An improved anharmonic force field of difluoromethanimine, F2C=NH

The anharmonic force field of difluoromethanimine, F2C=NH. has been reinvestigated theoretically using a coupled-cluster singles and doubles approach, augmented for structural optimization and harmonic force field by a contribution of connected triple excitations, CCSD(T). The cubic and quartic force constants have been obtained by numerical derivatives computed from analytical quadratic force constants calculated by second-order Moller-Plesset perturbation theory, MP2. The quadratic force constants and the equilibrium structure of F2C=NH have then been scaled by a global least-squares fitting procedure to the spectroscopic data and parameters experimentally determined for this molecule. This force field, obtained in the internal coordinates space and therefore valid for all isotopmers of difluoromethanimine. yields a complete set of spectroscopic molecular constants providing a critical assessment of the experimental rotational and centrifugal distortion constants fundamentals. overtones, and combination bands determined so far for F2C=NH. In addition, the final force field can he used to make predictions of all important vibrational and rotational parameters which should he accurate and useful for new spectroscopic investigations, (C) 2002 Elsevier Science (USA).