Coupling Across Bonds: Ab Initio Calculations for the Anharmonic Vibrational Resonance Dynamics of the Coupled OH and CH Chromophores in Trans Formic Acid HCOOH

Abstract

We report high level ab initio calculations (treating correlation by second order Møller-Plesset perturbation theory, MP2) of various two-dimensional (2D), three-dimensional (3D) and four-dimensional (4D) normal coordinate subspaces of the full 12D coordinate space of trans formic acid, HCOOH. With the resulting potential and electric dipole hyper-surfaces accurate vibrational variational calculations are carried out using a discrete variable representation (DVR) for the anharmonically coupled modes. The full absorption spectra are calculated and the overtone spectra of the OH chromophore, which clearly dominates the entire absorption spectrum, are analyzed in detail with respect to their anharmonic resonance dynamics. We have investigated the OH/CH stretching coupling, which is an example for a coupling across bonds, in terms of direct and indirect coupling terms. A close resonance coupling of 5