Structural study of the hydrogen-bonded 1-naphthol⋅(NH3)2 cluster

The structure of the 1-naphthol⋅(NH3)2 cluster was investigated by rotational coherence spectroscopy (RCS), mass selective one- and two-color resonant two-photon ionization (R2PI) experiments and ab initio calculations. RCS measurements yielded rotational constants of 1-naphthol⋅(NH3)2 as A=1197, B=500, and C=413 MHz, as well as those for several isotopomers. The counterpoise-corrected second-order Møller–Plesset perturbation theory (MP2) method predicts two isomers A and B. Both structures have hydrogen bonded naphthol–OH⋯NH3⋯NH3 chains, with the second NH3 bent above the proximal aromatic ring and pointing towards the π-electron system and have nearly the same binding energy. The experimental rotational constants agree better with those calculated for structure B. The B3LYP and PW91 density functional methods also predict two isomers A, B with the rotational constants of B in acceptable agreement with experiment. Based on two-color R2PI experiments using low ionization frequency to suppress cluster fragmentation, the S1←S0 electronic origin region of the 1-naphthol⋅(NH3)(2–4) cluster series was reassigned, in agreement with the work of Dedonder-Lardeux et al. [Phys. Chem. Chem. Phys. 3, 4316 (2001)]. In one-color experiments, the 1-naphthol⋅(NH3)3 cluster fragments with nearly 100% efficiency into the 1-naphthol⋅(NH3)2+ mass channel.