Solid-state primary aliphatic amines are actually ammonium amides as revealed by infrared spectroscopy

The infrared spectra of primary alkylamines largely belong to one of two different types as found out by a systematic data search. This search was motivated by observing before an intriguing spectral pattern (type-I) in our studies of octadecylamine, with only one nu NH band, and, besides, the signature for ammonium groups. This particular type-I spectrum occurs whenever amines are determined to adopt a solid state, that is, as a rule, in the long-chain analogs. The other, so to speak classical type-II of amine spectra arising from fluid-phase or very diluted samples shows two nu NH bands and no ammonium markers, and exclusively this pattern is given in the current textbooks for generally representing primary amine. Concerning the new type-I amine spectra found by ourselves for octadecylamine and shown also by others, a clear phenomenological description is hitherto missing, not to mention a relevant interpretation. Here, we characterize these spectra for the first time in detail and propose that the chemical compound underlying them actually is ammonium amide. The relevant far-reaching implications involve then logically a proton transfer to proceed from one amino group to the other. Reasonably presumable driving forces for this self-dissociation reaction are discussed. (C) 2008 Elsevier B.V. All rights reserved.