High resolution S1←S fluorescence excitation spectra of hydroquinone. Distinguishing the cis and trans rotamers by their nuclear spin statistical weights

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Abstract
Hydroquinone (HYQ) has two rotational isomers, differing in the orientation of the two O–H bonds with respect to the benzene ring. In the S1←S0 fluorescence excitation spectrum of HYQ, two electronic origins, one for each rotamer, are present with a separation of 34.7 cm−1. Rotationally resolved spectra of both origins have been obtained and analyzed. This analysis reveals that the rovibronic lines in the two origins exhibit different alternating intensity patterns due to nuclear spin statistical weights, thereby providing a means for distinguishing the two rotamers. On this basis, we assign the origin at 33 500 cm−1 to the trans, C2h rotamer and the origin at 33 535 cm−1 to the cis, C2v rotamer of HYQ. Coordinate determinations of the hydroxy–hydrogen atoms of both rotamers, in both electronic states confirm this assignment.