The S1(n,π*) states of cyclopentanone and cyclobutanone in a supersonic nozzle beam

Abstract

Fluorescence excitation spectra of cyclopentanone and cyclobutanone have been observed for their (n,π*) transition in a pulsed supersonic nozzle beam using a high power tunable laser. A drastic reduction of hot bands has been attained, making it possible to discuss the vibronic assignments in more detail than the previous works. The C=O out-of-plane wagging mode was found to be active for both molecules. The ring twisting and flapping in cyclopentanone and the ring puckering in cyclobutanone were also active. The molecules are pyramidally distorted in the excited states with double minimum potentials in the out-of-plane displacement coordinates. The barrier to inversion (V) and the C=O out-of-plane angle at the potential minimum (θm) have been determined for the S1(n,π*) state; V=680±17 cm−1 and θm=34° for cyclopentanone, and V=1850±50 cm−1 and θm=42° for cyclobutanone. The puckering mode (ν20′) of cyclobutanone was also found to have a double minimum potential with V=16.9 cm−1. The rotational envelope of each vibronic band has been analyzed on the basis of a computer simulation for an asymmetric top molecule. In contrast with formaldehyde, the A-type (parallel) component, as well as the B-type, was shown to be important in these cyclic ketones. The mechanism of the vibronic intensity borrowing is discussed on the basis of the band-type considerations.