15min:

NATHAN R. PILLSBURY, STEPHEN DRUCKER, Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI 54702; JAEBUM CHOO, Department of Chemistry, Hanyang University, Ansan 425-791, Korea; JAAN LAANE, Department of Chemistry, Texas A & M University, College Station, TX 77843.

The room-temperature cavity ringdown absorption spectra of 2-cyclopenten-1-one (2CP) and deuterated derivatives were recorded near 385 nm. The very weak ( < 1 M^-1 cm^-1) band system in this region is due to the T S₀ electronic transition, where T is the lowest-energy ³(n, *) state. The origin band was observed at 25,963.6 cm^-1 for the undeuterated molecule and at 25,959.4 and 25,956.2 cm^-1, respectively, for 2CP-5-d₁ and 2CP-5,5-d₂. 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 ( ₂₉'=238.9/227.8 cm^-1), out-of-plane carbonyl deformation ( ₂₈'=431.8/420.3 cm^-1), and in-plane carbonyl deformation ( ₁₉'=346.3/330.2 cm^-1). The ring-bending ( ₃₀') levels for the T 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 T state from ab initio 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 T state, electron repulsion resulting from the spin flip favors nonplanarity. The S₀ and S₁ states have planar structures that are stabilized by conjugation.