A deuterium NMR study of molecular dynamics and ...

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Title A deuterium NMR study of molecular dynamics and geometry in two classes of onium salts:(CH3) 3E+ X-and C6H5M (CH3) 3+ I
Author(s) G. H. Penner, James M. Polson, S. I. Daleman, K. Reid
Journal Canadian Journal of Chemistry / Revue Canadienne De Chimie
Date 1993
Volume 71
Issue 4
Start page 417
End page 426
Abstract 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 activation energy for rotation of the trimethyl groups about the C3' axis increases with increasing size of the central atom. In the first class of compounds the C-E-C bond angle decreases with the size of the atom 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 calculations of the electric field gradients. At low temperatures the H-2 NMR spectrum of C6H5N(CH3)3+I- Can only be rationalized with a model in which trimethyl rotation is faster than methyl rotation. The H-2 NMR of ring (predominantly ortho and para)-deuterated C6H5N(CH3)3+I- is consistent with rapid n-site (n greater-than-or-equal-to 3) rotation of the phenyl 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.

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