STRUCTURAL RELAXATION IN NITROMETHANELIKE MOLECULES

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