Vibrational force fields and amplitudes, and zero-point average structures of (CH 3) 3Y molecules (Y = N, P, As, Sb, Bi) : A Combination of electron-diffraction and spectroscopic data

Abstract

Following the development of methods for placing electron-diffraction and spectroscopic geometrical parameters on a common basis, available data on (CH 3) 3Y molecules (Y = N, P, As, Sb, Bi) have been used to derive force fields, r.m.s. amplitudes of vibration u along the internuclear vectors, and perpendicular amplitude correction coefficients K for these molecules. For trimethylamine, the amplitudes are similar whether or not off-diagonal elements are included in the force field; hence, only diagonal elements are considered for the other molecules. Among the interesting trends, as group V is descended, is that the C-Y r.m.s. amplitudes increase only from 0.049 to 0.058 Å, whereas the C-Y stretching force constant decreases by over 60% from 5.3 to 1.8 mdyn Å −1. There is evidence for an increasing tendency for torsional motion of methyl groups, as group V is descended. For each of the molecules, the amplitude data were used to derive zero-point average (r o α) structures and to make estimates of a partial equilibrium ( r e) structure. For trimethylamine the results suggest a systematic error in the electron-beam wavelength of the literature study, and the structural parameters were appropriately revised. The r o α lengths of the C-Y bonds in the five molecules are 1.458 ± 0.002, 1.844 ± 0.003, 1.979 ± 0.010, 2.169 ± 0.010 and 2.263 ± 0.004 Å, respectively. The estimated r e parameters for the bonds in trimethylamine agree well with the microwave r s structure.