Effects of anions on the NMR relaxation of pyridinium and di-tert-butylpyridinium ions in acid solution. Implications for chemisorption on solid acids

The line shape of the NMR signals of protons bonded to nitrogen shows that the longitudinal relaxation of N-14 (T-1N) is much faster for di-tert-butyl pyridinium ions (DTBPH+) than for pyridinium (PyH+) in solution. The relaxa tion times for ring carbons (T-1C) indicate that the difference comes from a different rate of tumbling in solution, rather than from a difference in the electric field gradient. Computer modeling gives ratios of relaxation t imes (T-1N = 1/R-1N) Of 10-20, Ion pairing has an opposite effect upon the two ions: it accelerates the relaxation of PyH+ but slows down the relaxati on of DTBPH+. In the absence of electrostatic interactions with the solvent , ion pairing should increase the correlation time tau(c) (decrease T-1) fo r the anion positioned in the plane of the ring and should have only a smal l effect on tau(c) for the anion perpendicular to the ring (along the z axi s). The anion in the ion pair of PyH+ is positioned on the x axis (the C-2 axis of the ring) for maximum hydrogen bonding with the N-H group. The inab ility of DTBP to form hydrogen bonds at nitrogen was confirmed by the equal ity of its N-15 chemical shifts in methyl tert-butyl ether, dry and contain ing water. B3LYP/6-31C* calculations indicate that the positioning of an an ion along the z axis of DTBPH+ induces a charge redistribution that reduces the electrostatic interaction of the cation with the solvent dipoles in th e xy plane, thus decreasing the tumbling correlation time, tau(c), and incr easing the NMR relaxation time, T-i. These data suggest that chemisorption of pyridine on acid sites on solid surfaces occurs with the nitrogen facing the surface but that DTBP is chemisorbed on the side (flatwise) with its d egree of hydronation depending on the degree of curvature of the surface ar ound the site.