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V, ., :,.._ ,. . . nix); ;. axe...» . ....J u. . . ......... 2....97. . ..z .A ,..;. . ..1. flirt .' . . ,....,..,.dyf)..._ .. C . . . . ......fv.......¢arvo...... darerx.“VI/1)........v....f4./._3.7.5.. . a}? (I .19711/133‘ (.93. r} . A. a. I- ’Ill mun-nu . ..m. __ i p. bafili‘higtm State University hi ‘1" . } ' Oman“! av IIIJAG 8: 8015' “.3995. F'AFEEBUE ABSTRACT 13C NUCLEAR MAGNETIC RESONANCE STUDIES OF 2-ARYL-2—NORBORNYL CATIONS By Bing Lun Lam A plot of the 13C(l) versus 13C(3) chemical shifts for a series of 2-ary1-2-norborny1 cations é, shows a change in slope for cations less 2 stable than 2-mech1orophenyl-Z-norbornyl cation. The result is consis- tent with a change from classical ion to non-classical ion character. It is also shown that the 13C nmr shifts of p-halogen substituted norbornyl cations do not show the marked deviation from linearity shown in the earlier PMR studies. This result is consistent with an earlier proposal that these ions are in rapid equilibrium with dimeric ions which show large magnetic anisotropic effects on the PMR chemical shifts. l3C NUCLEAR MAGNETIC RESONANCE STUDIES OF 2-ARYL-2-NORBORNYL CATIONS By Bing Lun Lam A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemistry 1973 4k '5 [16‘ To my parents for their affection and to my brother and sister-in-law DEDICATION for their moral support. 11 ACKNOWLEDGMENT I am grateful to Professor Donald G. Farnum who has provided me with the opportunity to complete this work. His guidance, interest and intellectual inspiration are deeply appreciated. I am thankful to members of "the group" for their helpful assistance. Particular thanks are extended to Dow Chemical Co., Midland, Michigan and Varian Associate Inc. for use of their instruments. I also acknowledge financial support from the Department of Chemistry in the form of teaching assistantship throughout the course of my study. iii TABLE OF INTRODUCTION . . . . . . . . . . . . RESULTS AND DISCUSSIONS EXPERIMENTAL . . . . . . . . . . . . 13CNMR Spectra . . . . . . . . Preparation of Carbocations . . CONTENTS Preparation of 2-aryl-Z-endo-norbornanols Typical Procedure for Preparing Alcohols using the ImIR MethOd O O C O O O O 0 Properties of 2-Ary1—2-endo-norbornanols REFERENCES . . . . . . . . . . . . . iv Page 18 18 18 18 19 20 21 LIST OF TABLES Table Page 1 13C Chemical Shifts for Cation z . . . . . . . . . . . . 8 2 130 Chemical Shifts for C(1), C(3) of Cation 2 . . . . . 12 3 Listing of 0+ Values Versus C(2) Chemical Shift in 2—ary1-2-norbornyl Cations é . . . . . . . . . . . . l4 Figure 1 LIST OF FIGURES Page Graph of H(1) vs. H(3) chemical shifts in 2-ary1-2-norborny1 cations é. . . . . . . . . . . . . 5 Graph of H(1) vs. H(3) chemical shifts in 2—ary1-2-bicyclo[2.2.2]octyl cations Q . . . . . . . 6 13CNMR spectrum of 2-p—chlorophenyl-Z-norbornyl cation 1. . . . . . . . . . . . . . . . . . . . . . . 9 Graph of 13C(l) vs 13C(3) chemical shifts in Z-aryl-Z-norbornyl cations z . . . . . . . . . . . . 13 vi INTRODUCTION INTRODUCTION An early application of strong acid solvents was the use of 100% H2804 for the preparation of stable solutions of carbonium ions such as the triphenylcarbonium ion (C6H5)3C+.1 More recently a large variety of different carbonium ions has been prepared and observed in various highly acidic media. For example, t-butanol is completely converted to the trimethylcarbonium ion in HSO F-SbF -SO at -60°.2 The potential of 3 5 2 this method to generate and observe stable carbonium ions at variable temperature by spectroscopic methods has been applied to the controver- sial question of whether the norbornyl cation is an equilibrating pair of classical ions 1 or a o delocalized non-classical ion g.3 A. % Schleyer, Saunders and Olah in 1964 first examined the 2-norbornyl