The Jet-Cooled Fluorescence Excitation Spectra, Potential Energy Functions, and Conformations of 2-Cyclopenten-1-one, Propanal, and Propynal in the S1(n,π*) Electronic Excited State

Abstract

The jet-cooled fluorescence excitation spectra of 2-cyclopenten-1-one and its 5,5-d2 isotopomer have been recorded in the 370 to 340 nm region and a portion of the spectrum for the undeuterated species is shown in Figure 1. The electronic origin for this molecule occurs at 27210 cm−1 for the S1(n,π*) electronic excited state. More than fifty separate frequencies were observed and compared to earlier absorption measurements [1]. Many of the transitions involve combinations with die ring-puckering, ring-twisting, or the carbonyl bending modes. The vibrational frequencies for die three carbonyl motions and the nine ring modes were determined for the excited state, and these are compared to ground state values [2] in Table I. 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, and the transitions corresponding to these are shown in Figure 2.