HIGH-RESOLUTION S(1)[-S(0)FLUORESCENCE EXCITATION-SPECTRA OF HYDROQUINONE - DISTINGUISHING THE CIS AND TRANS ROTAMERS BY THEIR NUCLEAR-SPINSTATISTICAL WEIGHTS

Hydroquinone (HYQ) has two rotational isomers, differing in the orient ation of the two O-H bonds with respect to the benzene ring. In the S1 <-- S0 fluorescence excitation spectrum of HYQ, two electronic origin s, one for each rotamer, are present with a separation of 34.7 cm-1. R otationally resolved spectra of both origins have been obtained and an alyzed. This analysis reveals that the rovibronic lines in the two ori gins exhibit different alternating intensity patterns due to nuclear s pin statistical weights, thereby providing a means for distinguishing the two rotamers. On this basis, we assign the origin at 33 500 cm-1 t o 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 ato ms of both rotamers, in both electronic states confirm this assignment .