S. CASTELLANO, H. GUNTHER,AND S. EBERSOLE
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Nuclear Magnetic Resonance Spectra of 2,2’-Bipyridyl
by S. Castellano, H. Ginther, and S. Eberaole Mellon Institute, Pittsburgh, Pennsylvania
(Received June 14, 1066)
The n.m.r. spectra of 2,2’-bipyridyl (I) in solution in 11 different solvents (CCI4, CCI3H, CHZOH, CzH50H, (CH&CC=CH, CH2ClCH20H, CH&OOH, C2H,COOH, CH3CHBrCOOH, C3H7COOH, CF,COOH) and in binary mixtures of solvents (CCI4-CCI3H, CClr CHIOH, CCLH-CF3COOH) have been recorded and completely analyzed in terms of the fundamental n.m.r. parameters. The n.m.r. spectrum of the iron complex [FeI1(CloN2H8)3]CI2(11) in CH3OH solution has also been recorded and exactly analyzed. The n.m.r. parameters of I show remarkable changes with the nature of solvent used; in particular, the behavior of the chemical shift of the proton 3 (and 3’) indicates the existence of a strong deshielding effect exerted by the nitrogen atom of the adjacent ring. The several effects which determine the low-field occurrence of the resonance of protons 3 (and 3’) are discussed and their contribution to the shift calculated. It is shown that the van der Waals dispersion forces may cause, at the 3 position, downfield shifts of 0.200.30 p.p.m. in the trans-planar conformation of I. As a result of the calculations, different conformations are assigned to 2,2’-bipyridyl in inert and proton donor solvents. The conformations of the mono- and diprotonated cations are also discussed and orders of magnitude of the dihedral angle between the two pyridine rings estimated. The chemical shifts of the protons of I1 are interpreted and calculated in terms of the anisotropy and dispersion force effects which arise because of the relative orientations of the bipyridyl groups in the molecule.
Introduction The n.m.r. spectrum of 2,2’-bipyridyl has been already described in general terms112and analyzed by a first-order approximation by Kramer and West3 and by ~i1.4 Our interest in the study of the n.m.r. spectrum of this molecule has been motivated by the previous observations of the large deshielding effects exerted by the nitrogen atom of aromatic heterocyclic systems a t the ortho protons of a phenyl ring attached a t the a position. Data obtained in this laboratory from the n.m.r. spectra of 2,4-diphenyl-6-methyl-s-triazine,3,6-diphenyltetrazine, and 2-phenyl~yridine~clearly suggested the existence of a strong interaction between the nitrogen and the opposed aromatic protons. A comparison of the chemical shifts of the ortho protons of the phenyl groups in 2-benz~ylpyridine~ and benzophenone’ suggests that the interaction occurs through space and is still present even if a carbonyl group is inserted between the two rings. The Journal of Phfjsical Chemistry
A preliminary investigation showed that a similar interaction is also present in 2,2’-bipyridyl whose n.m.r. spectrum shows an unusual downfield shift for the resonance of protons 3 and 3’. The spectrum of I appeared to be much simpler than the spectra of any one of the previously mentioned compounds and was therefore considered better suited for a study of the observed phenomenon. Since the latter appeared to be strongly dependent on the nature of the solvent used, it was felt that a systematic investigation of the
(1) M. Freymann and R. Freymann, Arch. Sei. (Geneva), 13, 506
(1960). (2) M.Freymann, R. Freymann, and D. Libermann, Compt. rend., 250, 2185 (1960). (3) F. A. Kramer, Jr., and R. West, J. Phys. Chem., 69,673 (1965). (4) V. M. S.Gil, MOL Phys., 9, 97 (1965).
(5) H.Gtlnther and S. Castellano, unpublished results. (6) S. Castellano and A. A. Bothner-By, J . Chem. Phgs., 41, 3863 (1964). (7) S. Castellano and J. Lorenc, Chim. I n d . (Milan), 47,643 (1965).
N.M.R.
SPECTRA O F 2,2'-BIPYRIDYL
4167
behavior of the spectral parameters of 2,2'-bipyridyl in different solvents could be highly rewarding in terms of a better understanding of the nature of the studied interaction. Accordingly, we have performed the analysis of the n.m.r. spectra of 2,2'-bipyridyl in different media and in mixtures of different solvents; the experimental results found in this study and their interpretation form the body of this paper.
Experimental Section Materials. Samples of 2,2'-bipyridyl were of commercial origin (Calbiochem Co. and Eastman Organic Chemicals). Solvents used were Merck, Fisher Certified, Baker Analyzed, and Matheson Coleman Spectroquality chemicals. All compounds were used without further purification. I n no case were extraneous peaks detected in the n.m.r. spectra, and this was considered as a sufficient criterion of purity for all materials. Repeated experiments with samples from different manufacturers gave identical results. No attempt was made to determine traces of water possibly present in the acids. The iron complex [ F ~ " ( C I O N ~ H S )was ~ ] Cprepared ~~ according to standard procedures.8 The n.m.r. spectrum of this compound showed barely detectable signals from proton-containing impurities. Sample Preparation and N.m.r. Spectra. Solutions of I were prepared by weighing the compound directly in standard 5-mm. n.m.r. tubes and by adding the solvent with a calibrated syringe. All tubes were degassed on a vacuum line and sealed after addition of about 1% of tetramethylsilane (TMS), which was used as an internal reference. For the solution of I1 in methanol, the concentration (less than 0.03 mole fraction) is not exactly known. No TMS was added to the solution; the shifts were measured in this case from the methyl peak of the solvent and referred thence to TMS. Proton spectra were obtained on a Varian A-60 spectrometer. Calibration of the spectra was by means of the audio side band technique. Peak positions were the average of at least four measurements, two made with increasing and two with decreasing field using a 50-C.P.S.full-sweep width. Spectral Analysis. The n.m.r. spectra of I and I1are of the type ABCD since no long-range coupling between the two aromatic rings is detectable experimentally. The spectra often show the characteristic first-order pattern of a 2-substituted pyridine ring,6#1031* and the assignment of the resonances of the different protons is straightforward. However, in the course of this study, several cases were met in which the resonances of two protons overlap and the spectrum con-
Figure 1. Experimental and calculated n.m.r. spectrum of [Fe"( ClaN,H&]C1, in CH,OH solution.
