Laser induced fluorescence spectra and carbonyl wagging potential energy functions for the S-1(n, pi*) excited states of tetrahydrofuran-3-one and tetrahydrothiophen-3-one: Correlation between inversion barrier and angle strain for cyclic ketones

The jet-cooled fluorescence excitation spectra of the S-1(n, pi*) states of tetrahydrofuran-3-one, CH2OCH2CH2C=O, and tetrahydrothiophen-3-one. CH2SCH2CH2C=O, 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, pi*) state of 1152 cm(-1) (13.8 kJ/mol) while the thiophenone has a barrier of 659 cm(-1) (7.9 kJ/mol). The two molecules have their potential energy minima for the S1(n, pi*) state at carbonyl wagging angles of 26 degrees and 20 degrees, 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. (C) 1997 American Institute of Physics.