THE GAOH-HGAO POTENTIAL-ENERGY HYPERSURFACE AND THE NECESSITY OF CORRELATING THE 3D ELECTRONS

The ground state potential energy hypersurface of the GaOH-HGaO system has been investigated using high level ab initio molecular electronic structure theory. The geometries and physical properties of two equil ibrium structures, one isomerization transition state and one inversio n transition state were determined at the self-consistent field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD wi th perturbative triple excitations [CCSD(T)] levels of theory with fou r sets of basis functions. It has been found that freezing the 3d elec trons of the Ga atom in the correlation procedures is not appropriate for this system. For the energy difference Delta E (GaOH-HGaO) the fre ezing of the 3d electrons results in an error of 25 kcal/mol! The dipo le moments, harmonic vibrational frequencies, and infrared (IR) intens ities are predicted for the four stationary points. At the highest lev el of theory employed in this study, CCSD(T) using triple zeta plus do uble polarization with higher angular momentum and diffuse functions [ TZ2P(f,d) + diff] basis set, the bent GaOH was found to be 41.9 kcal/m ol more stable than the linear HGaO species; with the zero-point vibra tional energy (ZPVE) correction, the energy separation becomes 40.4 kc al/mol. The classical barrier height for the exothermic isomerization (1,2 hydrogen shift) reaction HGaO-->GaOH is determined to be 44.5 kca l/mol and the barrier height with the ZPVE correction 42.3 kcal/mol. T he classical barrier to linearity for the bent GaOH molecule is determ ined to be 1.7 kcal/mol and the barrier height with the ZPVE correctio n to be 1.2 kcal/mol. The predicted dipole moments of GaOH and HGaO ar e 1.41 and 4.45 Debye, respectively. The effects of electron correlati on reduce the dipole moment of HGaO by the sizable amount of 1.2 Debye . The two equilibrium species may be suitable for microwave spectrosco pic investigation. Furthermore, they may also be detectable by IR tech niques due to the relatively large intensities of their vibrational mo des. The geometrical and energetic features are compared with those of the valence isoelectronic HXO-XOH systems, where X is a group IIIA at om and the HXO(+)-XOH(+) systems, where X is a group IVA atom. (C) 199 6 American Institute of Physics.