JET-COOLED FLUORESCENCE EXCITATION-SPECTRA, CONFORMATION, AND CARBONYL WAGGING POTENTIAL-ENERGY FUNCTION OF CYCLOPENTANONE AND ITS DEUTERATED ISOTOPOMERS IN THE S-1 (N,PI-ASTERISK) ELECTRONIC EXCITED-STATES

The jet-cooled fluorescence excitation spectra of cyclopentanone and its 2,2,5,5-d4 isotopomer have been recorded in the 305-335 nm region. In addition, the spectra of d1, d2, and d3 species were obtained from isotopic mixtures. The electronic band origin of the d0 molecule for the S1 (n,pi*) state of A2 symmetry occurs at 30 276 cm-1, while that of the d4 molecule is at 30 265 cm-1. More than 100 fluorescence bands were assigned for each species. These arise from combinations of v3 (C=0 stretch), v11 (ring-angle bending), v18 (ring twisting), v25 (C=O out-of-plane wag), v26 (ring bending), and v36 (C=O in-plane wag) and their vibrational excited states. The vibrational frequencies for v3, v11, and v36 are significantly lower in the S1 state than the S0 ground state. However, the out-of-plane ring modes v18 and v26 are only slightly shifted. A progression observed for v26 does indicate that in the S1 state, the bent ring conformation lies about 500 cm-1 above the ring-twisting minimum and corresponds to a saddle point in the two-dimensional ring-twisting/ring-bending potential energy surface. Band progressions for v8 can be used to calculate the ring-twisting barriers (the barriers to planarity) for the d0 and d4 isotopomers to be 1433 and 1240 cm-1, respectively. Because of limited data, however, these values may be as much as several hundred cm-1 too high. The energies for the C=O out-of-plane wagging states up to v25 = 9 for each isotopomer were determined for the S1 state and these were used to calculate the C=O wagging potential energy functions for each. In the S1 state, the barrier to inversion of the C=O group is 672 +/- 10 cm and the wagging angle is 22-degrees +/- 1-degrees.