Knowledge Bank

The room-temperature cavity ringdown absorption spectra of 2-cyclopenten-1-one (2CP) and deuterated derivatives were recorded near 385 nm. The very weak () band system in this region is due to the $T\leftarrow S0$ electronic transition, where T is the lowest-energy $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}(n,\pi \ast )$ state. The origin band was observed at $25,963.6cm-1$ for the undeuterated molecule and at 25,959.4 and $25,956.2cm-1$, 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 $+500cm-1$ 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 ($\nu 29\prime =238.9/227.8cm-1$), out-of-plane carbonyl deformation ($\nu 28\prime =431.8/420.3cm-1$), and in-plane carbonyl deformation ($\nu 19\prime =346.3/330.2cm-1$). The ring-bending ($\nu 30\prime$) levels for the T state were determined to be at 36.5, 118.9, 213.7, 324.5, and $446.4cm-1$ for the undeuterated molecule. These drop to 29.7, 101.9, 184.8, 280.5, and $385.6cm-1$ 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 $43.5cm-1$. 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.