to be dependent on substituent positions as well. The photoproducts of PCB reactions arose through the same excited state yja_dechlorination and/or nucleophilic substitution. F. Further Experiments The following experiments are examples of possible ways towards acquiring more information on PCB photochemistry: 1. Solid state photolysis of individual isomers could be carried out in soils containing environmental triplet sensitizers such as mercury salts and quinones. 2. The effect of chlorine substituents in haloterphenyls, one ring apart could be studied to determine the extent of transannular effects from the para_position on one ring to the ortho-in another. 3. Other electron donating substituents could be examined to study their effects on triplet lifetime and reactivity. EXPERIMENTAL PART I. MATERIALS AND PROCEDURES A. Preparation and Purification of Materials Purity is of critical importance in determining photochemical rate constants, since even small amounts of quenching or sensitizing sub- stances can have large effect on slow rates. Determination of the properties of the excited state such as lifetime, and intersystem crossing can be affected. Therefore all the compounds used in the photolyses described were carefully checked by vpc after purification to insure against such occurrences. 1. Polychlorinated Biphenyls a. 2,2',4,4'-Tetrachlorobiphenyl (I). 2,4-Dichloroaniline was diazotised and upon addition of potassium iodide it yielded 2,4-dichloro- iodobenzene (85% yield).57 2,4-Dichloroiodobenzene (109, 0.036 mole) was dissolved in 50 ml of refluxing dimethyl formamide (DMF) and copper powder (109, 0.16 mole) was added with stirring over a two hour period. 58 The reaction The mixture was refluxed under nitrogen for 20 hours. mixture was then poured into 400 ml of ice water and allowed to stand overnight under refrigeration. Vacuum filtration yielded a brown solid which was then extracted with 200 m1 of boiling acetone. The undissolved residue was filtered off and discarded. The acetone extract was concentrated to 30 ml in the rotary evaporator and upon cooling a yellow precipitate formed. This solid was recrystallized twice from ethanol and the white crystals obtained were dried under vacuum. The yield was 4.1 g (78% of theory). The melting point was 44-45°C. The nmr spectra showed signals at a 7.25 (2H, multiplet) and 7.50 (1H, triplet). The mass spectrum showed a parent peak at m/e 290. 57 58 M + 2, M + 4, M + 6 and M + 8 peaks were present. Fragments were observed at m/e 255, 220, 185 and 150 corresponding to dechlorinated species. Vpc analysis (COLl) showed a single peak and the sample was considered to be > 99.9% pure. b. 2,2',5,5'-Tetrachlorobiphenyl (II). Preparation was analogous to that of I. The crude product was recrystallized from acetone and ethanol. The white crystals were dried under vacuum and their melting point determined (86-87°C). The yield was 4.2 g (80% of theory). The nmr spectrum showed a multiplet at 6 7.35. The mass spectrum was similar to that of I. Vpc analysis analysis showed a single peak (COLl). c. 2,2',3,3'-Tetrachlorobiphenyl (III). Preparation was analogous to the ones previously described. The white crystals obtained after recrystallization from ethanol and acetone were dried and sublimed (50°C, 0.2 mm). Their melting point was 120-121°C. The nmr spectrum showed multiplets at 6 7.18 (2H) and 7.42 (1H). The mass spectrum had the same fragmentation patterns as those of I and II. The compound was found to be pure by vpc (COLl). d. 3,3',5,5'-Tetrachlorobiphenyl (V). Using the same method white crystals were obtained which melted at 168-169°C. The yield was 4.8 g (91% of theory). Its nmr spectrum showed a