Structure relaxation during internal rotation of the nitro group in nitrobenzene

Abstract

An analysis of the dependence of the observed rotational constants on the quantum number ν₃₆ of the NO₂ torsional mode of nitrobenzene is presented. It is first shown that the empirical data available cannot be explained by the exact solution of the semi-rigid rotation-internal rotation model; in particular, the decrease of the rotational constant A with increasing ν₃₆ deviates in a characteristic way from predictions of this model. A satisfactory interpretation, however, is obtained if relaxation of the ONO angle is included in the rotation-internal rotation theory. The relaxation effects are discussed by a perturbation treatment of the rotation-internal rotation theory. Since nitrobenzene represents a C _2ν frame-C _2ν top system, either the phenyl or the nitro group may be chosen as a frame. The interrelation of the two hamiltonians is given and their symmetry groups are shown to be outer automorphs.