HIGH RESOLUTION ELECTRONIC SPECTROSCOPY OF P-VINYLPHENOL IN THE GAS PHASE

Author Institution: Department of Chemistry, University of Pittsburgh, PA 15260Recently, a controversy has developed over the proper assignment of the electronic spectrum of trans-p-coumaric acid (\emph{tP}CA), the chromophore in photoactive yellow protein. Ryan \emph{et al.}} {\bfseries 124}, 6194 (2002).} claim that two closely spaced peaks near 33,200 cm$^{-1}$ are the S$_1\leftarrow\,\,$S$_0$ origin bands of \emph{tP}CA, whereas de Groot and Buma} {\bfseries 109}, 6135 (2005).} argue (based on REMPI results) that the spectrum should be contributed to the decomposition product \emph{p}-vinylphenol (\emph{p}VP). We have addressed this issue by recording the fully resolved spectra of these two bands. The derived values of the rotational constants show unambiguously that the carrier of these bands is \emph{p}VP; the two conformers are due to the two possible orientations of the -OH group with respect to the vinyl group. With the aid of theoretical calculations, the origin at 33,207.3 cm$^{-1}$ has been assigned to \emph{trans-p}VP and the origin at 33,211.8 cm$^{-1}$ to \emph{cis-p}VP. These results confirm the thermal decarboxylation of \emph{tP}CA