ELECTRON TRANSITION CURRENT-DENSITY IN MOLECULES .3. AB-INITIO CALCULATIONS FOR VIBRATIONAL TRANSITIONS IN ETHYLENE AND FORMALDEHYDE

We present the first examples of ab initio calculations of electron tr ansition current density (TCD) for vibrational transitions in molecule s, The non-Born-Oppenheimer (non-BO) theoretical expressions for TCD, derived in the first paper in this series, are implemented at the nb i nitio level for the 6 vibrational transitions of formaldehyde and the 12 vibrational transitions of ethylene. Vector field calculations of t he TCD were carried out with 6-31G(d) and larger basis sets and displa yed with the AVS visualization software program package. TCDs for vibr ational transitions arise from the non-BO correlation of electron curr ent density to nuclear velocities, The formalism used to express this correlation is the complete adiabatic approximation in which the elect ronic wave function carries an explicit dependence on the nuclear velo cities as well as the usual dependence on positions. Vibrational TCDs provide a unique, unambiguous visualization of electronic motion in mo lecules that accompanies the vibrational nuclear motion. Patterns of c alculated TCD in formaldehyde and ethylene are analyzed in terms of th eir group theoretical properties and allowed multipole transitions. Tw o principal classes of TCD motion are observed. One is motion that ref lects linear, laminar current densities that lead to changes in electr on probability density in response to nuclear displacements. A second is circulatory motion of TCD about atomic centers that appears be asso ciated primarily with the lateral motion of two adjacent atomic center s. The latter motion does not lend to changes in electron probability density, but gives rise to magnetic dipole moments, and is likely impo rtant in the generation of vibrational circular dichroism intensity.