coincidental singlet at 6 7.50. Its mass spectrum was in agreement with its structure, showing the presence of four chlorines and the biphenyl skeleton. Only one peak was detected by vpc. e. 2,2',6,6'-Tetrachlorobipheny1_(IV). All attempts to prepare this compound failed. Ullman coupling of 2,6-dichloroiodobenzene yielded only unreacted starting material. Photochemical coupling in 21 benzene at 300 nm yielded only l,3-dichlorobenzene and 59 2,6-dich10robiphenyl. Compound IV was finally obtained from Analabs, Inc. (North Haven, Conn.). The sample was recrystallized twice from ethanol and its mass spectrum checked for authenticity. Vpc analysis showed a single peak (COLl). f. 3,3',4,4'-Tetrachlorobiphenyl (VI). This compound was pre- 59 pared using a modified form of the procedure described by Tsutsui. 3,4-dichloroiodobenzene was obtained from Pfaltz and Bauer, Inc., Flushing, N. Y.) and 15 g (.055 moles) of it reacted with 1.3 g of magnesium metal (0.055 mole) in refluxing anhydrous ether (50 ml). One hour after the magnesium was consumed 4 g (0.03 mole) of copper (II) chloride was added and refluxing continued for 2 hours. The product was extracted with 100 ml of ether and the extract concentrated to 20 m1. Upon cooling a white solid precipitated. Upon recrystallization from ethanol and vacuum drying the melting point was determined (179-180°C). The nmr spectrum showed signals at 6 7.80 (1H) and 7.61 (2H). The mass spectrum was similar to that of I-V. Vpc analysis showed one peak (COLl). g. 2,4,6-Trichlorobiphenyl (VII)_was obtained commercially (Analabs) and purified as described above. h. 2,4,5-Trichlorobiphenyl (VIII) was obtained from Analabs and recrystallized from ethanol until pure by vpc. i. The same procedure described in (h) was used for all other PCB samples obtained from Analabs, Inc. j. 2,4,4-'Trichloro-Z'-methoxybiphenyl. 2,4-Dichloroiodobenzene and 2-methoxy-4-chloroiodobenzene (Aldrich) in equimolar amounts were coupled with copper powder in DMF in a manner analogous to I. The reaction yielded 28% I, 51% trichloromethoxybiphenyl and 8% dichloro- dimethoxybiphenyl. These products were separated by vpc (COL 5) and 60 identified by vpc-mass Spectrometric techniques. The mixture was used as standard for the identification of the photoproducts obtained from compound I in methanol. k. 2,2',4-Trichloro-4'-methoxybipheny]. Preparation was analo- gous to that of I. 2,4-Dichloroiodobenzene and 2-chloro-4-methoxy- iodobenzene (Aldrich) in equimolar amounts were coupled using copper powder in DMF to yield I (24%), 2,2'-dichloro-4,4'-dimethoxybipheny1 (11%), and 2,2',4-trichloro-4'-methoxybiphenyl (56%). These products were identified using gas chromatography-mass spectrometry (COLS) and the mixture used as standard for identification of the photoproducts of I in methanol. 2. Solvents a. Benzene (J. T. Baker, Chem. Co., GC-Spectrophotometric Quality) was washed with concentrated sulfuric acid until the acid layer no longer turned yellow. It was then washed with sodium hydroxide, saturated sodium chloride and distilled water. After drying over sodium sulfate it was distilled from P205. Only the center cut (70%) was retained. b. Toluene (Fisher Scientific Co.) purification was similar to that of benzene. c. Cyclohexane (Burdick and Jackson Labs., Glass Distilled) purification was similar to that of benzene. d. n-Hexane (Burdick and Jackson Labs., Glass Distilled) was fraction distilled twice. e. Methanol (Burdick and Jackson Labs., Glass Distilled) was fraction distilled twice over magnesium metal. f. Ethanol (Commercial Solvents Co., Absolute) was used as 61 received. 9. Acetone (Burdick and Jackson Labs., Glass Distilled) was used as received. 