A multinuclear NMR investigation of the effect of tert-butyl substituents on the rotation of the pyridine ring in acid solutions

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Dan Fărcasiu and Marta Lezcano


Abstract

The line-shape of the NMR signals of protons bonded to nitrogen indicated that the longitudinal relaxation of 14N is much faster for di-tert-butylpyridinium ions (DTBPH+) than for pyridinium (PyH+) in solution. Computer modeling showed ratios of relaxation times (T1=1/R1) of 10–20. A significant difference between the relaxation times (T1=1/R1) for the carbon atoms in β and γ positions (4.71 and 4.75 s for PyH+, 0.55 and 0.79 s for DTBPH+) was observed as well. Thus, the difference in longitudinal relaxation rates originates in a different rate of tumbling in solution, rather than a difference in the electrical field gradient. Calculations of the correlation times for the relaxation of molecules considered as ellipsoid-shape rotors in a medium of given viscosity indicate that the difference in size covers only a part of the difference in tumbling rates (lower τc for pyridine). The rest comes from specific interactions with the solvent, in the form of electrical double layers which have to be disturbed during the rotation. For PyH+, this electrical friction decreases when the acid strength increases, whereas for DTBPH+ the opposite effect is observed.


References

  1. R. A. Ogg, Discuss. Faraday Soc., 1954, 17, 215 RSC.
  2. (a) J. N. Shoolery, unpublished work cited in ref. 2(b); (b) J. D. Roberts, J. Am. Chem. Soc., 1956, 78, 4495 CrossRef CAS.
  3. D. Fărcaşiu and A. Ghenciu, Progr. NMR Spectrosc, 1996, 29, 129 Search PubMed.
  4. J. D. Baldeschweiler and E. W. Randall, Proc. Chem. Soc., 1961, 303 RSC Fig. 1 and 2 in that paper were, obviously, mislabeled.
  5. M. Allen and J. D. Roberts, J. Org. Chem., 1980, 45, 130 CrossRef CAS.
  6. S. N. Smirnov, N. S. Golubev, G. S. Denisov, H. Benedict, P. Schach-Mohammedi and H. Limbach, J. Am. Chem. Soc., 1996, 118, 4094 CrossRef CAS.
  7. M. Witanowski, L. Stefaniak and G. A. Webb, Ann. Rep. NMR Spectrosc., 1993, 25, 70 Search PubMed.
  8. (a) S. Watanabe and I. Ando, J. Mol. Struct., 1981, 77, 283 CrossRef CAS; (b) M. Witanowski, L. Stefaniak and G. A. Webb, Annu. Rep. NMR Spectrosc., 1986, 18, 192 and Table 130 Search PubMed.
  9. (a) E. Grunwald and E. K. Ralph, Acc. Chem. Res., 1971, 4, 107 CrossRef; (b) E. Grunwald and D. Eustace, in Proton Transfer Reactions, ed. E. F. Caldin and V. Gold, Chapman and Hall, London, 1975, ch. 4 Search PubMed; (c) V. Gold and R. A. Lee, J. Chem. Soc., Chem. Commun., 1984, 1032 RSC.
  10. (a) J. A. Pople, Mol. Phys., 1958, 1, 168 CAS; (b) M. Suzuki and R. Kubo, Mol. Phys., 1964, 7, 201 CAS; (c) M. Witanowski and G. A. Webb, Annu. Rep. NMR Spectrosc., 1972, SA, 414 Search PubMed.
  11. The calculated curves were generated with the program Sigma-Plot, developed by Jandel Scientific, currently part of SPSS, Inc. USA, http://www.spss.com/software/science/SigamaPlot/.
  12. E. D. Becker, High Resolution NMR. Theory and Chemical Applications, 2nd ed., Academic Press, Orlando, FL, (a)p. 192; (b)pp. 186–7 Search PubMed.
  13. R. D. Vold, J. S. Waugh, M. P. Klein and D. E. Phelps, J. Chem. Phys., 1968, 48, 3831 CrossRef CAS.
  14. (a) D. H. Waldeck, personal communication; (b) N. Balabai, A. Sukharevsky, I. Read, B. Strazisar, M. Kurnikova, R. S. Hartman, R. D. Coalson and D. H. Waldeck, J. Mol. Liq., 1998, 77, 37 CrossRef CAS.
  15. We think that for the charged species the electrostatic interaction with the dipoles of surrounding solvent molecules describes better the phenomenon than the dielectric friction, which is the interaction with a uniform dielectric medium: (a) D. S. Alavi and D. H. Waldeck, J. Chem. Phys., 1991, 94, 6196 CrossRef CAS; (b) R. S. Hartman, W. M. Konitsky, D. H. Waldeck, Y. J. Chang and E. W. Castner, Jr., J. Chem. Phys., 1997, 106, 7920 CrossRef CAS; (c) B. Bordes, F. Coletta, A. Ferrarini, F. Gottardi and P. L. Nordio, Chem. Phys., 1998, 231, 51 CrossRef CAS.
  16. D. Fărcaşiu and A. Ghenciu, J. Am. Chem. Soc., 1993, 115, 10901 CrossRef CAS.
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