volutes remarkably. For all the cases here reported, the analyses of the spectra were performed with the aid of the LAOCOON I1 programusing a7090 IBM computer. As a typical example of the fitting of the experimental spectra achieved in this way, we have reported in Figure 1 the experimental and calculated n.m.r. spectrum of 11. For the majority of the spectra the probable error of any parameter came out to be about A0.05 C.P.S. I n some cases, however (solution of I in a-bromopropionic and trifluoroacetic acids, solution of I1 in methanol), severe broadening of the spectra was observed, and the resulting parameters are thought For one of to be accurate only to within ~ 0 . C.P.S. 2 the solution in CClBH-CF&OOH mixtures, the fit of the experimental spectrum was achieved by varying only the chemical shifts6and using for the coupling constants (not reported in Table 11)the averages of the values obtained in solutions at lower and higher acid concentrations.
Results The parameters resulting from the analyses of the experimental spectra are reported in Tables I and 11. I n Figures 2 and 3, the chemical shifts of the protons of I are plotted vs. the compositions of the binary mixtures used as solvents. Other tables and figures (8) F. Blau, Monatsh. Chem., 19, 647 (1898). (9) Traces of paramagnetic solid material had, however, to be removed from the solution by centrifugation. (10) W. BrUgel, Z. Elektrochem., 66, 159 (1962). (11) V. J. Kowalewski and D. G . de Kowalewski, J. C h .Phys., 37, 2603 (1962).
Volume 69, Number 1.9 December 1966
4168
S. CASTELLANO, H. G U N T H E R ,
AND
S. EBERSOLE
Table I: N.m.r. Parameters of 2,2'-Bipyridyl in Different Solvents"
Solvent
Mole fraction of CiaNnHa
CCll CCla (CHa)aCCsCH CClaH CC13H CaHiCOOH CHaOH CHaOH CzH,OH CHIOH CZHSCOOH CHzClCHzOH CHzClCHzOH CHaCOOH CH3CHBrCOOHb CFsCOOHb CFaCOOHb
0.203 0.089 0.091 0.161 0.075 0.092 0,084 0.074 0.100 0.040 0.089 0.091 ,]-that their contributions to the chemical shifts are small Clz the cis-planar conformation of the pyridyl groups and do not invalidate the internally consistent interhas been found perfectly in line with the measured pretation of the results presented in this paper. values of the chemical shifts: From the latter data a metal-nitrogen distance of 2 A. has been derived. Acknowledgments. We are profoundly indebted to Finally, we want to point out that, although only Dr. A. A. Bothner-By for many helpful and construcstatic molecular models have been used in the calculative suggestions given to us during the course of the tions, the dihedral angles proposed must be interpreted present work as well as for reading this manuscript. as averaged positions of equilibrium over all the posComputations were performed at the University of sible vibrational and torsional states present in the Pittsburgh Computer Center with the partial support molecule. Several secondary effects (reaction field of the National Science Foundation.
Dielectric Dispersion of Crystalline Powders of Amino Acids, Peptides, and Proteins'
by S. Takashima and H. P. Schwan Electromedical Division, Moore School of Electrical Enoineering, University of Pennsylvania, Philadelphia, Pennsylvania (Receiced June 17,1066)
The dielectric constants of crystalline powders of glycine, tyrosine, glycylglycine, and ovalbumin were measured in the frequency range of 20 C.P.S. t o 200 kc.p.s. It was found that dry crystals did not have an appreciable dielectric constant but that adsorbed water increased the dielectric constant markedly. The static dielectric constants, their dispersions, and the dielectric losses were measured with varied amounts of adsorbed water. The increase of the dielectric constant is proportional to the increase of water of adsorption until the first water layer is completed. The second and third layers are formed if the vapor pressure is increased. The dielectric constant, however, does not increase any more and practically levels off. The formation of multilayers does not seem to affect the dielectric constant of crystals. Apparently, only the first layer of water of adsorption makes the major contribution to the dielectric constant of wet crystals.
Introduction The dielectric constants of crystalline powders of amino acids, peptides, and proteins were measured by Bailey2 in the dry and wet state. His results indicate that these materials have very small dielectric constants when they are carefully dried. He observed, I
however, that the adsorption of water was accompanied by considerable increases in the dielectric constant and the dielectric loss. Unfortunately, Bailey
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T h e JOUTTd of Physical Chemistry
(1) This study was supported by National Institutes of Health Grant N ~ 1253. . (2) 8. T. Bailey, Trans. Faraday Soe., 47, 609 (1951).
