CAVITY RINGDOWN ABSORPTION SPECTRUM OF THE $T_1 (n,\pi^{*}) \leftarrow S_0$ TRANSITION OF 2-CYCLOHEXEN-1-ONE
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Date
2013
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Ohio State University
Abstract
The cavity ringdown (CRD) absorption spectrum of 2-cyclohexen-1-one (CHO) was recorded over the range 401.5--410.5 nm in a room-temperature gas cell. The very weak band system ($\epsilon \leq $ 0.02 dm$^3$ mol$^{-1}$ ${\rm cm}^{-1}$) in this region is due to the $T_1(n, \pi$*$) \leftarrow S_0$ electronic transition. The $0^0_0$ origin band was assigned to the feature observed at \mbox{24,558.6 $\pm 0.3$ ${\rm cm}^{-1}$}. We have assigned about 25 vibronic transitions in a region extending from \mbox{$-200$ to $+350$ cm$^{-1}$} relative to the origin band. From these assignments we determined fundamental frequencies for several vibrational modes in the $T_1$ excited state. %These include ring twisting %$\nu_{39}'$, ring bending (or inversion) %$\nu_{38}'$, and C=C twisting $\nu_{37}'$. The table below compares their frequencies to corresponding values measured for CHO vapor in the $S_0$ electronic ground state (via far-IR spectroscopy) {\bf 73}, 2518 (1980); M.~Z.~M.~Rishard and J.~Laane, \emph{J.~Molec.~Struct.} {\bf 976}, 56 (2010).} and the $S_1(n, \pi$*$)$ excited state (via near-UV CRD spectroscopy). {\bf 112}, 38 (2008).} \\begin{center} Low-frequency fundamentals (cm$^{-1}$) of CHO vapor \vspace{-.2in} \\end{center} \\begin{displaymath} \begin{tabular}{ccccc}\hline \rule[0mm]{0mm}{3mm}{\rm Mode} & {\rm Description} & $S_0$ & $S_1(n,\pi^*)$ & $T_1(n,\pi^*)$ \\ \hline \rule[0mm]{0mm}{3mm}39 & {\rm ring twist} & 99.2 & 122.1 & 99.5 \\ 38 & {\rm bend (inversion of C-5) } & 247 & 251.9 & 253.2 \\ 37 & {\rm C=C twist} & 304.1 & 303.3 & 247.8 \\ 36 & {\rm C=O wag} & 485 & 343.9 & 345.5 \\ \hline \end{tabular} \\end{displaymath} For $\nu_{39}$ and $\nu_{37}$, the differences between $S_1$ and \ $T_1$ frequencies are noteworthy. These differences suggest that the electron delocalization associated with the $\pi^* \leftarrow n$ chromophore in CHO is substantially different for singlet vs.\ triplet excitation.
Description
Author Institution: Department of Chemistry, University of Wisconsin-\mbox{Eau Claire}; Eau Claire, WI 54702