Vibrational spectroscopy of secondary amine salts: 1. Assignment of NH(2)(+) stretching frequencies in crystalline phases.

The NH(2)(+) stretching modes of secondary amine salts have been previously studied, but the band assignments are inconsistent between the various studies. This paper assigns characteristic NH(2)(+) group frequencies between approximately 2500 and 2400 cm(-1). Crystal structures of four diamine salts are reported here. Vibrational frequencies were calculated with the B3LYP hybrid Hartree-Fock/density functional method and the 6-31G(d) split-valence plus polarization basis set, and the results are in agreement with the experimental frequencies. Deuterium dilution experiments result in a group of sharply featured bands between the NH(2)(+) and the ND(2)(+) stretching bands. These bands, located between 2200 and 2100 cm(-1), are attributed to modes that contain contributions from coupled N-H and N-D stretching motions.