Structural relaxation in nitromethanelike molecules

In a molecule with a large-amplitude coordinate like the internal rotation angle of the methyl hydrogens relative to the CNO2 fragment in nitromethane the other structural parameters are not strictly constant, but vary slightly with the angle. This behavior is usually described by saying that relaxations of the vibrational coordinates take placet and ii is observed by the presence of special terms in the Hamiltonian needed in fitting the spectra. It is shown how relaxations can be treated theoretically by a perturbation method. Subsequent quantitative analysis of the effects in nitromethane Indicates that the dominant relaxations involve the rocking and wagging coordinates of the nitro group. The maximum rocking relaxation in the staggered configuration is found to be theta(r) approximate to 4 degrees, whereas the maximum wagging relaxation in the eclipsed configuration is smaller, theta(k) approximate to 0.7 degrees.