Vibrational Spectroscopy of Secondary Amine Salts: 1. Assignment of NH2+ Stretching Frequencies in Crystalline Phases

The NH2+ stretching modes of secondary amine salts have been previously studied, but the band assignments are inconsistent between the various studies. This paper assigns characteristic NH2+ 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 NH2+ and the ND2+ 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.