THESIS I Z (:0 LIBRARY Michigan State University This is to certify that the thesis entitled PHOTOINDUCED CLEAVAGE OF CARBON-HALOGEN BOND FOR MONO-HALOGENATED 2’,4’,6’-TRI]VIETHYLBENZOPHENONES presented by Yijun Tang has been accepted towards fulfillment of the requirements for MS. Chemistry degree in Date 08/18/00 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution PLACE IN REfURN Box to remove this checkout from your record. To AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 6m cJCIRC/DateDuepes-p. 15 PHOTOINDUCED CLEAVAGE OF CARBON-HALOGEN BOND FOR MONO-HALOGENATED 2',4’,6’-TR|METHYLBENZOPHENONES By Yijun Tang A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemistry 2000 ABSTRACT PHOTOINDUCED CLEAVAGE OF CARBON-HALOGEN BOND FOR MONO-HALOGENATED 2’,4’,6’-TR|METHYLBENZOPHENONES By Yijun Tang One of the general reactions of mono-halogenated 2',4’,6’- trimethylbenzophenones is the carbon-halogen bond cleavage when the reactant is exposed to ultraviolet light source with proper wavelength. This type of reaction, on which my research has been focused, is believed to be occurring from the lowest triplet states. Quenching with triplet quenchers supports such thinking as well as provides triplet lifetime of reactant molecules. Photoinduced carbon-bromine bond cleavage and carbon-iodine bond cleavage have been studied by Professor Peter J. Wagner. By comparing with his study, some important conclusion can be made. It seems that halogenated 2’,4’,6’-trimethylbenzophenones have similar behavior as their non-alkyl- substituted analogues, in terms of reaction rate constant and triplet lifetime. Only apparent difference lies in the reactions of 3-bromo-benzophenones and 4-bromo benzophenones, 2’,4’,6’-trimethyl and non-substituted. For iodo-2’,4’,6’-trimethylbenzophenones, the carbon-iodine bond cleavage is the only major pathway of deactivation of the excited triplet reactant molecules. 2-iodo-2’,4’,6’-trimethy|benzophenone and 4-iodo-2’,4’,6'- trimethylbenzophenone have a larger carbon-iodine bond cleavage constant than 3-lodo—2’,4’,6’-trimethylbenzophenone, which is consistent with the electron density of n“ orbital, indicating 3(n,rt*) state is responsible for the reaction. As for bromo—2’,4’,6’-trimethylbenzophenones, the quantum yields of dehalogenated 2’,4’,6’-trimethylbenzophenone formation from 3-bromo-2’,4’,6’- trimethylbenzophenone and from 4-bromo-2’,4',6’-trimethylbenzophenone are very low. This may suggest some competing reactions. One likely possibility is hydrogen abstraction from solvent by the triplet ketones. 2-bromo-2’,4’,6’- trimethylbenzophenone undergoes more efficient carbon-bromine bond cleavage reaction with a much higher quantum yield compared to 3- and 4- bromo-2’,4’,6’- trimethylbenzophenones. Both the rate constants and the quantum yields of dehalogenated ketone formation are small in the case of chloro-2’,4’,6’-trimethylbenzophenones because of the relatively high carbon-chlorine bond energy. Dedicated to my parents, sister, wife and those who made this paper possible. For with You is the fountain of life; in Your light we see light. Psalm 36:9 ACKNOWLEDGMENTS First of all, I must thank Professor Peter J. Wagner. As the boss in this group and my advisor, his brilliant understanding of and invaluable experience in physical photochemistry has benefited my study so much that I never thought I would meet with any difficulties that could not be overcome in the course of my graduate life. No one got surprised when the information came that he had won the Inter-American Photochemical Society Award in Photochemistry. I also appreciate that he has guided me in selecting some critically useful courses. Professor Wagner is kind of a strict man, which comforts me in that I know I deserved whatever the title I achieved in this group. On the other hand, his sense of humor has impressed me a lot, especially when he releases those humorous remarks with a serious looking. I would also like to thank all other professors in my committee, who are Professor James E. “Ned” Jackson, Professor Gary J. Blanchard, and Professor David P. Weliky. In the past three years, we had several enjoyable party-like committee meetings, which will be shining pearls worthy of recalling in the string of my life. I am very grateful to the precious discussion that they have offered. I owe a lot to the following co-workers in our group: Mr. Jong-lll Lee, Dr. Hyo-Jung Yoon, and Ms. Varsha Govardhan. The good time we have spent together in the lab bears in my mind forever. At last but absolutely not the least, I would like to thank my loving wife Danyan for having been backing me up while we start our new life in the United States of