3. Quenchers a. cis-l,3-Pentadiene (cis-piperilene, Aldrich Chem. Co.) was passed through alumina followed by distillation. Pure by vpc (COL2). b. trans-1,3-Pentadiene (Aldrich) was used as received. c. 1,3-Cyclohexadiene (Aldrich) was used as received. d. Biacetyl (Aldrich) was fraction distilled. 4. Sensitizers a. Benzophenone (J. T. Baker) was recrystallized three times from ethanol. b. Valerophenone (Aldrich) was fraction distilled. 5. Internal Standards a. Hexadecane (Aldrich) purification was similar to that of benzene. b. Octadecane (Aldrich) was recrystallized from ethanol. c. Eicosane (Matheson, Coleman and Bell) was used as received. d. Docosane (Matheson, Coleman and Bell) was used as received. B. Photolysis Procedures 1. Preparation of Samples Class A volumetric flasks and pipettes were used exclusively to make up photolysis solutions. Stock solutions of PCB and internal standards in various solvents used were prepared in 25 ml volumetrics. For quantum yield determina- tions (PCB) concentrations ranged from 0.05M to 0.200M to insure absorption of 99.9% of the impinging light in the 290-310 nm range. For triplet lifetime measurements two stock solutions were prepared in each 2M concentration with an adequate concen- case. One contained PCB in 10' tration of internal standard and the other contained quencher (l,3- cyclohexadiene). One milliliter of PCB solution was then pipetted into each of five 10 ml volumetrics and 1,2,4,6, or 8 ml portions of quencher solution added. The flask was then filled to the mark with solvent. From each of these solutions three exactly 3 ml portions were withdrawn yja.a 10 ml syringe and injected into 13 x 100 mm Pyrex culture tubes which had been drawn into small capillaries about 2 cm from the open end to facilitate sealing after degassing. For photoproduct identification 10'3M solutions of I-XI were photolyzed in a 50 ml round bottom flask equipped with a magnetic stirrer and then concentrated for analysis. 2. Degassing In order to remove dissolved oxygen, sample tubes were attached to a vacuum line over no. 00 one-holed rubber stoppers on individual stop- cocks. The solutions were slowly frozen above liquid nitrogen and then immersed before opening to the vacuum. A minimum vacuum of 0.01 mm was 62 63 attained before closing the stopcocks and allowing the tubes to thaw. After the fourth freezing and evacuation the tubes were sealed off with a torch. In cases where only rough estimates of rate constants and quantum yields were needed, the solvents were placed in a ultrasonicator for 0.5 hours and then nitrogen was bubbled through for an additional 0.5 hours. 3. Irradiation Sample tubes were irradiated in parallel on a rotating merry-go- round apparatus60 to insure that the same amount of light impinged each sample, for quantum yield determinations and quenching studies. The light source used was a Rayonette Reactor (The Southern N.E. Ultraviolet Co.) fitted with RUL 3000 lamps having a peak output energy (90%) at 300: 10 nm (Figure 20). The light output was continuous over > 80 hour periods (Figure 21). The irradiation chamber temperature was 30°C. RELATIVE EN E RGY 64 230 300 400 WAVELENGTH (nm) Figure 20. Energy output distribution of RUL3000 UV lamps 65 ..1 1.0.. NERGY ourPu1(quqmum-It". IO" 1 Y o 20 40 00 TIME (nouns) Figure 21. Energy output of RUL 3000 lamps C. Analysis of Photolysate 1. Instruments All analyses for photoproducts were made in the following instru- ments equipped with flame ionization detectors. VPC-l Varian Aerograph, model 1400 with Sargent recorder. VPC-Z Beckman, model GC-65 with Beckman recorder and disc integrator. VPC-3 Beckman, model GC-4 with Bristol Dynamaster recorder and