A study of the molecular dynamics of compounds containing rotor groups of c(_3) symmetry

Abstract

Models for calculation of barriers to rotation from torsional mode frequencies were reviewed, and a new simple harmonic quantum mechanical treatment of two coaxial rotors with both internal and external barriers was developed. Torsional and librational mode frequencies of several sets of compounds containing hydrogenous rotor groups were obtained principally by incoherent inelastic neutron scattering, with reference to both new and old infrared and Raman studies. The internal and external potentials in the N(_2)H(_6)(^+2) halide salts were calculated using the new model. The external barriers were used to calculate the appreciable hydrogen bond strengths, and the internal barrier was found to be less than in ethane. The different phases of the monomethylammonium halide and PF(_6)(^-)salts were studied, including selective deuteration studies, and the new model again applied to obtain barriers. The model worked well for the chloride p-phase but began to break down for the bromide indicating loss of simple harmonic behaviour. The other phases all displayed non-harmonic behaviour. The internal barrier was found tobe intermediate between that of N(_2)H(_6)(^+2) and C(_2)H(_6). In the C(_6)H(_5)NH(_3)(^+)C1(^-) and Br(^-) salts the internal barrier was found to be very small, and the external -NH(_3)(^+) torsional barrier for the chloride was in good agreement with that in N(_2)H(_6)(^+2)C1(_2)(6-). A large number of methyl-halogeno compounds of the group IV elements were studied. Multiple top torsional mode frequency splittings were observed and their neutron scattering intensities related to their mode degeneracies. Several previous I.R. determinations were shown to be in error. The di-, tri- and tetra-methylammonium halide salts were studied and torsional mode splittings were again observed. Barriers to methyl rotation were found to be relatively high, and steric effects due to the short C-N bond length, and external influences, were found to be quite important.