Laser-induced fluorescence excitation spectra of indoline: low-frequency vibrations of the pseudo-four-membered ring

Abstract

The laser-induced fluorescence excitation (FE) and single vibronic level luminescence (SVL) spectra of indoline have been studied by a pulsed seeded jet technique. The vibronic spectrum bears the signature of two low-frequency vibrations involving coupling of C₂-puckering and N-inversion. A low-frequency (122 cm^-1 regular progression in the FE spectrum has been assigned to a coupled vibration involving C₂-puckering and N-inversion. The corresponding ground state frequency, as obtained from SVL spectra, is 160 cm^-1. Another series of bands with irregular intervals has been fitted with a double-well potential barrier of 33 cm^-1; this corresponds to the second coupled vibration fo C₂-puckering and N-inversion. On substituting the H atom attached to the N atom by a D atom, a large isotope shift is obtained. This is explained as being due to a large difference in N=H vibrational frequencies of S₀ and S₁ states.