Vibrational potential energy functions and conformations of 3-cyclopenten-1-one, tetrahydrothiophen-3-one, and tetrahydrofuran-3-one in their electronic excited states

Abstract

The jet-cooled fluorescence excitation spectrum of 3- cyclopenten-l-one has been recorded in the 308-330 nm region, and the electronic origin for the SI(n,7r) state was observed at 30229 cm-'. The observed spectrum consists of more than eighty bands involving primarily P3 (carbonyl stretch), V29 (carbonyl inversion), and V30 (ring bending). Bands were also assigned to combinations with seven other vibrational modes. The energies for the v = 0 to I 1 quantum states of v2g were measured and used to determine a one-dimensional potential energy function. This function had energy minima at inversion angles of +24' and a barrier to interconversion of 939 cm-'. Four bands associated with V30 were observed and used to determine an asymmetric single-minimum one- dimensional ring-bending potential energy function. The ring-bending energy levels in the V29 vibrational excited state are little changed from the v = 0 state indicating that there is little interaction between the carbonyl inversion and ring-bending motions. Theiet-cooled fluorescence excitation spectra of tetrahydrothiophen-3-one and tetrahydrofuran-3-one have been recorded in the 308-333 nm region. Tetrahydrothiophen-3-one and tetrahydrofuran-3-one have electronic origins for their S,(n,ir*) states at 30028 cm-' and 30180 cm-', respectively. Each spectrum consists of more than sixty bands assigned primarily to vibronic transitions involving the carbonyl inversion, ring bending, and carbonyl stretching vibrations. A progression of carbonyl inversion transitions was observed for each molecule, and was used to determine its one-dimensional potential energy function for this vibration. For tetrahydrothiophen-3-one, this function had minima at carbonyl inversion angles of +20' and a barrier to interconversion of 659 cm-'. The corresponding function for tetrahydrofuran-3-one had minima at inversion angles of +26' and an interconversion barrier of 1152 cm-'. These results are consistent with previous work done on similar molecules.