America. vi TABLE OF CONTENTS Abstract ................................................................................................................. ii Acknowledgements .............................................................................................. vi List of Tables ....................................................................................................... viii List of Figures ....................................................................................................... ix List of Abbreviations .............................................................................................. x INTRODUCTION ................................................................................................... 1 A. Mechanism of carbon-halogen bond cleavage for halogenated aromatic ketones ..................................................................... 1 B. Competitions with carbon-halogen bond cleavage ....................................... 2 81. Competition at bond cleavage stage: Hydrogen abstraction ................. 2 82. Competition at hydrogen abstraction stage: Recoupling ....................... 4 C. Electronic states of benzophenones ............................................................ 5 D. Kinetics and quenching study ...................................................................... 6 E. Goals of research ....................................................................................... 10 RESULTS AND DISCUSSIONS ......................................................................... 11 A. Quantum yield ............................................................................................ 11 B. Quenching study ........................................................................................ 12 C. Conclusion ................................................................................................. 17 EXPERIMENTAL ................................................................................................ 18 A. Synthesis ................................................................................................... 18 A1 . Chloro-2’,4’,6’-trimethylbenzophenones .............................................. 18 A2. Bromo-2’,4’,6’-trimethylbenzophenones .............................................. 20 A3. lodo-2’,4’,6’-trimethylbenzophenones .................................................. 22 B. Identification ............................................................................................... 25 C. Quantum yields .......................................................................................... 25 D. Quenching study ........................................................................................ 27 REFERENCES .................................................................................................... 29 vii LIST OF TABLES Table 1. The quantum yields of some BPs in cyclopentane ................................ 11 Table 2. Room temperature photokinetics for some XM3BPS in cyclopentane....15 Table 3. Melting points of XMaBPs ...................................................................... 24 Table 4. Spectroscopic features of some benzophenones .................................. 25 viii LIST OF FIGURES Figure 1. Mechanism of photoinduced cleavage of Carbon-halogen bond for olBP ............................................................... 1 Figure 2. Fate of triplet halo-benzophenone molecules in cyclopentane ............... 3 Figure 3. Benzocyclobutenol formation from 2,4,6- trialkylbenzophenones ............................................................................ 3 Figure 4. Fate of halogen phenyl radical pair after bond cleavage ........................ 4 Figure 5. Energy diagram of excited triplets of halophenyl ketones ...................... 5 Figure 6. Mechanistic representation with rate constants ..................................... 7 Figure 7. Quenching by naphthalene of M3BP formation from oBrM3BP ............. 13 Figure 8. Quenching by naphthalene of M3BP formation from mBrMaBP ............ 13 Figure 9. Quenching by naphthalene of M3BP formation from pBrMaBP ............. 13 Figure 10. Quenching by naphthalene of M38P formation from olM3BP ............. 14 Figure 11. Quenching by naphthalene of M38P formation from mlMaBP ............ 14 Figure 12. Quenching by naphthalene of M3BP formation from legBP ............. 14 Figure 13. Synthesis of XMaBP ........................................................................... 