Internal rotation dynamics of trifluoronitromethane and related compounds

Abstract

Internal rotation in CF 3NO 2 has been studied by ab initio methods. The influence of geometry relaxation on the torsional energy levels has been assessed using both a one dimensional torsional Hamiltonian and a two dimensional Hamiltonian which incorporates the NO 2 wagging mode of the molecule. Torsion and torsion/wag potential energy surfaces, together with related geometric data have been derived at the 6–311G ∗ basis set level with electron correlation included at the MP2 level. Comparison is drawn between a model for internal rotation formulated in terms of internal coordinates and geometry relaxation and that published recently by Tolles (W.M. Tolles, J. Chem. Phys., 99 (1993) 5718). It is demonstrated that vibrational coupling between the high frequency NO 2 wagging vibration and the much lower frequency torsional mode is, as expected, very small indeed. It is also shown that geometry relaxation provides a mechanism by which an interaction of the type described by Tolles can occur between two vibrational modes with widely differing frequencies. Results are presented for CH 3NO 2 which suggest that the conclusions drawn here regarding torsional motion in CF 3NO 2 may apply to CX 3NO 2 species in general.