cation and observed its PMR spectrum in solution starting from 2-fluoro- exo-norbornane in SbFB-SO2 at room temperature and -140°.4 They con- cluded that even at -1403 the PMR method does not necessarily differen— tiate between the rapidly equilibrating classical structures or the l non-classical structure of the 2—norbornyl cation because its time scale is much slower than the expected rapid exchange of equilibrating forms. More recently Olah obtained the low temperature Carbon-13 (13C) spectrum of the norbornyl cation by the Internuclear Double Resonance (INDOR) method and provided a strong case for a non-classical structure 3. -70° -150° *5 13 :c =c =c - +101.8 c =c +70 c 1 2 6 1 2 03- +162.5 c6 +173 c4- +156.1 At -150° the observed two carbon resonance at 70 ppm is not compatible with the expected value of 16.5 ppm from two equilibrating classical carbonium ion centers. The 0 delocalized non-classical nature is further supported by a photoelectron spectrum of the norbornyl cation.6 According to Winstein a basic tenet of the non-classical theory is that the more stable the cationic center the less stabilization that center will require from the rest of the molecule and the less important participation should be.7 Consequently we should expect to find a marked decrease in those characteristics associated with participation as we move along a series to more and more stable cations. Therefore,one is not entirely surprised by Olah's study and conclusion that the 2-methyl- 2-norbornyl cation is closer to the classical than to the completely non— classical structure.8 One might point out that,in going from the parent norbornyl to the 2-methyl-2-norborny1 system, one is comparing a secondary with a tertiary center. Now if one changes the 2-substitutent in the * In parts per million from 13C82. 3 norbornyl system to a phenyl group, one would expect the cationic center to be greatly stabilized. Indeed, that 2-pheny1-2-norbornyl cation is classical has been demonstrated by an extensive PMR study9 and confirmed by Olah's 13CMR studies.8 At this point, a dilemma in our trend of thought is presented by 10,11 H.C. Brown and K. Takeuchi. They correlated the rate of solvolysis of a number of 2-ary1-2-exo-norborny1 derivatives 3 with 0+ values. B They found that the correlation was linear with none of the curvature 4‘, expected if a delocalization became an important factor for any of the derivatives. Such a plot would not be linear if 0 delocalization were operative even though the study covered only 60% of the reactivity range from Z-p-anisyl-Z-norbornyl to the 2-norborny1 system. It seems intui- tively unreasonable that 2-norbornyl cation would be the only norbornyl derivative to exhibit a delocalization. To resolve this puzzling inconsistency, A.D. Wolf carried out an extensive PMR study of a number 12 of 2-aryl-2-norbornyl cations 2 in HSOBF. 7 4 The aryl group is varied to decrease electron donation to the carbonium ion center (C2) which, in turn, increases the electron demand on the norbornyl skeleton--especially at C(1) and C(3). Such an effect will show up in a down field shift of H(l) and H(2) in the PMR spectrum. Therefore, providing there is no other mechanism for leaking charge to C(1) and C(3), the charge effect felt by C(1) and C(3) would be directly proportional to the variation of charge density at the carbonium ion center C(2). In other words, a linear relationship would be expected by plotting the chemical shift of H(1) versus that of H(3) for the different 2-aryl-2- norbornyl cations, if the cations are classical. As it turned out, a linear relationship was not observed for the complete series, but a change in slope was obtained for cations less stable than 2-phenyl-2— norbornyl cations (Figure 1). By contrast, the behavior of the H(1) and Q H(3) chemical shift for a