A DEUTERIUM NMR-STUDY OF MOLECULAR-DYNAMICS AND GEOMETRY IN 2 CLASSESOF ONIUM SALTS - (CH3)3E- AND C6H5M(CH3)3+I-(X)

Deuterium NMR measurements are reported for two types of onium salts: (CH3)3E+I-, where E = O (counterion is BF4-), S, Se, or Te, and C6H5M( CH3)3+I-, where M = N, P, or As. Within each class of compounds the ac tivation energy for rotation of the trimethyl groups about the C3' axi s increases with increasing size of the central atom. In the first cla ss of compounds the C-E-C bond angle decreases with the size of the at om E. In addition the magnitude of the quadrupolar coupling constant, chi, varies with E, ranging from 160 kHz for E = O to 190 kHz for E = Te. This is in qualitative agreement with molecular orbital calculatio ns of the electric field gradients. At low temperatures the H-2 NMR sp ectrum of C6H5N(CH3)3+I- Can only be rationalized with a model in whic h trimethyl rotation is faster than methyl rotation. The H-2 NMR of ri ng (predominantly ortho and para)-deuterated C6H5N(CH3)3+I- is consist ent with rapid n-site (n greater-than-or-equal-to 3) rotation of the p henyl ring above 390 K. Below 390 K spectra characteristic of two-site , 180-degrees, flips of the phenyl ring are observed. Below 280 K the motion of the phenyl ring is in the rigid lattice limit.