disc integrator Model 202. A variety of vpc columns were utilized, as designated below. COL-1 50' x 1/16" o.d. 0.02“ i.d. stainless steel S.C.O.T. (Support Coated Open Tubular) SE-3O column (Perkin-Elmer) Used at temp- eratures in the 150-210°C range, nitrogen flow 6-8 ml/min. COL-2 32' x 1/8" stainless steel column containing 25% 1,2,3-tris- (2-cyanoethoxy)propane on 60/80 chromosorb P. Used at temp- eratures in the 40-60°C range. COL-3 6' x 1/8“ glass column containing 3% 0V-210 on 60/80 mesh Gas Chrom 0. Used at temperatures in the 130-200°C range. COL-4 6' x 1/8" stainless steel column containing 3% Apiezon L on 60/80 mesh Gas Chrom Q. COL-5 6' x l/8" stainless steel column containing 15% QF-l, 10% DC-200 on Gas Chrom 0. (60/80 mesh). Mass spectra were obtained from a DuPont 21-490 apparatus interfaced with VPC-2 (COLl and 5) or the sample was introduced by direct probe. 2. Product Identification PhotOproducts were identified primarily by their vpc retention times, mass spectra and comparison to authentic standards. The photolysate of 66 67 I-XI always contained starting materials and dechlorination products. Their retention times (Rt) increasing with greater chlorine content and amount of vicinal substitution. The detector response did not vary appreciably for PCB containing 2-4 chlorines. Using COL-l the methoxylated products obtained in methanol had only slightly greater Rt than their chlorinated counterparts. Using COL-5 much better separation was obtained. In the case of 2,2',4,4'—tetrachlorobipheny1 enough conversion occurred so that an nmr spectra of the major photoproduct, 4,4'-dichloro- biphenyl, could be obtained. The nmr matched that of an authentic sample (6 7.31, coincidental singlet). 3. Standardization Vpc detector response was checked for linearity using varying concentrations of PCB. To determine the remaining PCB concentration after photolysis the vpc area ratio (PCB)t/(PCB)0 was multiplied by the initial concentration. The use of an internal standard insured that the injections were the same. D. Actinometry 1. Quantum Yields of Reaction (¢r) All sample tubes for which quantum yields were measured had PCB concentrations sufficient for complete absorption of the incident light. Qis-tran§_isomerizations of gig-piperilene were employed to monitor the light output. Actinometer tubes containing 0.05-0.10 M concentrations of ben20phenone and 0.1-0.2 M gjsfpiperilene in benzene solution were prepared as described before and irradiated in parallel with the sample tubes. The triplet state of benzophenone formed quantitatively from the singlet (100% isc), is completely quenched by piperilene, the excited 61 piperilene then decays to both gi§_and trans isomers in a known ratio. The light intensity (Einstein 1t'1) Ia can be calculated from the following relationship: * Ia = (pip3 ) = (g; pip)o ln(0.555/O.555-% trans) (31) Due to the length of time some samples had to be irradiated, specially those with very small quantum yields, several sets of actinometer tubes had to be used in series. 2. Intersystem Crossing Quantum Yields (¢’isc) Measurement of isc was attempted by comparison of the benzophenone and PCB sensitized isomerization of gig-piperilene. Only compound VI yielded reproducible results, indicating 100% isc. Probably radical reactions with the quencher resulted in marked variance of the results. Measurement of isc was successfully accomplished by sensitizing the phosphorescence of biacetyl (512 nm) with either benzophenone (isc = l) 68 69 or PCB. The intensities obtained were compared to each other and the ratio provided a direct measure of intersystem crOSSing in PCB. After each measurement the degassed sample was aerated and