Vibrational relaxation in jet-cooled p a r a-alkylanilines

Abstract

Absorption and fluorescence spectra have been obtained for the ¹B₂(ππ*)←¹A₁ transition of a series of para‐alkylanilines cooled in a supersonic free jet. The series consists of all p‐n‐alkylanilines through n‐butyl‐ as well as p‐isopropylaniline and aniline itself. For those members of the series having an alkyl chain length of three or greater, two distinct molecular conformations are found to be populated in the jet with roughly equal likelihood. Spectra of both the extended chain conformation [p‐n‐(t)‐alkylanilines] and the coiled chain conformation [p‐n‐(g)‐alkylanilines] show that two optically active aniline vibrations are largely unaffected by changes in the length and nature of the n‐alkyl chain. One of these vibrations (the inversion motion of the –NH₂ group) would be expected to be particularly weakly coupled to the torsions and bends of p‐alkyl chain. Nonetheless, fluorescence spectra obtained when this vibration is excited in the ¹B₂ state show that intramolecular vibrational relaxation occurs into the chain modes within a nanosecond.