18 LIST OF ABBREVIATIONS BP ................................................................................................... benzophenone BrBP ..................................................................................... bromo-benzophenone BrM3BP ......................................................... bromo-2’4’6’-trimethylbenzophenone CIBP ..................................................................................... chloro—benzophenone CIM3BP ........................................................ chloro-2’,4’,6’-trimethylbenzophenone IBP .......................................................................................... iodo-benzophenone IMaBP ............................................................. iodo-2’,4’,6’—trimethylbenzophenone M38P ...................................................................... 2’,4’,6’-trimethylbenzophenone mBrBP ............................................................................... 3-bromo-benzophenone mBrM3BP ................................................... 3-bromo-2’4’6’-trimethylbenzophenone mClBP ............................................................................... 3-chloro-benzophenone mClMgBP .................................................. 3-chloro-2',4’,6’-trimethylbenzophenone mIBP .................................................................................... 3-iodo-benzophenone mlMgBP ....................................................... 3-iodo-2’,4’,6’-trimethy|benzophenone mXBP ................................................................................... 3-halo—ben20phenone MXM3BP ......................................... 3-halogenated 2’,4’,6’-trimethylbenzophenone oBrBP ................................................................................ 2-bromo-benzophenone oBngBP .................................................... 2-bromo-2’4’6’-trimethylbenzophenone oClBP ................................................................................ 2-chloro-benzophenone OCIM3BP ................................................... 2-chloro—2’,4’,6’-trimethy|benzophenone olBP ..................................................................................... 2-iodo-benzophenone OIM3BP ........................................................ 2-iodo-2’,4',6’-trimethylbenzophenone oXBP .................................................................................... 2-halo-benzophenone OXM3BP .......................................... 2-halogenated 2’,4’,6’-trimethylbenzophenone pBrBP ................................................................................ 4-bromo-benzophenone pBngBP .................................................... 4-bromo-2’4’6’-trimethylbenzophenone pClBP ................................................................................ 4-chloro-benzophenone pClMgBP ................................................... 4-chloro-2’,4’,6’-trimethylbenzophenone pIBP ..................................................................................... 4-iodo-benzophenone leaBP ........................................................ 4-iodo-2’,4’,6’-trimethylbenzophenone pXBP .................................................................................... 4-halo-benzophenone pXM3BP .......................................... 4-halogenated 2’,4’,6’-trimethylbenzophenone Q .............................................................................................................. quencher XBP ......................................................................................... halo-benzophenone XM3BP ............................................... halogenated 2’,4’,6’-trimethylbenzophenone INTRODUCTION Ketones are always the objects of study in photochemistry because the longest wavelength of absorption for the carbonyl group in ketones exceeds 280 — 300 nm.2 Wavelength in that area can be obtained easily in most photochemistry laboratories. Carbon-halogen bond cleavage for mono-halogenated benzophenones occurs from the triplet state because of rapid intersystem crossing?"4 Study of the reactivity of halophenyl ketones helps us to understand the triplet state of a molecule. A. Mechanism of carbon-halogen bond cleavage for halogenated aromatic ketones. Carbon-halogen bond cleavage in halogenated aromatic ketones has been studied widely.5 The mechanism has been proposed by P. J. Wagner. 