Lowest n,π* triplet state of 2-cyclopenten-1-one: Cavity ringdown absorption spectrum and ring-bending potential-energy function

Abstract

The room-temperature cavity ringdown absorption spectra of 2-cyclopenten-l-one (2CP) and deuterated derivatives were recorded near 385 nm. The very weak (epsilon < 1 M-1 cm(-1)) band system in this region is due to the T-1 <-- S-0 electronic transition, where T-1 is the lowest-energy (3)(n,pi*) 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 (nu'(29) = 238.9/227.8 cm(-1)), out-of-plane carbonyl deformation (nu'(28) = 431.8/420.3 cm(-1)), and in-plane carbonyl deformation (nu'(19) = 346.2/330.2 cm(-1)). The ring-bending (nu'(30)) levels for the T-1 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 T-1 state, electron repulsion resulting from the spin flip favors nonplanarity. The S-0 and S-1 states have planar structures that are stabilized by conjugation.