Dan Fărcasiu and Marta Lezcano
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.