A systematic study of molecular vibrational anharmonicity and vibration—rotation interaction by self-consistent-field higher-derivative methods. Asymmetric top molecules

Abstract

Recently developed analytic third-derivative methods for self-consistent-field (SCF) wavefunctions have made it possible to theoretically determine a number of anharmonic molecular properties, including vibration—rotation interaction constants, vibrational anharmonic constants, fundamental vibrational frequencies, sextic centrifugal distortion constants, and rotational constants which include zero-point vibrational and centrifugal distortion corrections. Application has been made here to a variety of asymmetric top molecules including H 2O, H 2S, H 2CO, HCO, CH 2( 3B 1), CH 2( 1A 1), CH 2( 1B 1), C 2H 4, and several isotopomers. The formulas employed for the various spectroscopic constants are those derived from standard spectroscopic perturbation theory. For most of these molecules the anharmonic molecular constants which are available from experiments are well reproduced theoretically using a double-zeta plus polarization (DZP) basis set. Particularly good agreement is found for fundamental vibrational frequencies obtained by combining configuration interaction singles and doubles (CISD) harmonic frequencies with SCF anharmonic corrections within the same basis set. This work also provides insight into the applicability of standard spectroscopic perturbation theory to such molecules. Moreover, the methods used here are immediately applicable to molecules significantly larger than those studied here.