The GeOH–HGeO system: Are the 3d electrons core or valence?

Abstract

The 2A′ ground state of GeOH–HGeO system has been investigated by ab initio electronic structure theory. The equilibrium geometries and physical properties including dipole moments, harmonic vibrational frequencies, and associated infrared (ir) intensities for GeOH, HGeO, and the isomerization (1,2 hydrogen shift) transition state are determined at the self-consistent-field (SCF) and configuration interaction with single and double excitations (CISD) levels of theory with four basis sets. There appear to be two minima for the bent HGeO (isomers A and B) on its SCF and CISD potential energy hypersurfaces. At the Hartree–Fock level the structure with HGeO angle near 90° (isomer B) lies lower, but correlated methods show that the structure with HGeO angle near 120° (isomer A) actually lies lower. At the optimized CISD geometries, the single point energies of coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] methods are also determined. In the correlated procedures three different types of frozen core orbital approximation (15 frozen core, 10 frozen core, and 6 frozen core orbitals) have been examined. The energetics based on the first (15 frozen core orbitals) approximation present errors of about 1 kcal/mol compared to more accurate second (10 frozen core orbitals) and third (6 frozen core orbitals) approximations. At the highest level of theory employed in this research, CCSD(T) with triple zeta plus double polarization with diffuse and higher angular momentum functions [TZ2P(f,d)+diff] basis set, the bent GeOH molecule is predicted to be lower in energy than the bent HGeO molecule by 28.5 kcal/mol. This energy separation becomes 25.7 kcal/mol with the zero-point vibrational energy (ZPVE) correction. The classical energy barrier for the exothermic isomerization reaction [HGeO(B)→GeOH] is determined to be 11.8 kcal/mol and the activation energy (with the ZPVE correction) 10.7 kcal/mol. The theoretically predicted isotope shifts for the GeO stretching vibrational frequency of GeOH agree very well with experimental assignments by Withnall and Andrews [J. Phys. Chem. 94, 2351 (1990)].