similiar series of 2-aryl-2-bicycloocty1 cations Q is linear from p-methoxyphenyl through meta-chlorophenyl, (Figure 2). Such deviation from the initial linearity leads to the con- clusion that one is observing the onset of non-classical character in these 2-ary1-2-norbornyl cations. The recent development of Fourier Transform (FT) Nuclear Magnetic Resonance Spectroscopy13 has made versatile 13CNMR studies not only practical but also comparable with PMR in terms of experimental ease and Hl(r) .7 9 , . + 3,5-(C1)2-4-N(CD3)2H . / b " ’ I * 3,5;(CF3)2 3 " m-Cl Jr I 1 A4 1 J 6.6 6 4 6.2 6 0 5.8 Figure 1.8 Graph of H(1) vs. H(3) chemical shifts in 2-aryl-2-norborny1 cations é. 5.2 P * 395-(CF3)2 / / ' / + p—CF 3 5.4 .— 5.6 ,- H1(t) 5.8 .. Figure 2.8 Graph of H(l) vs. H(3) chemical shifts in 2-aryl-2-bicyclo[2.2.2]octyl cations Q. 7 quality. In particular the sensitivity of the 13C chemical shift to charge distribution14 offers another challenging testing ground for the conclusion drawn from Wolf's PMR studies. With this in mind, we set forth with the following studies. RESULT AND DISCUSS IONS RESULTS AND DISCUSS ION The feasibility of studying various cations at probe temperature was studied using the PMR spectrum as a measure of approximate lifetime. It was concluded that the 13CNMR spectrum could be obtained at probe tempera- ture for the 2-aryl-2-norbornyl cations where the aryl groups are: p-meth- oxyphenyl; 2,3-dimethylpheny1; p-chlorophenyl; mebromophenyl and p-iodo— phenyl. Spectra of the other cations were obtained at low temperature. In all cases hydrogen decoupled spectra were obtained. An off-reso- nance decoupling experiment was performed on a few samples to help make spectral peak assignments. In these experiments, non-protonated carbon, CH, CH2, and CH3 were observed as a singlet, doublet, triplet and quartet respectively. A 13CNMR spectrum of 2-p-chlorophenyl-2-norbornyl cation 1 in FSO3H- FSO3D is displayed in Figure 3. The chemical shift assignments are shown in Table 1. TABLE 1. 13C Chemical Shifts for Cation Z Carbon 6(TMS) C(1) 59.9 C(2) 257.5 C(3) 50.9 C(4) 40.2 C(S) 34.7 C(6) 42.0 C(7) 25.9 C(8,lO,12) 133.5 C(11) 162.2 1 C(9,13) 142.2 .M coaumu Haauonuoalmlahsonaouoanuualm mo asuuoonm MZZOmH .m madman a: a m o Assoc o o u Amvo Ama.mvu sm.m~ m.p.m om .. oo.vm ..cmo .w o. n..ov ..nom on m vo.~o m.oem on m mm.om b.s.o. mm b WM.MM. w.%mn. .. . . . . . m m. m Ana OH mvo om.mn. m.0pm~ 00. v -.~v. m.vqm~ be m .wnwo. v.6qmm o. m mm nmm m.om.m um _ no.7 a..m. m e and um 10 The order of assignment of the 13C chemical shifts for the norbornyl skeleton illustrated in Table 1 remains constant in the series of cations studied. Since the total 13C spectra of 2-substituted—Z—norbornyl cations have not been reported in the literature, direct comparison of our assign- ments with others is not possible. However the 13C chemical shifts of C(1) and C(2) of the 2-phenyl-2-norbornyl cation in FSO 15 3H were obtained by Olah using the INDOR method and were reported to be 62:2 ppm* and 258.6:2 ppm* respectively. Our FT 13C chemical shift values of C(1) and C(2) are 59.8 ppm and 257.8 ppm respectively. These values compare favora- bly with those reported by Olah. Returning to Figure 3 and the 13C chemical shift values (Table 1) for 2-p-chlorophenyl-Z-norbornyl cation, a comment would be appropiate concern- ing our assignment of the lower field singlet in the aromatic region to C(ll) rather than 0(8) of the aromatic ring. The relative importance of the resonance structures a and IQ'(Scheme l) to the stabilization of Cation 1,13 not obvious. In a recent study of para—substituted triphenyl- l6 carbonium ions 8 it was found that the chemical shift value for C(S) * Reported here as 6(TMS) using the conversion value 6C82 = 192.8. decreased while C(2) changed much less as the EEEE substituent