Large-amplitude vibrations and their effects on lowering onset of intramolecular vibrational redistribution in jet-cooled N-methylaniline

Abstract

Laser-induced fluorescence (LIF) spectroscopy of isolated jet-cooled N-methylaniline (NMA) is reported in detail for the first time. The low-frequency, large-amplitude vibrations involving internal methyl rotation, N–aryl torsion and inversion of the methylamino NHCH₃ group have been examined especially and their potential barrier heights determined. We present two hypotheses to describe the behaviour of the lone pair of electrons on the nitrogen atom in the methylamino group and to explain the conjugation between the nitrogen lone pair and methyl group as well as the benzene ring system from which the observed spectra are well interpreted and the geometry of the molecule determined. A link between changes in the torsional potential and π-electron density on the methyl group and the effect of large-amplitude motions on relaxation processes such as intramolecular vibrational redistribution (IVR) is illustrated. A rather low onset of IVR has been observed, which is mainly attributed to the activity of the methylated amino group.