Lowest n,π Triplet State of 2-Cyclopenten-1-one:  Cavity Ringdown Absorption Spectrum and Ring-Bending Potential-Energy Function

The room-temperature cavity ringdown absorption spectra of 2-cyclopenten-1-one (2CP) and deuterated derivatives were recorded near 385 nm. The very weak (ε < 1 M-1 cm-1) band system in this region is due to the T1 ← S0 electronic transition, where T1 is the lowest-energy 3(n,π*) state. The origin band was observed at 25 963.55(7) cm-1 for the undeuterated molecule and at 25 959.38(7) and 25 956.18(7) cm-1 for 2CP-5-d 1 and 2CP-5,5-d 2, respectively. For the -d 0 isotopomer, about 50 vibronic transitions have been assigned in a region from −500 to +500 cm-1 relative to the origin band. Nearly every corresponding assignment was made in the -d 2 spectrum. Several excited-state fundamentals have been determined for the d 0/d 2 isotopomers, including ring-twisting (ν‘29 = 238.9/227.8 cm-1), out-of-plane carbonyl deformation (ν‘28 = 431.8/420.3 cm-1), and in-plane carbonyl deformation (ν‘19 = 346.2/330.2 cm-1). The ring-bending (ν‘30) levels for the T1 state were determined to be at 36.5, 118.9, 213.7, 324.5, and 446.4 cm-1 for the undeuterated molecule. These drop to 29.7, 101.9, 184.8, 280.5, and 385.6 cm-1 for the -d 2 molecule. A potential-energy function of the form V = ax 4 + bx 2 was fit to the ring-bending levels for each isotopic species. The fitting procedure utilized a kinetic-energy expansion that was calculated based on the structure obtained for the triplet state from density functional calculations. The barrier to planarity, determined from the best-fitting potential-energy functions for the -d 0, -d 1, and -d 2 species, ranges from 42.0 to 43.5 cm-1. In the T1 state, electron repulsion resulting from the spin flip favors nonplanarity. The S0 and S1 states have planar structures that are stabilized by conjugation.