changed from a methoxy to a Fluoro group. Our chemical shifts are close to those reported for C(2) and C(5) of the appropriately substituted triaryl- carbonium ions. Included in Table 2 is a listing of the 13C chemical shifts of interest, i.e., C(1), and C(3). A plot of the 13C chemical shift of C(1) versus C(3) is shown in Figure 4. In this graph the 13C chemical shifts for the 3,5-bis(trifluoromethylphenyl) substituted cation are plotted as a range of values. Setting aside the significance of the graph in Figure 4 for later discussion, let us focus on several features of this graph. As stated in the introduction, a strong electron releasing group places relatively little positive charge at C(2) of the norbornyl system while a strong electron withdrawing group places relatively more positive charge at C(2). An increase in charge density at C(2) causes the 13C chemical shift of C(3) and C(1) to shift to lower field. This trend is apparent from the ordering of points with the electron releasing capacity of the substituent as indicated by their 6+ values.17 In fact a similar empirical relationship exists for the cationic center C(2) chemical shift and 0+ values (refer to Table 3). However a quantitative correlation is not obtained when one plots 0+ values for the different substituents on phenyl for cation 5 versus the 13C(2) chemical shift. The observed scatter of the points may be an inherent deviation obtained when one 12 C(3) 47.2 49.6 50.8 50.87 50.47 50.86 51.27 52.23 52.4-53.9 3 l 2 R E 13 . * - . TABLE 2. C Chemical Shifts for C(1), C(3) of Cation g Cation 5 R C(1) a p-OCH3 53.5 R 3,4(CH3)2 57.1 8 p—cl 59.86 g p-I 59.76 21 H 59.34 g m-Br 59.83 51 erl 61.45 1 h p-CF3 63.75 2 i 3,5(CF3)2 66.7-67.4 * ppm from capillary TMS. 1) Determined at -80°. 2) C(1) and C(2) are reported as a range of values, spectrum was determined at -90°. C(1) in ppm 68 66 64 62 60 58 56 54 52 13 L- / / 3,5 CF -—————~ '1 _. I /’ I / l I” / F— p-CF3———P / (I / 1 I / 2 I ... I / 1' / l I __ - I! _.____.m-Cl p Cl '11—- E _L m-Br 2 P (6 P / ‘— 3’ ...,2 IE] -—-——-—- p-OCH3 J n I 1 l 1 47 48 50 52 54 56 58 C(3) in ppm Figure 4. Graph of 13C(1) vs 13C(3) chemical shifts in 2-aryl-2-norborny1 cations g. 14 TABLE 3. Listing of 0+ Values Versus C(2) Chemical Shift in 2-aryl-2—norbornyl Cations 2. Cation z Aryl Group C(2) 0+ Q p-OCH3C6HS 223.3 -0.764 R 3,4(CH3)2C6H5 230.6 ~O.375 g p—C1C6H5 257.5 0.112 g C6H5 257.8 0. f m—BrC6H5 257.8 0.391 Q p-IC6H5 258.9 0.132 g m—C1C6H5 262.44 0.373 i 3,5(CF3)2 263.2 1.04 h p-CF3C6H5 264.7 0.61 15 extrapolates d+values, which were determined under solvolytic conditions, to these strong acidic media. It has been shown in Wolf's8 proton NMR studies that there was some unusual effect operating in the p-halogen substituted cations which caused these points to "fall off the line" in the 2-aryl-2-norbornyl cation series (Figure 1). Interestingly enough, such gross deviations are not observed for the p-halogen substituted cations in this 13CNMR study. A simple resolution to this puzzling discrepency may be found in the differ— ent response of PMR and 13CNMR to diamagnetic effects. Available calcula- tions agree that the paramagnetic term normally dominates 13C chemical shifts and the anisotropy term, which depends only on the spatial relation— ship between the nucleus being observed and the anisotropic group, is relatively insignificant in 13CNMR.18 Therefore, a logical explanation is that there exists some sort of neighboring anisotropy effect for the p—halogen substituted cations in FSO3H. This explanation is consistent with Wolf's suggestion of a dimer 11 in equilibrium with the cation 2g. h 2 ‘0 Q 0 16 I Another possibility is the reorientation of the aromatic ring in Cation 2g such that the protons on C(3) are affected by its anisotropy. What- ever the explanation, one thing is