Interplanar torsion in the $S1←S$ electronic spectrum of jet cooled 1-phenylimidazole

The $S1←S0$ transition of 1-phenylimidazole (1PI) has been studied in a supersonic jet expansion by resonant two-photon ionization. The origin band at $36 075 cm−1$ is accompanied by a low frequency progression associated with torsion about the bond connecting phenyl and imidazole groups. Torsional potentials have been determined for both states. In $S0,$ $φmin=37.2±0.5°$ and the planar barrier is $339±20 cm−1,$ while in $S1,$ $φmin=17.6±0.5°$ and the planar barrier is $57±2 cm−1.$ The transition moment alignment is observed to be consistent with an excited state of $Lb$ character, in spite of the “off-axis” conjugation provided by the imidazole ring. These results are compared with ab initio calculations on both states, performed using Hartree–Fock, Møller–Plesset second-order perturbation, density functional theory with the Becke3–Lee–Yang–Parr functional, time-dependant density functional theory, configuration interaction singles, and complete active space self-consistent field methods. Solution-phase UV spectra of neutral and protonated 1PI are also reported.