15min:

M. G. SPRINGER, S. P. JAGUSCH, N. C. HLAVACEK AND S. DRUCKER, Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI 54702.

The cavity ringdown absorption spectrum of 4-cyclopenten-1,3-dione (CPD) was recorded near 487 nm in a room-temperature gas cell. The very weak band system ( < 0.05 dm³ mol^-1 cm^-1) in this region is due to the T₁(n, *) S₀ electronic transition. The origin band was observed at 20,541\pm 1 cm^-1. We have assigned 15 vibronic transitions in a region extending to about +700 cm^-1 relative to the origin band. From these tentative assignments we determined fundamental frequencies for several vibrational modes in the T₁ excited state. These include the out-of-plane ring modes ₁₉' (b₁) and ₁₄' (a₂). The table below compares their frequencies to corresponding values in the S₀ electronic ground state and the S₁(n, *) excited state.

\begincenter Ring frequencies (cm^-1) of CPD in its lowest electronic states \vspace-.2in \endcenter \begindisplaymath \beginarrayccccc\hline \rule[0mm]0mm3mm Mode & Description & S₀ & S₁ & T₁
\hline \rule[0mm]0mm3mm ₁₉ & pucker & 99 & 160 & 106
₁₄ & twist & 239 & 307 & 299
\hline \endarray \enddisplaymath The increases in these ring frequencies upon electronic excitation signify that the nominal n ^* chromophore is delocalized to include the conjugated ring atoms. It is noteworthy, though, that the ₁₉ fundamental increases by only 7 cm^-1 upon T₁ S₀ excitation, compared to 61 cm^-1 for the S₁ excitation. Other cyclic conjugated enones show differences (S₁ vs. T₁ ring frequencies) of similar magnitude. These findings are attributable to the configuration mixing that each of the excited states undergoes. The two n, ^* excited states may undergo mixing within distinctly different manifolds of spin-orbitals.