Lowest n,π* Triplet State of 2-Cyclopenten-1-one: Cavity Ringdown Absorption Spectrum and Ring-Bending Potential-Energy Function †
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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-d1 and 2CP-5,5-d2, respectively. For the -d0 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 -d2 spectrum. Several excited-state fundamentals have been determined for the d0/d2 isotopomers, including ring-twisting (v′29 = 238.9/227.8 cm-1), out-of-plane carbonyl deformation (v′28 = 431.8/420.3 cm-1), and in-plane carbonyl deformation (v′19 = 346.2/330.2 cm-1). The ring-bending (v′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 -d2 molecule. A potential-energy function of the form V = ax4 + bx2 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 -d0, -d1, and -d2 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.
author list (cited authors)
Pillsbury, N. R., Choo, J., Laane, J., & Drucker, S.