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The cavity ringdown absorption spectra of 2-cyclohexen-1-one (2CHO) and a deuterated derivative were recorded near 380 nm in a room-temperature gas cell. The weak band system ($ϵ\approx$ 20 $M-1$ $cm-1$) in this region is due to the $S1(n,\pi$*$)\leftarrow S0$ electronic transition. The origin band was observed at \mbox{26,081(1) $cm-1$} for the undeuterated molecule and at \mbox{26,076(1) $cm-1$} for 2CHO-2,6,6-$d3$. For the $d0$ isotopomer, about 40 vibronic transitions have been assigned in a region from $-300$ to \mbox{$+700$ $cm-1$} relative to the origin band. Nearly every corresponding assignment was made for the $d3$ species. Several fundamental vibrational frequencies in the $S1$ state, as well as the five lowest ring-puckering (or inversion) energy levels in the $S1$ state, have been determined for the $d0/d3$ isotopomers. The spectroscopic results are summarized below (frequencies in cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, uncertainties $\pm 0.5$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$), along with results of a DFT calculation of the $d0$ isotopomer: \begin{center}Vibrational frequencies of 2CHO in its $S1$ state \end{center} \vspace{-3mm} \begin{displaymath} \begin{array}{cccccccc}\hline {\rm mode} & {\rm description} & d_0 & d_0 \hspace{0.02in}(\rm DFT \hspace{0.05in} calc) & d_3 & v'{39} & d_0 & d_3 \ \hline \rule[0mm]{0mm}{3mm} \nu'{39} & {\rm inversion} & 122.1 & 120.8 & 114.4 & 1 & 122.1 & 114.4 \ \nu'{38} & {\rm ring \hspace{0.05in} bending} & 251.9 & 249.9 & 236.9 & 2 & 243.8 & 228.6\ \nu'{37} & {\rm C\hspace{-0.03in}=\hspace{-0.03in}C \hspace{0.05in} twisting} & 303.3 & 298.4 & 294.6 & 3 & 364.5 & 341.8\ \nu'_{36} & {\rm carbonyl \hspace{0.05in} deformation} & 343.9 & 341.9 & 332.0 & 4 & 485.3 & 455.3\ & & & & & 5 & 603.6 & 565.7\ \hline \end{array} \end{displaymath} The inversion-level spacings in the $S1$ state indicate a barrier to planarity that is significantly higher than the 2000-cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ barrier height of the ground electronic state. Work is in progress to fit an $S1$ inversion potential to the spectroscopic data.