Infra-red solvent shifts and molecular interactions. Part 1.—Relative strengths of primary and secondary nitrogenous bases

Abstract

A detailed study of the N-H stretching modes of a number of bases is reported and a correlation established between the solvent dependency of v(N-H) and the dissociation constants of their conjugate acids. It is proposed that the AV/V solvent slope be adopted as a measure of the intrinsic proton-accepting power of a base. Such values provide an acidity scale which is independent of solvent.Recent work ~2 has shown that the solvent shift of an infra-red-active hydrogenic vibrational frequency is primarily due to solute-solvent dipolar association, 40 d-8+ d-Pergamon Press), in press.

Associated XH Frequencies, the Hydrogen Bond

“…Cutmore and Hallam [164] give Vs and Vas values for a series of substituted anilines in a variety of solvents, including acetone, ether and pyridine. Cutmore and Hallam [164] give Vs and Vas values for a series of substituted anilines in a variety of solvents, including acetone, ether and pyridine.…”

“…In a I : I complex the antisymmetric band moves only slightly whilst the symmetric band is moved considerably more. Data on N-substituted anilines in various bases are given by Cutmore and Hallam [164] and by Perrier, Saumagne and Josien [169]. One result of this is that the separation of the two bands is markedly greater in the 1 : 1 complex, and this can be used as a pointer for the occurrence of hydrogen bonding [299,301].…”

Infrared Spectrometry

“…Cutmore and Hallam (129) reported a detailed study of the NH stretching mode of a number of bases and established a correlation between the solvent dependence of y(NH) and the dissociation constants of their conjugate acids; they proposed that the relative frequency shift be adopted as a measure of the intrinsic proton-accepting power of a base to provide an acidity scale which is independent of solvent. The usual relative-frequency shift relations described by Bellamy, Williams, and Hallam (54) involve the use of the vapor-phase frequencies.…”