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

The line-shape of the NMR signals of protons bonded to nitrogen indicated t hat the longitudinal relaxation of N-14 is much faster for di-tert-butylpyr idinium ions (DTBPH+) than for pyridinium (PyH+) in solution. Computer mode ling showed ratios of relaxation times (T-1=1/R-1) of 10-20. A significant difference between the relaxation times (T-1=1/R-1) for the carbon atoms in beta and gamma positions (4.71 and 4.75 s for PyH+, 0.55 and 0.79 s for DT BPH+) 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 correlat ion times for the relaxation of molecules considered as ellipsoid-shape rot ors in a medium of given viscosity indicate that the difference in size cov ers only a part of the difference in tumbling rates (lower tau(c) for pyrid ine). The rest comes from specific interactions with the solvent, in the fo rm of electrical double layers which have to be disturbed during the rotati on. For PyH+, this electrical friction decreases when the acid strength inc reases, whereas for DTBPH+ the opposite effect is observed.