THE JET-COOLED FLUORESCENCE EXCITATION SPECTRUM AND RING-BENDING POTENTIAL-ENERGY FUNCTION AND CONFORMATION OF 2-CYCLOPENTEN-1-ONE IN THE S1(N,PI-STAR) ELECTRONIC EXCITED-STATE

The jet-cooled fluorescence excitation spectra of 2-cyclopenten-1-one and its 5,5-d2 isotopomer have been recorded in the 370-340 nm region. The electronic origin for the undeuterated species occurs at 27 210 cm-1 for the S1 (n,pi*) electronic excited state. The vibrational frequencies for the three carbonyl motions and the nine ring modes were observed for the excited state. Bands at 67, 158, and 256 cm-1 for the d0 species, at 63, 147, and 240 cm-1 for the 5-d1 isotopomer, and at 59, 138, and 227 cm-1 for the d2 species were assigned to the ring-puckering motion in the S1 state. A single one-dimensional potential-energy function accurately fits the data for all three isotopomers. This function is nearly purely quartic in character and shows the ring to be planar in the electronic excited state. However, it has become less rigid, and this is ascribed to a decrease in initial angle strain within the ring. The C = O and C = C stretching frequencies occur at 1418 and 1357 cm-1 for the d0 molecule. The ring-twisting frequency for the S1 state occurs at 274 cm-1. Previous electronic absorption measurements had resulted in a misassignment for this motion.