JET-COOLED FLUORESCENCE EXCITATION-SPECTRA AND CARBONYL WAGGING AND RING-PUCKERING POTENTIAL-ENERGY FUNCTIONS OF CYCLOBUTANONE AND ITS 2,2,4,4-D(4) ISOTOPOMER IN THE S-1(N,PI-ASTERISK) ELECTRONIC EXCITED-STATE

The jet-cooled fluorescence excitation spectra of cyclobutanone and its 2,2,4,4-d(4) isotopomer have been recorded in the 305-335 nm region. The electronic band origin of the d(0) molecule for the S-1(n,pi*) state of A(2) symmetry occurs at 30 292 cm(-1) (30 265 cm(-1) for the d(4) molecule). The observed spectra consisting of more than 50 bands for each isotopomer involve v(7), v(8), and v(9) (the three A(1) ring vibrations) as well as v(20)(C=O in-plane wag), v(26) (C=O out-of-plane wag), and v(27) (ring puckering). Five bands associated with the excited vibrational states of v(26) in the S-1(n,pi*) electronic state were observed for each isotopic species, and these were used to determine the one-dimensional potential energy functions for the C=O out-of-plane wagging. The C=O wagging angle was determined to be 39 degrees and the barrier to inversion is 2149 cm(-1) (2188 cm(-1) for the deuteride). For the ring-puckering in the S-1 state the lowest three vibrational energy spacings were found to be 106, 166, and 185 cm(-1) as compared to values of 35, 57, and 65 cm(-1) in the So ground state. Several ring-puckering potential energy functions with varying degrees of asymmetry are capable of reproducing the observed results. In all cases, however, the v=0 puckering state lies above any barrier to planarity. A two-dimensional potential function which fits the observed data was also determined in terms of the wagging and puckering coordinates.