# THE LOWEST ${^{3}}(n,\pi^{\ast})$ STATE OF 2-CYCLOPENTEN-1-ONE: CAVITY RINGDOWN ABSORPTION SPECTRUM AND RING-BENDING POTENTIAL ENERGY FUNCTION

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 Title: THE LOWEST ${^{3}}(n,\pi^{\ast})$ STATE OF 2-CYCLOPENTEN-1-ONE: CAVITY RINGDOWN ABSORPTION SPECTRUM AND RING-BENDING POTENTIAL ENERGY FUNCTION Creators: Pillsbury, Nathan R.; Drucker, Stephen; Choo, Jaebum; Laane, J. Issue Date: 2003 Abstract: The room-temperature cavity ringdown absorption spectra of 2-cyclopenten-1-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 $T \leftarrow S_{0}$ electronic transition, where T is the lowest-energy ${^{3}}(n, \pi^{\ast})$ 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 $2CP-5-d_{1}$ and $2CP-5,5-d_{2}$. 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 ($\nu^{\prime}_{29} = 238.9/227.8 cm^{-1}$), out-of-plane carbonyl deformation ($\nu^{\prime}_{28} = 431.8/420.3 cm^{-1}$), and in-plane carbonyl deformation ($\nu^{\prime}_{19} = 346.3/330.2 cm^{-1}$). The ring-bending ($\nu^{\prime}_{30}$) 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 $-d_{2}$ 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 T state from ab initio 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 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. URI: http://hdl.handle.net/1811/20991 Other Identifiers: 2003-TD-09