a 0 intensity reading obtained. All phosphorescence studies were carried out in an Aminco Spectrofluorophotometer. E. Ultraviolet Spectroscopy Uv spectra were obtained from a Beckman DB-G grating spectro- 4 photometer. Solutions 0.1-5.0 x 10' M in I-XI were prepared in cyclo- hexane and scanned from 320-220 nm. The sample was contained in a standard 1 cm quartz cell. EXPERIMENTAL PART II. KINETIC DATA A. Reaction Rate Constants for Polychlorinated Biphenyls in Cyclohexane Solution . General Comments: (PCB)o refers to the starting tetrachlorobiphenyl concentration. (PCB)t is the concentration at time t. The concentra- tion of PCB is plotted vs time to obtain the rate constant (k) from the slope. Cn is the internal standard. Every figure given for (PCB)o - (PCB)t is the average of at least two simultaneously irradiated samples. Correlation values were obtained by the least squares method. 70 Table 15. Reaction Rate Constants for Polychlorinated Biphenyls (I-VI) in Cyclohexane Solution 71 Part A. 2,2',4,4'-Tetrachlorobiphenyla (PCB)°,M (PCB)t,M (PCB)°-(PCB)t,M time,1055ec 0.190 0.185 0.005 0.67 0.190 0.160 0.030 4.02 0.190 0.145 0.045 6.00 a0.100 M C20 standard used. Correlation 1.0000 Part B. 2,2',5,5'-Tetrachlorobiphenyla o t o t . 5 (PCB) ,M (PCB) ,M (PCB) -(PCB) ,M t1me,lO sec 0.078 0.074 0.004 5.40 0.078 0.070 0.008 10.70 0.078 0.068 0.010 13.30 30.100 M C20 standard used. Correlation 1.0000 Part c. 2,2',3,3'-Tetracn1orobipneny1a o t o t . 5 (PCB) ,M (PCB) ,M (PCB) -(PCB) ,M t1me,10 sec 0.079 0.076 0.003 6.12 0.079 0.075 0.004 8.20 0.079 0.072 0.007 14.60 a0.100 M C20 standard used. Correlation 0.9999 72 Part D. 2,2',6,6'-Tetrachlorobiphenyla (PCB)°,M (PCB)t,M (PCB)°-(PCB)t,M time.105sec 0.045 0.044 0.001 2.70 0.045 0.043 0 002 5.20 0.045 0.042 0 003 8.30 a0.050 M C22 standard used. Correlation 1.000 Part E. 3,3',4,4'-Tetrachlorobipheny1a (P08)°.M (PCB)t,M (PCB)°-(PCB)t,M time,105sec 0.012 0.011 0.001 3.6 0.012 0.0105 0.0015 5.4 0.012 0 010 0.002 6.8 a0.010 M C22 standard used. Correlation 0.9980. Part F. 3,3',5,5'-Tetrachlorobiphenyla (PCB)°,M (PCB)t,M (PCB)°-(PCB)t,M time,1055ec 0.0230 0.0225 0.0006 5.0 0.0230 0.0220 0.0010 8.3 0.0230 0.0210 0.0020 15.3 a0.020 M C22 standard used. Correlation 0.9986. B. Quantum Yield of Reaction for Polychlorinated Biphenyls in Cyclohexane Solution General Comments: (PCB)0 and (PCB)t represent the initial and final concentrations of starting material, as determined by vpc analysis. Cn represents the internal standard used in each case. Ia (light absorbed) values were obtained from gig-piperilene actinometry as described previously and are averages of at least two tubes per period. All quantum yield (¢r) determinations required several sets of actinometer tubes irradiated in series. Table 16. Quantum Yields of Reaction for I-VI. Part A. 2,2',4,4'-Tetrachlorobiphenyla 0 t -1 (PCB) ,M (PCB) ,M (PCB)reacted’M Ia,E 1t ¢r 0.190 0.144 0.046 0.456 0.101 0.150 0.105 0.045 0.456 0.100 a 0.100 M C20. Part B. 2,2',5,5'-Tetrachlorobipheny1a o t -1 (PCB) ,M (PCB) ,M (PCB)reacted’M Ia,E 1t ¢r 0.078 0.073 0.005 0.456 0.011 0.120 0.116 0.004 0.456 0.009 a 0.100 M C20. 73 74 Part c. 2,2',3,3'-Tetracn1orobipneny1a 1 o t ~ (PCB) ,M (PCB) ,M (PCB)reacted’M Ia,E 1t 1r 0.079 0.076 0.003 0.456 0.007 0.095 0.091 0.004 0.456 0.008 . T o 100 M C20° ; Part 0. 2,2',6,6'-Tetrachlorobiphenyla E (PCB)° M (PCB)t M (PCB) M I E 1t" a ’ ’ reacted’ a’ r 0.045 0.042 0 003 0.502 0.006 0.090 0.087 0.003 0.502 0.006 a 0.100 M 020- Part E. 3,3'5,5'-Tetrachlorobiphenyla (PCB)° M (P08)t M (PCB) M I E 1t“1 6 ’ ’ reacted’ a’ r 0.0230 0.0218 0.0012 0.626 0.002 0.0115 0.0104 0.0011 0.626 0.002 a 0.010 M C22. 