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

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 τ_c 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.