Knowledge Bank

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. The electronic origin for the undeuterated species occurs at 27210 $cm-1$ for the $S1(\Pi ,\pi \bullet )$ electronic excited state. The vibrational frequencies for the three carbonyl motions and the nine ring modes were observed for the excited state. Bands at 67, 158, and $256cm-1$ 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. A single one-dimensional potential energy function accurately fit the data for all three isotopomers. This function is nearly purely quartic in character and shows the ring to be planar in the electronic excited state. However, it has become less rigid, and this is ascribed to a decrease in initial angle strain within the ring. The C=0 and C=C stretching frequencies occur at 1418 and 1357 $cm-1$ for the $d0$ molecule. The ring-twisting frequency for the $S1$ state occurs at 274 $cm-1$. Previous electronic absorption measurements had resulted in a misassignment for this motion. The vibrational data for the electronic excited state will be compared to ground state values recently obtained by far-infrared $spectroscopy.1$