Abstract
The jet-cooled fluorescence excitation spectra of the S1(n,π*) 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=ax4−bx2 or V=cx2+d exp(−fx2). The furanone was found to have an inversion barrier in the S1(n,π*) 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,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.
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