Infrared Spectroscopic Study on Trimethyl Amine Radical Cation: Correlation between Proton-Donating Ability and Structural Deformation

Barrierless intermolecular proton transfer from a CH bond has recently been reported in the vertical ionization of the trimethyl amine (TMA) dimer. This result indicates the remarkable enhancement of the proton-donating ability of the CH bond in its cationic state. In the present study, we have carried out an infrared spectroscopy of the neutral and cationic TMA in the CH stretch region and their theoretical calculations to investigate the mechanism of enhancement of the proton-donating ability in the cationic state. In the spectrum of the cation, the CH stretch band shows a long tail of up to 2600 cm^-1. This tail component is attributed to the CH bond hyperconjugated with the nonbonding orbital at the nitrogen atom through geometry deformation (excitation of molecular vibrations) with the excess energy upon photoionization. This hyperconjugation causes the delocalization of the σ electron of the CH bond to the singly occupied nonbonding orbital so that the proton-donating ability of the CH is enhanced. It is shown that the excitation of the CN stretching vibration is especially effective in promoting the hyperconjugation.