0 O o 0| h - O I H Figure 1. Mechanism of photoinduced cleavage of carbon-halogen bond for olBP1 The mechanism is similar for other XBPs and XMaBPs. Upon excitation, the carbon-halogen bond breaks and a radical pair is formed. This radical pair is believed to be an “in-cage” tight one and the two components may diffuse apart to form free halogen atom and phenyl radical. The phenyl radical abstracts hydrogen from solvent molecule; thus, dehalogenated benzophenones are formed. B. Competitions with carbon-halogen bond cleavage. Mechanism of cleavage indicates a two-step reaction: bond cleavage and hydrogen abstraction. Competitions may occur at either step. B1. Competition at bond cleavage stage: Hydrogen abstraction. The possibility of competition from physical decay is out of the question because the research indicates that the rate of bond cleavage is on the order of 108 s'1 in cyclopentane, which is much faster than typical phosphorescence emission process.“'7 Besides, physical radiationless decay process of ketone triplet cannot compete efficiently either.1 Thus, I will only discuss chemical competition in this paper. In addition to bond cleavage, another decay of ketone triplet is hydrogen abstraction, intermolecular or intramolecular. When phenyl ketone molecules abstract hydrogens from their environment instead of experiencing carbon halogen bond cleavage, (X-hYdI'OXY radicals are formed. Pinacols then are produced by coupling of such hydroxy radicals.‘3 Figure 2 shows the procedure. I pinacols Figure 2. Fate of triplet halo-benzophenone molecules In cyclopentane. (X=Br or I)1 The rate of hydrogen abstraction from the environment is based on the identity of the phenyl ketones and the character of solvent molecules. When the ketone molecules contain y-hydrogens, like benzylic hydrogen in XM3BPS, intramolecular hydrogen abstraction may also occur. The following figure shows the process. R O _ OH _ R Ph R c, "H2 \ 2 R 2 R1 R2 _ R1 .. R1 xylylenol benzocyclobutenol Figure 3. Benzocyclobutenol formation from 2,4,6-trialkylbenzophenones' The significance of those two kinds of competition can be illustrated by the concentrations of pinacol and of benzocyclobutenol in the product mixture if there is any. 82. Competition at hydrogen abstraction stage: Recoupling. A contact radical pair is formed immediately after the carbon-halogen bond breaks. The fate of halogen phenyl radical pair is shown in the following figure. Again, we will just use olBP as an illustration. 0 diffusion /a pair of free radicals 0 "contact" radical pair recoupling I Figure 4. Fate of halogen phenyl radical pair after bond cleavage Recoupling of halogen phenyl radical pair competes with the diffusion apart of those two radicals, which is reflected in the quantum yield of dehalogenated benzophenone formation."4 C. Electronic states of benzophenones. As good sensitizers, benzophenone molecules induce rapid intersystem crossing between triplet states and singlet states of themselves; thus the reactivity of benzophenone is confined to the triplet states,‘ especially the lowest triplet states because of the lower energy compared to higher level triplet states. The rate of vibrational relaxation from higher triplet states to t.” the lowest triplet states is very fas For this reason, I will only talk about the triplet states of benzophenone. 38—X3 3(nX!n*) m ::= . _ (1t,1t*) 3(n.n*) C, O. circa—5,: GS Figure 5. Energy diagram of excited triplets of halophenyl ketones‘ The lowest triplet state of benzophenone is the 3(n,It*) state.1 3(n,n*) state is only a few kcal/mol higher than the lowest triplet state3 and also plays an important role in the reaction. Because of the small energy gap between 3(n,1t*) and 3(1:,7t") states, both of them are considered as “reactant”.4 However, the dissociative states are 3(043*) and 3(nx,o*).° The coupling of “reactant” states with dissociative states is needed in the course of carbon-halogen bond cleavage. The 3mm“) is the only state responsible for the C-X bond cleavage for iodinated benzophenones,"6 while the 3(1t,1r*) states dominates the C-X bond cleavage for brominated benzophenones.‘ The coupling of the 3(n,rr*) state with the 3(o,o*) state explains the behavior of carbon-iodine bond cleavage for iodinated benzophenones.” For the isomers of brominated benzophenones, the lone pair p orbital on bromine atom couples with benzoyl 7t system, producing comparable coupling of “reactant” state with dissociative state.4 D. Kinetics and quenching study. Figure 6 shows the pathway of the C-X bond cleavage for XM3BPs. —7F—_ CDT 1", XA “(heal pair free radicals A, .X \kd‘ OX kI-I t A-H + S. A- S-H 2k, kq[Q] kr so, XM3BP or XA Figure 6. Mechanistic representation with rate constants (D1 is the quantum yield of triplet formation, which can be considered as 1 in C-X bond cleavage of benzophenones.1o kx is the rate constant of C-X bond cleavage and kq is the rate constant of quenching. Q is the quencher. 