FTIR and DFT studies on the hydrogen bond interactions in liquid formamide and its solutions with Quinoline

The carbonyl stretching bands of pure Formamide (FA), and its solutions at three different molar ratios with quinoline (QUI) have been analysed by the experimental and theoretical means. The possibility for the presence of various FA associates, and FA-QUI as well as few FA associates in pure formamide, and solutions, respectively, is indicated by the carbonyl stretching peaks. However, the associates like cyclic dimers with anti-parallel dipolar alignment and the structures which can be approximated to this type of alignment dominate in liquid FA as per the effective Kirkwood correlation factor while the linear and cyclic ring structures are least probable. The negligible change in the number of carbonyl fundamentals on dilution by QUI and the existence of few FA associates which are free from heterointeraction in the solutions are indicative of the stability of FA associates against dilution. The FA concentration dependent dielectric relaxation time of the solutions depicts that 1:1 (FA:QUI) as well as 2:2 complexes are present in 1:1 (FA:QUI), and 1:2 solutions, while 2:1 complexes are prominent in 2:1 solution. In literature, the hydrogen bond energy has been reported to be determined with considerable accuracy using the experimental proton donor stretching frequency shift. Theoretical frequency shifts are as good as experimental shifts to yield the hydrogen bond energies. The electron density at the bond critical point gives hydrogen bond energy which is closer to the value obtained from frequency shift of proton donor bond.