A theoretical investigation of the vibrational states of HCO2− and its isotopomers

Abstract

Making use of the coupled cluster variant CCSD(T) and the aug-cc-pVQZ basis set a six-dimensional (6D) potential energy surface has been calculated for HCO₂⁻, a fundamental organic anion. Therefrom, a variety of vibrational term energies and wavefunctions has been obtained by means of the discrete variable representation in an approach termed DVR(6). Calculated wavenumbers of the fundamentals of HCO₂⁻ and DCO₂⁻ agree with recent experimental values from neon matrix isolation IR spectroscopy within 15 cm⁻¹. The out-of-plane bending vibrations v₄ are predicted at 1030 and 894 cm⁻¹. Moderately strong Fermi resonance interaction is calculated between vibrational states v₁ and 2v₄ of DCO₂⁻. Excellent agreement with experiment (differences less than 0.7 cm⁻¹) is observed for the ¹³C and ¹⁸O isotopic shifts. Accurate ground-state rotational constants are predicted for eight different isotopomers of the formate ion and the dissociation process HCO₂⁻ → H⁻ + CO₂ is investigated in some detail, with the dissociation energy D₀ predicted to be 216 kJ mol⁻¹.