The structure of the NH2 group and its basicity

The fundamental symmetric (v^s) and antisymmetric (v^as) NH₂ stretching frequencies of some amine-containing compounds (aliphatic and aromatic amines, benzyl amines, and amides) have been measured in dilute CCl₄ solutions. The s-character of the hybrid orbitals of the N-H bonds (b²) has been calculated from v^s and v^as. The basicities (pK_BH⁺) of a number of compounds have been determined. The relationship between pK_BH⁺ and the amount of s-character of the N-H, bonds of the amino groups of the corresponding compounds has been obtained. Separate linear relationships have been found between the indicated values for each group of compounds. For anilines, which have the most highly electron-accepting substituents, there are deviations from the linear dependence b² = f(pK_BH⁺). Differences in the correlation character b² = f(pK_BH⁺) have been explained by a combination of the variations of the N-atom hybrid state and the population of the orbital's lone pair under the influence of substituents.