2k; is the sum of the rate constants of decay pathways other than C-X bond cleavage and quenching. Actually, the only apparent competition results from intermolecular and intramolecular hydrogen abstraction (refer to Part 81 of INTRODUCTION) based on previous studies of carbon-halogen bond cleavage from benzophenones;""° therefore, 23k; can be replaced with kH1[SH] and kHz, where km is the rate constant of intermolecular hydrogen abstraction and kHz is the rate constant of intramolecular hydrogen abstraction. kd is the rate constant of diffusion apart of the “contact” radical pair which is formed immediately after C-X bond cleavage. kr is the rate constant of radical recoupling from the “contact” radical pair. kH is the rate constant of hydrogen abstraction of phenyl radicals from solvent. The quantum yield of M38P formation can be expressed by the following equafion. k k, -X T k_,+kq[Q]+2:k, k, +k, (1) As CDT is almost 11° and Zki can be replaced with kH1[SH] + kHz the above equation is revised as follows. k k, ”I (I) = . (2) k—x +kq[Q]+kHl[SH]+kH2 kd +kr When there is no quencher added, the quantum yield is denoted as (Do. The following equation can be easily derived from Equation 2. k k, “I (D0 = ' (3) k_x +k,,,[SH]+kh,2 kd +k, Professor P.J. Wagner has studied the factors that affect (Do and has found that it increases with increasing kd. Viscous solvent produces smaller kd value, which decreases (1)0.” From Equations 2 and 3, Stem-Volmer equation is obtained, which is shown as Equation 4. (D -;° = 1+ k,r[Q] (4) T is the triplet lifetime when there is no quencher. The expression of ‘t is shown in Equation 5. l =k_, +k,,,[SH]+kH2 (5) 2' By varying‘the concentration of quencher, [Q], a series of (Do/(D value can be obtained. The plot of (Do/(D against [Q] is a straight line with a slope of m. As the value of kq, kH1[SH] and kHz can be obtained by reliable 10.11.14 estimate, the triplet lifetime, therefore kx, can also be obtained. 1 k :;_kHl[SH]_kHZ (6) "I For benzophenones in cyclopentane, the value of kH1[SH] is less than 0.1x106 s'1 for oXBPs and oXMgBPs; 7.6x106 s'1 for mXBPs and mXMgBPs, and 6.3x106 3'1 for pXBPs and pXM3BPs.“ The value of kHz is 1.7x106 s".‘° E. Goals of research. Carbon-halogen bond cleavage of XBPs has been well studied by Professor P.J. Wagner’s group; however, the same kind of cleavage of XMaBPS has not been fully studied. The only related reports are limited to the graduation dissertation of Martin Sobczak.8 The purpose of my research is to understand the chemistry, in particular carbon-halogen bond cleavage of XM3BPs. The benzene ring with three methyl groups in an XM3BP molecule is almost perpendicular to the carbonyl group. We are interested in how that twisted benzene ring affects carbon-halogen bond cleavage. We also expect that such research might provide information that helps to understand the nature of triplet states of organic molecules. 10 RESULTS AND DISCUSSIONS A. Quantum yield. The quantum yields of M38P formation in cyclopentane for various XMaBP were measured at room temperature under irradiation of 313 nm. The results are listed in Table 1. Table 1. The quantum yields of some BPs ln cyclopentane Quantum oC 0.00668 0.009 0.0007 0.0013 0.16 0.21 0.34 oBrBP” 0.09 mBrBP" 0.002 0.003 mlBP" 0.28 0.27 a. in cylclohexane b. ref. 4 c. ref. 1 The quantum yield decreases in the order of olMaBP, oBrMaBP, and oClMaBP, which is consistent with C-X bond strength. This indicates that the weaker the C-X bond is, the more readily it breaks. Former researches also indicate the similar decreasing trend of quantum yield.1"'5'“ The C-Cl bond does not break for ClBPs, and the quantum yield of BrBPs is smaller than that of the corresponding lBPs. 11 Some interesting conclusion may be made when the quantum yields of XM38PS are compared with those of XBPs. For BrBPs, the ortho isomer has a much larger quantum yield than the meta or para isomers. The fact is also true in the case of BngBPs. This has been explained by the direct through space electron transfer between bromine atom and the adjacent carbonyl group."15 B. Quenching study. The quenching study was carried out in about 0.01M cyclopentane solution under exposure of 365 nm irradiation with naphthalene as the quencher. The reaction of acetophenone formation from 0.01M valerophenone was the actinometer. The value of kg is gleaned from Stern-Volmer plot. The value of kq for 2,4,6-trimethylbenzophenone in benzene has been measured, which is 5x109 M"s’1."