Ab Initio Study of the Structures and π* ← <i>n</i> Electronic Transition in Formic Acid−(Water)<i>_n</i> (<i>n</i> = 3, 4, and 5) Hydrogen Bonded Complexes

Abstract

Hydrogen-bonded complexes of formic acid-(H 2 O) n ( n = 3, 4, and 5) have been studied using multiconfigurational SCF and second-order perturbation theory, (CASSCF/CASPT2). Equilibrium geometries in the ground electronic state S 0, and vertical S 1 ← S 0 transition energies have been calculated for the 1:3, 1:4, and 1:5 complexes. Similar results for energetics and geometry of the ground state were obtained with Becke3LYP calculations. The vertical transitions are blue-shifted with respect to the corresponding monomer transition, and their dependence on the number of water molecules is not monotonic.