Hydrogen bonding in electronically excited states: a comparison between formic acid dimer and its mono-substituted thioderivatives

Abstract

A multi-state complete active space second order perturbation theory (MS-CASPT2) study on the valence singlet electronic excited states of formic acid dimer is presented. The electronic spectrum of this dihydrogen bonded system is dominated by nπ* and ππ* intramonomer and charge transfer excitations and consists of a very intense ππ* transition at 8.25 eV and three weaker nπ*(2×) and ππ*(1×) electronic excitations at 6.21 eV, 9.13 eV, and 9.93 eV, respectively. The characteristic nπ*–nπ*–ππ*–ππ*… pattern found in the formic acid dimer electronic spectrum is altered when a sulfur atom is introduced in the molecule. Furthermore, carbonyl-by-thiocarbonyl or hydroxyl-by-thiohydroxyl substitution is predicted to strongly affect the intensity of the above electronic transitions. The effect of oxygen-by-sulfur substitution on the geometry of the first excited state (S₁) has been investigated at the CC2 and CASSCF levels of theory. Although the two methods qualitatively predict the same geometrical changes upon nπ* excitation, the geometries of the S₁ state are found to differ considerably between the two levels.