“’ Quenching is a diffusion-controlled process; therefore, the value of kq decreases with increasing viscosity of solvent. The value of kq for halogenated 2’,4’,6’- trimethylbenzophenone in cyclopentane is estimated as 1010 M"s‘1 because the viscosity of cyclopentane is lower than that of benzene.11 Stern-Volmer plots are shown in Figures 7-12 and the result of quenching study is shown in Table 3. .12 10/0 0 WT FTTTTFTTTTIT TVTTTIfTTT‘ : : - . -1 I I i- I I . I 0811111111E41111111111111111111111141 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 [OLM Figure 7. Quenching by naphthalene of M;BP formation from oBrMaaP 7 h f T T T I T T T T I T T T T I T T T 1 I T T T v T T I T ‘ @m i- ' I I . I a p ' ' d 0 I. I ' -I .. .......................................... J ......................................... _I . z ' 4 I- . . 4 I- - I 4 >- ‘ ‘ -I ,— ..................................................................................... _. b - .1 I- : -I P I ,. I ....................................................... 1--.-...-.-....l........-... i I .......... ....................................................................... ..................................................................................... YTTITTTTITTVTT 1111111111111111111111 I l i ' I I 0 1 L 1 1 1 1 1 1 1 L4 1 1 1 1 1 1 1 1 1 1 1 1 1 l 1 1 14L 0 0.001 0.002 0.003 0.004 0.005 0.006 [0]. M Figure 8. Quenching by naphthalene of M3BP formation from mBrM3BP T—fiT T I T T T T I T T T T I T T T T I T T T T1 T T I] V I O I I Old) 0 I I I I I I I a I I I I J I I I I I I l i I o I I . I J I I I I c I I I I I l I I ~ - c I I I I I I l I n n n u I I I o a I a I I I I I I I I i u l I I I I I I 111111111 111111111111111 11111 NU‘UIOQNQ _II 0 0.001 0.002 0.003 0.004 0.005 0.006 [0]. M Figure 9. Quenching by naphthalene of M33P formation from pBngBP 13 (DIG) o 1111111111111111111 11111 0 9 : 1 1 1 1 1 L 1 1 1 1 1% 1 1 1 1 1 1 1 1 0 0.05 0.1 0.15 0.2 [O]. M Figure 10. Quenching by naphthalene of M38P formation from 01M3BP 0 0.005 0.01 0.015 0.02 0025 [Q]. M Figure 11. Quenching by naphthalene of M38P formation from mIM3BP 00/¢ 24 r r T 1 2.2 -------------------- d: 2 .............................. I 1.8 |> --------------------------------- F ------------------------------ -: 1.6 . .................................................. _- 1.4 — ------------------------------------------------------------------- . --------------- —- 1.2 4 ---------------- — E : 1 i ..................................................... .0. ............... .2. 0.8 . L 4 ‘ 0.5 [O]. M Figure 12. Quenching by naphthalene of M38P formation from leaBP 14 Table 2. Room temperature photokinetics for some XM3BPs ln cyclopentane” Benzophenones <5 mm") 1Ir(10°s'1) kH(10°s")d K1410" s")' oBrM3BP 0.009 173.5 0.58 0.017 0.56 mBrMaBP 0.0007 1228.7 0.081 0.093 <0.001 pBrM3BP 0.0013 1211.6 0.083 0.080 0.003 olMaBP 0.16 3.98 25.1 0.017 25.1 mIM3BP 0.21 308.7 0.32 0.093 0.23 le3BP 0.34 3.31 30.2 0.080 30.1 oBrBP” 0.09 - 0.6 <0.001 0.6 mBrBP" 0.002 - 0.076 0.076 0.001 pBrBP" 0.003 - 0.063 0.063 0.001 map“ 0.28 265 0.37 0.076 0.30 plBP" 0.27 3.0 32 0.063 32 a. kq = 10‘° M‘s" (ref. 11); b. ref. 4; c. ref. 1; d. for XBPs, kH=kH1[SH]; for XM3BPs, kH=kH1[SH]+kH2: kaSH] is assumed to be <0.001x108 s", 0.076x10a s", and 0.063x108 s'1 for oX(M3)BPs, mX(M3)BPs and pX(M3)BPs in that order (ref. 14); kHz = 0.017x108 3'1 (ref. 10); e. lax is obtained according to Equation 6. The photochemistry of XMaBPS is almost the same as that of XBPs. Meta brominated isomers have comparable C-X bond cleavage rate constants as para brominated isomers, while meta iodinated isomers C-X bond cleavage rate constants almost 100 times faster than para iodinated isomers. This phenomenon has been thoroughly explained by P.J. Wagner.""° Only the 3(n,n"’) state is responsible for the cleavage reaction for iodinated benzophenones}6 while 3(11,1:*) state dominates the dissociation process for brominated benzophenones.‘ A major difference between those two kinds of triplet states is that both meta and para electron-donating 15 substituents interact very strongly with the benzoyl 11 system in the 3(n,n*) states." Consequently, the strong interaction gives comparable coupling with dissociative states; thus carbon halogen bond cleavage rate constants for meta and para brominated benzophenones are almost the same. On the contrary, halogen atoms do not interact equally at meta and para positions for iodinated benzophenones since the ”reactant” state for iodinated benzophenones is 3(n,rr*). The spin density in the n* orbital is much larger at para position than at meta position,17 which is thought to be the reason that iodinated benzophenones shows great positional dependency} For IBPs and 1M33PS, the triplet decay rate constant is almost the same as the C-l bond cleavage rate constant since C-l bond cleavage is the only apparent decay process. However, hydrogen abstraction from solvent competes with C-Br bond cleavage for BrBPs (refer to Part 81 of INTRODUCTION). The hydrogen abstraction rate is on the order of 107 s'1 for BrBPs in cyclopentane,"“'8 which