HIGH-LEVEL AB-INITIO STUDY ON THE GROUND-STATE POTENTIAL-ENERGY HYPERSURFACE OF THE HCO+-COH+ SYSTEM

The ground state potential energy hypersurface of the HCO +-COH+ system has been reinvestigated systematically using high level ab initio electronic structure theory. The geometries and physical properties of the two equilibrium and one isomerization transition state structures 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 with perturbative triple excitations [CCSD(T)] levels of theory with nine basis sets. First, the optimized geometries of the three stationary points at twenty eight (28) levels of theory were discussed. Second, the characteristics and responses of the molecular orbitals (MOs) with respect to the normal coordinates have been elucidated via energy derivative analysis technique. Third, dipole moments, harmonic vibrational frequencies, and infrared (IR) intensities were described. Finally, the relative energies among the stationary points have been compared. At the highest level of theory, CCSD(T) using triple zeta plus double polarization with diffuse and higher angular momentum functions [TZ2P(f,d)+diff] basis set, the linear HCO+ molecule is found to be 39.7 kcal/mol [37.7 kcal/mol with zero-point vibrational energy (ZPVE) correction] lower in energy relative to the linear COH+ molecule. At the same level of theory the activation energy for the isomerization reaction HCO +→COH+ is predicted to be 76.9 kcal/mol (72.6 kcal/mol with the ZPVE correction). Once the COH+ species is formed, therefore, it should be reasonably stable with respect to the isomerization reaction at low temperatures. © 1994 American Institute of Physics.