Structure relaxation during internal rotation of the nitro group in nitrobenzene

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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.