demonstrated clearly. The pehalogen substituted cations are on the line. 16 In this study the linearity ranges from the p-methoxyphenyl to m— chlorophenyl substituted cation, while the "break" from linearity starts after the 2—phenylnorbornyl cation in Wolf's PMR studies. Again this difference in "break" point may be due to a neighboring anisotropy ex- perienced by the mrhalogen substituted cations 2. Such anisotropic effects would not be unexpected in view of the much larger effect that caused the p-halogen substituted cations to "fall off the line". Now we will turn our attention to the significant implication of Figure 4. The basic assumption in Wolf's PMR study8 is that the chemical shift difference between H(l) and H(3) will be constant for classical aryl-norbornyl cations. This hypothesis, though well illustrated by the Proton NMR studies, was weakened by the unaccountable behavior of the p-halogen substituted cations. The smooth linearity, ranging from the p-methoxy substituted to the mechlorophenyl substituted cation, shown in Figure 4 strengthens the hypothesis. Similar to the proton studies, a non-linear behavior in the graph of 13C(1) shifts versus 13C(3) shifts signifies charge leakage of the cationic center C(2) to C(1). Such deviation from initial linearity is demonstrated in Figure 4 with the "break" at the mechlorophenyl substituted cation. Thus the more reliable 13C chemical shift plot has confirmed the general picture obtained from previous PMR studies for a similar series of 2-ary1-2-norbornyl cations. In summary, our data have demonstrated the following points: (1) There is a qualitative relationship between the 0+ value and the charge density at the cationic center. (2) The marked deviation from linearity exhibited by p-halogen substituted cations in the PMR studies of WOlf is apparently due to 17 diamagnetic effects, since the 130 chemical shifts of the p-halogen substituted cations show the expected linearity. (3) The change in slope exhibited in the 13C nmr correlation of Figure 4 for cations more electron demanding than 2-m-chlorophenylnor— bornyl signifies a change from classical to non-classical ion character. EXPERIMENTAL EXPERIMENTAL 13CNMR Spectra Most 13CNMR spectra were obtained at 20 MHz on a Varian CFT-20 Pulse Fourier Transform (FT) Spectrometer using 20% FSO D as internal pulse 3 deuterium lock in 10 mm. O.D. sample tubes. Some spectra were obtained on a Varian XL-lOO Pulse FT spectrometer in 12 mm sample tubes through the courtesy of Dow Chemical Co., Midland, Michigan. All chemical shift assignments were made with reference to capillary TMS with a :_0.1 ppm tolerance. Preparation of Carbocations Appropriate amounts of carbocation precusor were dissolved in a chosen solvent, generally CFCl The solution was transferred to an NMR 3. sample tube and was cooled at -80° in a Dry Ice—isopropanol bath. After 3 minutes, a layer of SO ClF (25% by volume) was introduced without 2 mixing with the solution of precursor. Under N FSO H was pipetted into 2’ 3 the sample tube slowly and carefully to avoid mixing. After 10 minutes at -80°, the contents of sample tube were rapidly mixed well. For sam- ples that needed to be observed at probe temperature, the sample tube was put under very high vaccum to evacuate volatile components. In general, the carbocation solutions were prepared at approximately 20% weight con- centration. Preparation of 2-ary1-2-endo-norbornanols The alcohols used for this work were prepared by reacting the appropriate organometallic reagents with commercially available 18 19 2-norbornanone. The organometallic reagents were formed either by 19 20 Grignard reaction or by the halogen-metal interconversion reaction (HMIR) starting with aryl bromide except in the case of p-iodo-phenyl lithium, when aryliodide was used. Typical procedure