Jet-cooled fluorescence excitation spectra and carbonyl wagging and ring-puckering potential energy functions of cyclobutanone and its 2,2,4,4-d4 isotopomer in the S1(n,π*) electronic excited state

Abstract

The jet-cooled fluorescence excitation spectra of cyclobutanone and its 2,2,4,4-d4 isotopomer have been recorded in the 305–335 nm region. The electronic band origin of the d0 molecule for the S1(n,π*) state of A2 symmetry occurs at 30 292 cm−1 (30 265 cm−1 for the d4 molecule). The observed spectra consisting of more than 50 bands for each isotopomer involve ν7, ν8, and ν9 (the three A1 ring vibrations) as well as ν20(C=O in-plane wag), ν26 (C=O out-of-plane wag), and ν27 (ring puckering). Five bands associated with the excited vibrational states of ν26 in the S1(n,π*) 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° and the barrier to inversion is 2149 cm−1 (2188 cm−1 for the deuteride). For the ring-puckering in the S1 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 S0 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.