Solvent effects on isolated formamide and its monohydrated complex: observations from PCM study

Abstract

A polarizable continuum model (PCM), at the B3LYP/6-311++G (d,p) level of theory, is used to study solvent effects on isolated formamide and its monohydrated complex. Six varying kinds of solvent (viz.water, dimethyl sulfoxide, acetonitrile, ethanol, tetrahydrofuran, carbontetrachloride) are selected to model different polarity environments. The roles of non-specific solvation and specific H-bonding associated with the bound water in influencing geometries, vibrational frequencies, binding energies, ¹H chemical shifts and n →π* transition energies are discussed for formamide. Natural bond orbital (NBO) and atoms in molecules (AIM) theories are used to analyze the nature of H-bonding and the origin of solvent effects. Significant red- and blue-shifts are observed for the frequencies of formamide upon solvation and formation of hydrogen bonds, respectively. Both the solvation and the H-bonding increase the ¹H chemical shifts of amino protons and the n →π* transition energy, while the chemical shift of formyl proton is nearly insensitive to the two effects. The role of the specific H-bonding in influencing molecular properties is lessened by the solvation since the solvent lowers the binding energy of the complex, while the solvent effect is also modulated by the H-bonding through a specific intermolecular interaction. Compared with non-polar solvents, polar solvents have a more obvious effect on the properties examined. Furthermore, all the variations show a large dependency on the dielectric constant up to a value of ∼10, after which no further changes are observed.