is much faster than the rate constant of C-Br bond cleavage. Therefore, further investigation is needed to find out the rate constant of hydrogen abstraction for XBrMaBPs, in order that the carbon halogen bond cleavage rate constant can be figured out. 16 C. Conclusion We have found that XMaBPs bear similar photophysical and photochemical characters as XBPs although the molecular geometries are different. R O \ I B/—X R R X= Cl, Br, or i R= H or CH3 Benzene ring A is almost conjugated with the carbonyl group for XBPs while it is perpendicular to the carbonyl group for XM3BPS.8 It indicates that carbon-halogen bond cleavage on benzene ring 8 is not significantly affected when the geometry of benzene ring A changes from a coplanar ring to a perpendicular ring. In another word, the twisted ring A does not change carbon halogen bond cleavage a lot. 17 EXPERIMENTAL A. Synthesis. Generally speaking, XMaBPs were made according to the following scheme. 0 o 0 . \ SOCIZ I \ Cl Mesrtylene : x—'— \ X-r / X—. / AICI3 . / Figure 13. Synthesis of XMaBP The intermediate chlorides might be solids or oil substances. The final products are all solids. As the photochemical study does not require much amount of chemicals, l concentrated on purity of the product more than on the yield. In most occasions, several times of recrystallization were needed. A1. Chloro-2’,4’,6’-trimethylbenzophenones. 3.759 appropriate chlorobenzoic acid was refluxed with 30 mL thionyl chloride for about 12 hours. At this point, the reactant mixture was a clear solution. The excessive thionyl chloride was removed and crude acid chloride was obtained. The crude acid chloride was dissolved in a small amount of 1,2-dichloroethane. 18 A mixture of 3.89 aluminum chloride, 3.59 mesitylene and 25 mL 1,2- dichloroethane was cooled to 0°C. The acid chloride was then added into the mixture drop by drop. The reaction mixture was stirred for 24 hours. Then the reaction was quenched with a mixture of 15mL concentrated hydrochloric acid and 609 ice. The aqueous layer was extracted with dichloromethane twice. The combined organic layer was washed with water, 5M sodium hydroxide and water, twice each, and then was dried over magnesium sulfate. The solvent was removed and the crude product was recrystallized from petroleum ether. Charcoal might be needed if the color of product is not white. oClMaBP: 1H-NMR(benzene-d5, 300MHz, 8 ppm): 2.06 (s, 3H), 2.07 (s, 6H), 6.62 (s, 2H), 6.69 (m, 2H), 7.07 (dd, 5.7, 1.2 Hz, 1H), 7.38 (dd, 7.2, 1.8 Hz, 1H); 13C-NMR(CDCI3, 75MHz, 8 ppm): 19.7, 21.1, 126.8, 128.8, 131.5, 131.7, 132.7; FTIR (KBr, cm"): 761(s), 916(s), 1252(3), 1437(3), 1586(s), 1674(vs), 2363(w), 2916(br); GC-MS (m/z): 147.1(52.79%), 208.0(57.94%), 223.0(100%), 257.1‘(M*'), 259.2(M"). 19 pClM3BP: 1H-NMR (CDCI3, 300MHz, 6 ppm): 2.07 (s, 6H), 2.33 (s, 3H), 6.90 (s, 2H), 7.41 (d, 8.7 Hz, 2H), 7.74 (d, 8.7 Hz, 2H); 13C-NMR (CDCI3, 75MHz, 5 ppm): 19.3, 21.1, 128.4, 129.1, 130.7, 134.1; FTIR (KBr, cm"): 812(s), 903(3), 1091(3), 1167(s), 1274(8), 1572(5), 1655(3), 1925(m), 2907(br); GC-MS (m/z): 147.0(100%), 208.2(47.92%), 223.2(97.62%), 257.1(M+'), 259.2(M*'). A2. Bromo-2’,4’,6’-trimethylbenzophenones. 59 appropriate bromobenzoic acid was refluxed with 8 mL thionyl chloride and 25 mL chloroform for about 4 hours. At this point, the reactant mixture was a clear solution. The solvent and unreacted thionyl chloride were removed and crude acid chloride was obtained. The crude acid chloride was dissolved in a small amount of 1,2-dichloroethane. A mixture of 49 aluminum chloride, 3.69 mesitylene and 25 mL 1,2- dichloroethane was cooled to 0°C. The acid chloride was then added into the mixture drop by drop. The reaction mixture was stirred for 24 hours before the reaction was quenched by a mixture of 15mL concentrated hydrochloric acid and 609 ice. The aqueous layer was extracted with dichloromethane twice. The combined organic layer was washed with water, 5M sodium hydroxide and water, twice each, and then was dried over magnesium sulfate. The solvent was removed and the crude product 20 was recrystallized from ethanol. Charcoal might be needed if the color of product is not white. oBrMaBP: 1H-NMR (benzene-d5, 300MHz, 8 ppm): 2.01 (s, 3H), 2.07 (s, 6H), 6.61 (s, 2H), 6.67 (m, 2H), 7.30 (d, 2.4 Hz, 1H), 7.33 (dd, 7.8, 1.5 Hz, 1H); 13c-NMR (00013, 75MHz, 8 ppm): 19.8, 21.1, 127.3, 128.8, 131.8, 132.7, 134.9, 135.3; FTlR(KBr,cm'1):897(s), 1242(s), 1292(s), 1433(s), 1584(5), 1674(5); GC-MS (m/z): 147.1(80.60%), 208.1(97.01%), 223.2(100%), 302.2(M‘”), 304.2(M*'). mBngBP: 1H-NMR(benzene-d5, 300MHz, 8 ppm): 1.94 (s, 6H), 2.07 (s, 3H), 6.60 (m, 3H), 7.20 (m, 1H), 7.56 (d, 7.8 Hz, 1H), 8.21 (t, 1.8 Hz, 1H); 13c-NMR (00013, 75MHz, 8 ppm): 19.3, 21.1, 128.0, 128.4, 130.3, 131.9, 134.1, 136.3; FTIR (KBr, cm"): 1260(s), 1420(3), 1560(5), 673(s); GC-MS (m/z): 147.0(100%), 208.2(45.65%), 223.2(79.57%), 302.2(M"'), 304.2(M*'). 