Electronic spectra of jet-cooled isoindoline: Spectroscopic determination of energy difference between conformational isomers

Abstract

The electronic spectra of jet-cooled isoindoline between the electronic ground (S(0)) state and the pi pi(*) lowest-excited singlet state (S(1)) were observed by the fluorescence excitation and single-vibronic-level dispersed fluorescence methods. The low-frequency progression due to the puckering vibration appeared in both spectra. Analysis of dispersed spectra together with geometry optimization at the level of B3LYP/6-311+G(d) indicated the presence of conformational isomers possessing axial and equatorial N-H bonds with respect to the molecular plane. The 0-0 bands of the axial and equatorial conformers were measured at 37,022 and 36,761 cm(-1), respectively. Three common levels in the S(1) state accessible from the respective S(0)-state zero levels were observed. From their transition frequencies, the S(0)-state energy difference between the isomers was determined to be 47.7+/-0.2 cm(-1), where the axial conformer was more stable. In the S(1) state, the energy difference was 213.7+/-0.2 cm(-1), and the equatorial conformer was more stable. The cause of switching from a stable conformation upon excitation is discussed in terms of the electron conjugation between the pi(*) orbital in benzene and the lone pair orbital of nitrogen.