Hydrogen-bonding interaction of methyl-substituted pyridines with thioacetamide: steric hindrance of methyl group

The hydrogen-bonding interaction between a series of methyl-substituted pyridines as proton acceptors and thioacetamide as a proton donor in CCl4 has been investigated using near-infrared absorption spectroscopy. The stability of the 1:1 hydrogen-bonded complex increases with the number of methyl groups and depends on the position of methyl groups. The steric hindrance of ortho-methyl groups particularly reduces the stability of complex. The relative stability agrees with the ease of miscibility of pyridines with water for methyl and dimethyl homologs. The calculated proton affinities and the DFT association energies using 6-31+G(d, p) and 6-311++G(2d, 2p) basis sets reveal the steric hindrance of ortho-methyl groups.