75 Part F. 3,3',4,4'-Tetrachlorobiphenyla -1 o t (PCB) ,M (PCB) ,M (PCB)reacted’M IaE’ 1t ¢r 0.012 0.010 0.002 0.537 0.004 0.024 0.022 0.002 0.330 0.006 a 0.010 M C22. 76 C. Quenching Relative quantum yields of reaction for I-VI were measured as a function of quencher concentration for use in the Stern-Volmer quenching plots. Conversions (PCB reacted) were kept below 20% to insure linearity. Cn is the internal standard used. Table 17. 2,2',4,4'-Tetrachlorobiphenyl (2.9 x 10'3 M) in Methanol. Quenching with 1,3-Cyclohexadienea -3 -4 o (Quencher),10 M (PCB)reacted,]0 M ¢r/¢r 0.00 7.10 1.00 3.61 5.30 1.34 4.90 4.80 1.49 6.55 4.30 1.65 7.70 4.00 1.79 °czo, 0 005 M internal standard. Correlation 0.9990. Table 18. 2,2',5,5'-Tetrachororbiphenyl (2.02 x 10‘3 M) in Methanol. Quenching with l,3-Cyclohexadienea -2 -4 o (Quencher),10 M (PCB)reacted’10 M or /¢r 0.00 3.00 1.00 0.34 2.30 1.30 0.70 1.90 1.59 1.01 1.60 1.85 1.70 1.20 2.50 “020. 0.002 M internal standard. Correlation 0.9686. 77 Table 19. 2,2',3,3'-Tetrachlorobiphenyl (3.26 x 10'3 M) in Methanol. Quenching with l,3-Cyclohexadienea 3 -4 (Quencher), 10 M (PCB)reacted,lO M ¢rl¢r 0.00 8.20 1.00 2.69 6.90 1.19 5.38 5.50 1.49 8.07 4.70 1.74 13.50 3.50 2.33 aC20, 0.002 M internal standard. Correlation 0.9980. Table 20. 2,2',6,6'-Tetrachlorobiphenyl (2.65 x 10'3 M) in Methanol. Quenching with l,3-Cyclohexadienea (Quencher) 10'3M (PCB) 10’4M 4° lo ’ reacted’ r r 0.00 5.60 1.00 3.44 4.40 1.27 6.88 3.50 1.61 10.30 2.90 1.91 17.20 2.20 2.56 a020, 0.002 M internal standard. Correlation 0.9995 78 Table 21. 3,3',4,4'-Tetrachlorobiphenyl (3.28 x 10‘3 M) in Methanol. Quenching with l,3-Cyclohexadienea (Quencher),10'4M (Pcn)reacted,10'4 a: /or 0.00 7.00 1.00 2.00 6.50 1.07 6.10 5.90 1.18 10.10 5.50 1.28 14.30 5.01 1.40 °czz. 0.003 M internal standard. Correlation 0.9992. Table 22. 3,3',5,5'-Tetrachlorobiphenyl (1.49 x 10'3 M) in Methanol. Quenching with l,3-Cyclohexadienea (Quencher),10'4M (PCB)reacted’]O-4 9: /¢r 0.00 6.20 1.00 0.93 4.40 1.40 1.86 3.70 1.69 2.79 3.50 1.79 3.72 3.20 1.96 a022, 0.002 M internal standard. Correlation 0.9686. 79 Table 23. 2,2',4,4'-Tetrachlorobiphenyl (6.19 x 10'3 M) in Cyclohexadiene. Quenching with l,3-Cyclohexadienea -3 -4 o (Quencher),10 M (PCB)reacted’]0 M ¢r lor 0.00 9.00 1.00 3.84 7.40 1.22 5.76 6.30 1.43 7.68 5.70 1.58 9.60 5.30 1.70 aC20, 0.005 M internal standard. Correlation 0.9943. Table 24. 2,2',6,6'-Tetrachlorobiphenyl (2.25 x 10'3 M) in Cyclohexane. Quenching with l,3-Cyclohexadienea -3 - o (Quencher),10 M (PCB)reacted,lO 4M ¢r /o 0.00 4.00 1.00 1.92 3.10 1.28 5.76 2.50 1.56 7.68 2.50 1.56 a0 22’ 0.003 M internal standard. Correlation 0.9914. 0. Determination of Intersystem Crossing Quantum Yields Table 25. Triplet Sensitized gi§;trans Isomerization of Piperilenea b Sample 1 Sample 2 Sample 3 Average Compound % trans % trans % trans % trans Benzophenone 0.218 0.217 0.212 0.215 VI 0.211 0.215 0.210 0.212 30.38 M initial §j§;piperilene b0.20 M. Table 26. Triplet Sensitized Phosphorescence of Biacetyla b Average Average Average Compound Intensity Intensity Intensity Benz0phenone 47.4 -47.0 47.2 I 47.6 46.8 47.2 III 45.9 46.2 46.1 V 47.1 46.7 46.9 VI 46.9 46.9 46.9 a0.04 M Biacetyl in Benzene. b0.03-0.05 M. 80 LIST OF REFERENCES \I 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 010194» LIST OF REFERENCES New Scientist.33€ 612 (1966). R. W. Risebrough, P. Rieche, D. B. Peakall, S. G. Herman and M. N. Kirvan, Nature,g20, 1098 (1968). C. G. Gustafson, Environmental Science and TechnologyyA, 814 (1970). D. B. Peakall and J. L. Lincer, BioScience,20, 958 (1970). G. D. Veith and G. Lee, Water Research,4, 265 (1970). A. Richardson, J. Robinson, A. Crabtree and M. K. 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