for preparing Alcohols using the HMIR Method Ether was distilled into a dried 50 ml 3 neck flask containing a weighted amount of desired arylhalide to make up a 10-30% by volume solu- tion. The flask was equipped with a low temperature thermometer, a magne- tic stirring bar, N2 inlet and a 15 m1 constant pressure addition funnel. The arylhalide solution was cooled to -40°usinggiDry—Ice—acetone bath. A 10% molar excess (over halide) of commercial n—butylithium in hexane was added dropwise while keeping the reaction temperature between -30° and -40°. Then it was allowed to warm to -10° for 5 minutes and lowered back to -40°. A solution of norbornanone (one equivalent) in anhydrous ether was added at a rate such that the temperature of the reaction mix— ture didrunzrise above -30°. After addition of ketone was completed, the reaction mixture was warmed up to room temperature and hydrolyzed with cold saturated aqueous NH4C1 solution and extracted with ether. Drying the ether solution with MgSO4 followed by evaporation of the ether led to a crude oil or solid which was then purified by fractional molecular distillation or by recrystallization with hexane. 20 Melting points and boiling points reported for the following com— pounds are the same as recorded by Wolf8 unless indicated otherwise. (I) 2-p-methoxyphenyl-Z-endo—bicyclo[2,2,l]heptanol 38% yield from HMIR; b.p. 82° at 3mm (Rec. 100°—150° at 8 mm)8 (II) 2-(3.4.-dimethylpheny1)~2-endo-bicyclo[2,2,1]heptanol 55% yield from HMIR; b.p. 130°-145° at 0.5 mm. (III) 2-p-idodophenyl-2-endo-bicycloigg2,llheptanol 60% yield from HMIR; m.p. 99° (IV) 2-p-chlorophenyl-Z-endo-bicyclo[2,2,1]heptanol 68% yield from HMIR; m.p. 86° (V) 2-m-chlorophenyl—2-endo-bicyclo[2,2,llheptanol 27% yield from HMIR, b.p. 105° at 4.5 mm. (Rec. m.pt. 42-43.5°)8 (VI) 2-m-Bromophenyl-2-endo-bicyclo[2,2,llheptanol 40% yield from HMIR; m.p. 54.5°—56° (VII) Zip-trifluoromethylphenyl-Z-endo[2,2,1]heptanol 32.6% from HMIR; m.p. 65-66° (VIII) 2-[3,5,bis(trifluoromethylphenyl)152-endo-bicyclo[2,2,l]heptanol 54% yield from HMIR; m.p. 75°-76° (IX) 2-phenyl-2-endo-bicyclo[2,2,l]heptanol 50% yield from HMIR; m.p. 40-4l° REFERENCES 10. ll. 12. 13. 14. 15. 16. 17. 18. LIST OF REFERENCES A. Hantzsch, Z. Phys. Chem., 62, 41 (1908). G.A. Olah, J. Amer. Chem. Soc., 8], 3586 (1967). For reviews of this subject see: . (a) H.C. Brown, Chem. Brit., 199 (1966). (b) Ibid, Chem. Eng. News., 45, 87 (Feb. 13, 1967). (c) P.D. Bartlett, "Nonclassical Ions", N.A. Benjamin, New York, New York 1965. (d) G.D. Sargent, Quart. Rev. Chem. Soc., 20, 299 (1966). P. Von Schleyer, M. Saunders and G.A. Olah, J. Amer. Chem. Soc., 86, 5680 (1964). G.A. Olah, A.M. White, J.R. DeMember, g£_al., J. Amer. Chem. Soc., 2%, 4627 (1970). G.A. Olah, G.D. Mateescu, J.L. Riemenschneider, J. Amer. Chem. Soc., 24, 2549 (1972). S. Winstein, et_ 31,, J. Amer. Chem. Soc., 74, 1113 (1953). G.A. Olah, J.R. DeMember, 35 31., J. Amer. Chem. Soc., 21 1442 (1969). D.G. Farnum and G. Mehta, J. Amer. Chem. Soc., 91, 3256 (1969). H. C. Brown and K. Takeuchi, J. Amer. Chem. Soc., 20, 2691 (1970). K. Takeuchi and H.C. Brown, J. Amer. Chem. Soc., 20, 2694 (1970). A.D. Wolf, Ph.D. Thesis, Michigan State University (1972). R.R. Ernst and N.A. Anderson, Rev. Sci. Inst., 31, 93, (1966). M. Karplus and J.A. Pople, J. Amer. Phys., 38, 2803 (1963). G.A. Olah, J.R. DeMember, 35 al., J. Amer. Chem. Soc., 21, 3959 (1969). R.J. Kurland, E£.Elr’ Tet., 21, 735 (1971). H.C. Brown and J. Okamoto, J. Amer. Chem. Soc., 80, 4979 (1958). G.C. Ley and G.L. Nelson, "13CNMR for Organic Chemist Wiley- Interscience, N.Y., 1972, p. 22-24. 21 22 19. D.C. Kleinfelter and P. Von R. Schleyer, J. Org. Chem., 26, 3740 (1961). 20. R.G. Jones and H. Gilman, "Organic Reaction", Wiley, New York, N.Y. 1951 p. 339. CHIGAN STATE UNIVERSITY LIBRARIES 3 1293 030 0555 56