Ion-dip spectroscopy and structure of 5-phenyl imidazole, 4-phenyl imidazole and its 1:1 hydrate

Analysis of the two-photon ionisation (R2PI) and ion-dip `hole-burnʹ spectra of 4-phenyl imidazole (4-PI), its 1:1 hydrate and its tautomer, 5-phenyl imidazole (5-PI), establish: a near-planar structure for 4-PI, an N–H hydrogen-bonded structure for its hydrate and a twisted structure for the tautomer. The ion-dip spectrum of 5-PI displays either even (symmetric) or odd (anti-symmetric) members only, of the extended torsional progression in its S1←S0 transition, to provide a new diagnostic for the existence of a double minimum potential. The transition is predicted (and the type-a band contour of 4-PI confirms) the population of a 1La excited state.