Infrared Studies of Nitriles as Proton Acceptors in Hydrogen Bond Formation

Abstract

The hydrogen-bond formation between phenol, methyl alcohol, and pyrrole as proton donors and acetonitrile, propionitrile, butyronitrile, acrylonitrile, 3-butenenitrile, and cyanogen bromide as proton acceptors has been studied by the infrared spectroscopic method. Spectra were recorded (CaF2 prism) for the OH and NH stretching modes in binary and ternary solutions. CCl4 was used as the solvent for the latter. In ternary solutions bands for both bonded and unbonded molecules were observed. Temperature dependent spectra were used to obtain the heat of formation of the hydrogen bonds. A correlation between the energy of formation and the corresponding frequency shift, Δν (=νfree—νassoc) could be made. In the binary system with a donor compound as solute and an acceptor compound as solvent, only one band corresponding to the bonded molecule was observed. The intensity of this band increased monotonically with the solute concentration, and no other band was observed even for a concentration of 4 moles/liter. The results indicate that the solute—nitrile interactions are stronger than solute—solute interactions, and that only one species of complex molecules is formed, which presumably is a single solute molecule bonded with a single nitrile molecule. A comparison shows that in every case the Δν measured in a weak ternary system was of considerably lower value than that in the corresponding binary system. This is explained as due to the environmental effect of the nitriles which are known to have very high dielectric constants (∼40).