Hydrogen bonding in the pyrimidine/formamide system: a concentration-dependent Raman and DFT study

Abstract

High-resolution Raman spectra of pyrimidine (PD) and formamide (FA) mixtures with different compositions recorded in the ring breathing region of PD (nu(1) similar to 991 cm(-1)) are presented. The dilution of PD with FA leads to the appearance of a new band at nu(1)' similar to 994 cm(-1), which is assigned to hydrogen-bonded PD: FA species. From a quantitative analysis of the concentration-dependent Raman spectra, the average number of FA molecules in the first solvation sphere of PD is determined as being equal to 2. This value is supported by density functional theory (DFT) calculations: a symmetric 1 : 2 complex is the most stable species among various hydrogen-bonded PD: FA clusters with stoichiometries ranging from 1 : 1 to 1 : 4. A qualitative explanation for the blue shift of the nu(1) mode upon complexation is given. Additionally, we have observed not only similarities but also some differences with respect to the PD : water system. Copyright (C) 2010 John Wiley & Sons, Ltd.