21 pBngBP: 1H-NMR (CDCI3, 300MHz, 8 ppm): 2.04 (s, 6H), 2.31 (s, 3H), 6.87 (s, 2H), 7.56 (d, 9.0 Hz, 2H), 7.64 (d, 8.7 Hz, 2H); 13C-NMR (CDCI3, 75MHz, 6 ppm): 19.2, 21.1, 128.4, 130.8, 132.1, 134.0, 136.0, 136.1, 138.8; FTIR (KBr, cm"): 834(3), 892(3), 1051(3), 1158(3), 1265(3), 1440(3), 1585(3), 1658(3), 1931(w), 2917(br); GC-MS (m/z): 147.0(81.36%), 208.0(67.80%), 223.1(100%), 302.1(M"'), 304.1(M"). A3. Iodo-2’,4’,6’-trimethylbenzophenones. 59 appropriate iodobenzoic acid was refluxed with 8 mL thionyl chloride and 25 mL chloroform for about 4 hours. At this point, the reactant mixture was a clear solution. The solvent and unreacted thionyl chloride were removed and crude acid chloride was obtained. The crude acid chloride was dissolved in a small amount of 1,2-dichloroethane. A mixture of 3.29 aluminum chloride, 2.99 mesitylene and 25 mL 1,2- dichloroethane was cooled to 0°C. The acid chloride was then added into the mixture drop by drop. The reaction mixture was stirred for 24 hours before the reaction was quenched by a mixture of 15mL concentrated hydrochloric acid and 609 ice. The aqueous layer was extracted with dichloromethane twice. The combined organic layer was washed with water, 5M sodium hydroxide and water, twice each, and then was dried 22 over magnesium sulfate. The solvent was removed and the crude product was recrystallized from petroleum ether. Charcoal might be needed if the color of product is not white. olMaBP: 1H-NMR (CDCl3, 300MHz, 8 ppm): 2.09 (s, 6H), 2.29 (3, 3H), 6.85 (s, 2H), 7.13 (m, 1H), 7.31 (m, 2H), 8.04 (d, 7.2 Hz, 1H); 13c-NMR (00013, 75MHz, 8 ppm): 19.7, 21.1, 128.0, 128.7, 131.6, 132.7, 135.2, 142.0; FTIR (KBr, cm"): 889(s), 1003(3), 1275(3), 1415(3), 1607(3), 1666(3); GC-MS (m/z): 147.0(54.60%), 208.0(80.46%), 223.0(100%), 350.0(M*'). mlMgBP: 1H-NMR(CDCI3, 300MHz, 8 ppm): 2.05 (s, 6H), 2.31 (s, 3H), 6.88 (s, 2H), 7.15 (t, 7.8 Hz, 1H), 7.66 (d, 7.8 Hz, 1H), 7.88 (d, 7.8 Hz, 1H), 8.16 (t, 1.8 Hz, 1H); “C-NMR (CDCI3, 75MHz, 8 ppm): 19.4, 21.1, 94.8, 128.4, 128.7, 130.5, 134.2, 137.8, 138.8,139.0,142.2; FTIR (KBr, cm"): 847(3), 1169(s), 1261(3), 1419(3), 1555(3), 1607(3), 1666(3), 2335(w); GC-MS (m/z): 147.0(100%), 208.0(56.34%), 223.0(88.73%), 350.1(M*'). 23 leaBP: 1H-NMR(CDCI3, 300MHz, 8 ppm): 2.04 (s, 6H), 2.30 (s, 3H), 6.87 (s, 2H), 7.48 (d, 8.4 Hz, 2H), 7.79 (d, 8.4 Hz, 2H); 13C-NMR (CDCI3, 75MHz, 8 ppm): 19.5, 21.3, 102.5, 128.6, 130.9, 134.3, 136.7, 138.3, 138.9 FTIR (KBr, cm"): 818(3), 977(3), 1141(3), 1259(3), 1437(3), 1553(3), 1666(3), 1922(m), 2952(br); GC-MS (m/z): 147.0(59.84%), 208.0(65.35%), 223.0(100%), 350 Table 3. Melting points of XM38P8 Benzophenones M.P. (°C) M.P. MIC) oClMlBP 101 .0~101 .5 99~101.8 "’ pClMgBP 68.0~68.5 67~69.5 "' oBrM3BP 110.9~111.0 113~115’" mBerBP 84.5~85.5 86~87 7‘ pBrM3BP 70.5~71.2 70~73 2‘ olMaBP 94.5~95.0 97~98 2“ mlMgBP 84.0~84.5 85~86 ‘3 leaBP 78.0~79.0 72~73 2" UV and phosphorescence XM38P3 have shown similar UV and phosphorescence spectroscopic characters as corresponding XBPs. 24 Table 4. Spectroscopic features of some benzophenones ° Benzophenones A“, n, 11* A...“ Phosphorescence 0, 0 (run) (nm) in nm (kcallmol) oClMaBP 214.8 335 - oBrMaBP 214.4 338 - olM3BP 220.2 342 - mlM3BP 226.2 341.9 441 (64.9) le;BP 206.0 341.2 437 (65.5) oBrBP” - - 412 (69.5) mlBP‘= - 347 419 (68.3) plBP“ - 348 422 (67.8L a. The sample concentration is about 0.01M in cyclopentane for UV spectra and about 0.0001M in 1:1 MeOH/EtOH for phosphorescence. b. ref. 4 c. ref. 1 8. Identification. Varian 3400 gas chromatograph and Hewlett-Packard 3392A integrator were used. Product from C-X cleavage was identified by comparing its retention time with the retention time of authentic MaBP. The column was DB-210 50% tri-fluoropropyl methyl polysiloxane. The temperature of column was programmed as: 3 minutes at 80°C, increasing to 200 at the rate of 10°C/min., and then staying at 200°C infinitely. The temperature of the injector was 200°C and that of the detector was 220°C. The pressure of carrier gas helium was set at 80 lbs per square inch. The injection amount was 2 9L. The coincidence of retention time indicated the C-X cleavage product was dehalogenated 2,4,6-trimethyl benzophenone. C. Quantum yields. Quantum yields depict efficiency of certain process such as formation of some product and some decay pathways of excited states. In my 25 research, I studied the quantum yield of dehalogenated benzophenone formation. The quantum yield is defined as the following equation.