A comparative theoretical analysis of the hydrogen bond in dimers containing substituted carbonyl compounds

Abstract

Ab-initio SCF calculations have been performed with a minimal ST0-3G basis set to determine the structures and energies of the dimers H2O-HFCO and H2O-F2CO, with H2O as the proton donor. A comparison of the hydrogen bond energies of these dimers with the energy of the hydrogen bond in H2O-H2CO shows that the hydrogen bond strength decreases in order of decreasing sigma electron density at the oxygen. The equilibrium structures are well described by the general hybridization model for the hydrogen bond. The relative orientation of the dipole moments of proton donor and acceptor molecules is also found to be significant in the structures of these dimers. Configuration interaction (CI) calculations have also been performed to determine the excitation energies for the lowest vertical excited singlet states in the monomers and dimers. The vertical excitation energies in the dimers are greater than the n +π* transition energies of the respective proton acceptors, and therefore the experimentally observed blue shift is reproduced. The blue shift is quantitatively attributed to the additional energy needed to break the hydrogen bond in the excited singlet states of these dimers.