O–H torsional vibrations in the S and S1 states of catechol

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Abstract
A spectroscopic study of supersonic jet‐cooled catechol (1,2‐dihydroxybenzene) and its d 1‐ and d 2‐isotopomers, deuterated at the hydroxy groups, was performed by resonant two‐photon ionization (R2PI) and fluorescence emission techniques, and supplemented by molecular‐beam hole‐burning experiments. The latter prove that one single rotamer of catechol is dominant under molecular beam conditions. The complicated vibrational structure in the S 0→S 1 spectrum from the 00 0 band to 400 cm−1 above is not due to three different rotamers, as previously thought, but is due to the excitation of a vibrational progression associated mainly with the torsion of the hydroxy groups. The torsional bands are very prominent in the R2PI spectra, but are weak in the emission spectra. Detailed analysis of the torsional bands was based on a fit to the S 1 and S 0 state frequencies and the Franck–Condon factors in absorption and emission, using a double‐minimum potential for the S 1 state and a harmonic potential for the S 0 state. In the S 1 state one of the two –O–H torsional mode frequencies is lowered from τ2≊250 to ≊50 cm−1, and the molecule is only quasiplanar with respect to the –O–H torsional coordinates.
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