Intermolecular bonding and vibrations of 2‐naphthol⋅H₂O (D₂O) Available to Purchase

A combined experimental and theoretical study of the 2‐naphthol⋅H₂O/D₂O system was performed. Two different rotamers of 2‐naphthol (2‐hydroxynaphthalene, 2HN) exist with the O–H bond in cis‐ and trans‐position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6‐31G(d,p) basis set, fully energy‐minimized geometries were computed for both cis‐ and trans‐2HN⋅H₂O of (a) the equilibrium structures with trans‐linear H‐bond arrangement and C_s symmetry and (b) the lowest‐energy transition states for H atom exchange on the H₂O subunit, which have a nonplanar C₁ symmetry. Both equilibrium and transition state structures are similar to the corresponding phenol⋅H₂O geometries. The H‐bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenol⋅H₂O at the same level of theory. Extension of the aromatic π‐system therefore hardly affects the H‐bonding conditions. The barrier height to internal rotation around the H‐bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6‐31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and M‐matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a′ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β₂ wag mode shows the largest anharmonic contributions. For the torsional mode τ (H₂O H‐atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonant‐two‐photon ionization and dispersed fluorescence emission spectra of 2HN⋅H₂O and d‐2HN⋅D₂O were measured. A detailed assignment of the bands in the intermolecular frequency range is given, based on the calculations. The predicted and measured vibrational frequencies are compared and differences discussed.