15min:

E. J. GILLES, L. K. AUSMAN, E. A. BROWN, S. DRUCKER, Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI 54702; S. LEE, J. CHOO, Department of Chemistry, Hanyang University, Ansan 425-791, Korea; M. RISHARD, J. LAANE, Department of Chemistry, Texas A & M University, College Station, TX 77843.

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 ( ~ 20 M^-1 cm^-1) in this region is due to the S₁(n, *) S₀ electronic transition. The origin band was observed at 26,081(1) cm^-1 for the undeuterated molecule and at 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 +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^-1, uncertainties \pm 0.5 cm^-1), along with results of a DFT calculation of the d₀ isotopomer: \begincenterVibrational frequencies of 2CHO in its S₁ state \endcenter \vspace-3mm \begindisplaymath \beginarrayccccc||ccc\hline mode & description & d₀ & d₀ \hspace0.02in( DFT \hspace0.05in calc) & d₃ & v'₃₉ & d₀ & d₃
\hline \rule[0mm]0mm3mm '₃₉ & inversion & 122.1 & 120.8 & 114.4 & 1 & 122.1 & 114.4
'₃₈ & ring \hspace0.05in bending & 251.9 & 249.9 & 236.9 & 2 & 243.8 & 228.6
'₃₇ & C\hspace-0.03in=\hspace-0.03inC \hspace0.05in twisting & 303.3 & 298.4 & 294.6 & 3 & 364.5 & 341.8
'₃₆ & carbonyl \hspace0.05in deformation & 343.9 & 341.9 & 332.0 & 4 & 485.3 & 455.3
& & & & & 5 & 603.6 & 565.7
\hline \endarray \enddisplaymath

The inversion-level spacings in the S₁ state indicate a barrier to planarity that is significantly higher than the 2000-cm^-1 barrier height of the ground electronic state. Work is in progress to fit an S₁ inversion potential to the spectroscopic data.