Ab initio study of the structures and π* ← n electronic transition in formic acid-(water)n (n=3, 4, and 5) hydrogen bonded complexes

Hydrogen-bonded complexes of formic acid-(H2O)(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.