Jet-cooled fluorescence spectra and carbonyl wagging potential energy functions of tetrahydrofuran-3-one and tetrahydrothiophen-3-one in their S 1(n,π∗) excited states

Abstract

The jet-cooled fluorescence excitation spectra of states of tetrahydrofuran-3-one and tetrahydrothiophen-3-one have been recorded and analyzed. The carbonyl inversion bands, which arise from double-minimum potential energy functions in the excited states, were fit with functions of the form V = ax 4 − bx 2 or V = cx 2 + d exp(− fx 2). The furanone was found to have an inversion barrier in the S 1(n,π∗) state of 1152 cm −1 (13.8 kJ mol −1) while the thiophenone has a barrier of 659 cm −1 (7.9 kJ mol −1). The two molecules have their potential energy minima for the S 1(n,π∗) state at carbonyl wagging angles of 26 and 20°, respectively. The results here, together with previous data for several other cyclic ketones, demonstrate that the inversion barrier increases with the ring angle strain at the ketone carbon atom.