15min:

ROBERTO LINGUERRI, ISABELLE NAVIZET AND PAVEL ROSMUS, Laboratoire de Chimie Théorique, Université de Marne la Vallée, F 77454 - Champs sur Marne, France; STUART CARTER, Department of Chemistry, University of Reading, Reading RG6 2AD, U.K.; JOHN P. MAIER, Department of Chemistry, University of Basel, Klingelbergstr. 80 CH-4056 Basel, Switzerland.

Two independent approaches are applied to study the intra-molecular dynamics in B₄ electronic ground state. First, a double minimum six-dimensional potential energy surface (PES) is determined in symmetry coordinates for the most stable rhombic (D_2h) B₄ isomer in its ¹A_g electronic ground state by fitting to energies calculated by the CCSD(T) approach. The PES exhibits a barrier to the D_4h square structure of 255 cm^-1. The vibrational levels (J = 0) are calculated variationally using the MULTIMODE code, which involves the Watson kinetic energy operator expressed in normal coordinates. The pattern of about 65 vibrational levels up to 1600 cm^-1 for all stable isotopomers is analyzed. Analogous to the inversion in ammonia-like molecules, the rhombus rearrangements lead to splittings of the vibrational levels. In B₄ it is the b_1g (D_4h) mode which distorts the square molecule to its planar rhombic form. It is shown that the rhombic ground states of B₄⁺ and B₄^- consist of two vibronically coupled electronic states. The PES for B₄ is used also in time-dependent molecular dynamics calculations using the multi-configurational time-dependent Hartree (MCTDH) method. The 6-D wavepackets are analyzed and the results are compared with time-independent results.