Modeling the electronic structure of formamide: an acid/base amphoteric solvent

Abstract

H-bond formation of formamide, in which acidity centers interactions with fluoride ions and basicity centers with hydrofluoric acid, was used to determine its Lewis acidity and basicity. For this purpose, several methods and basis sets were tested, and the B3LYP hybrid functional in conjunction with a 6-311++G(d,p) basis set was selected as the one which gives high-quality results with reasonable computational requirements. Lewis acidity parameter (LAP) for the acidity center and Lewis basicity parameter (LBP) for the basicity center were determined as energies of the strongest H-bonds in the molecule. The obtained quantities, LAP = −33.38 kcal/mol and LBP = −12.32 kcal/mol, are in line with the empirical solvent parameters as Reichardt’s ET and Kamlet–Taft’s basicity parameter β. Changes in the π-electron structure of the NCO fragment were observed as a result of H-bonding (HOMA in the range between 0.521 and 0.969) and can be easily explained with the contribution of resonance structures. Several QTAIM-based parameters allowed us to estimate such contributions and also to discuss the acid/base character of formamide. It was demonstrated that the complexation (via H-bonding) could be directly linked with a charge transfer (CT, data from both QTAIM and NBO) between the interacting fragments. It appeared that the QTAIM-based local properties of H-bonds were linearly correlated with CT, despite the fact that characteristics of the bonds formed by different atomic centers were used.