High resolution S1←S fluoescence excitation spectra of the 1- and 2-hydroxynaphthalenes. Distinguishing the c i s and t r a n s rotamers

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Abstract
Both fluorescence excitation and dispersed emission techniques have been used to study the S1←S0 electronic spectra of 1‐ and 2‐hydroxynaphthalene (1/2HN) in the collision‐free environments of a supersonic jet and a twice‐skimmed molecular beam, using both pulsed and high‐resolution cw lasers operating in the ultraviolet. In the jet experiments, we observe that each molecule exhibits two electronic origins, separated by 274 cm−1 in 1HN and by 317 cm−1 in 2HN. In the beam experiments, we resolve the rotational structure of each of the four bands and determine the inertial constants of all eight zero‐point vibrational levels, accurate to ±0.1 MHz. We also determine the orientations of the four optical transition moments in the molecular frame. Significant differences in both the inertial constants and the transition moment orientations are observed in each band. Similar experiments have been performed on the hydroxy‐deuterated 1/2HN (1/2DN). A comparison of the results obtained for 1/2DN with those for the corresponding bands in 1/2HN makes possible the determination of the center‐of‐mass coordinates of the hydroxy hydrogen in both electronic states, accurate to ± 0.02 Å. Differences in these coordinates reveal that the two electronic origins in each spectrum are caused by the presence of two N–O–H(D) rotamers in both 1H(D)N and 2H(D)N, one with a cis (or syn) geometry and one with a trans (or anti) geometry with respect to the naphthalene frame. We make an unambiguous assignment of each origin to a specific rotamer. The lower energy origin in the spectrum of 1HN is that of the cis rotamer, whereas the lower energy origin in the spectrum of 2HN is that of the trans rotamer. We then use these results, together with those of ab initio calculations on the ground electronic states of all four isomers, to explore the reasons for the differences in their energies, to account for the orientations of their transition moments, and to specify other features of the S0 and S1 potential energy surfaces along the cis–trans isomerization coordinate. Motion along this coordinate requires significant displacement of the oxygen atom and selected ring hydrogens as well as rotation about the C–O bond, in both electronic states.