Accurate dissociation energies of two isomers of the 1-naphthol⋅cyclopropane complex

The 1-naphthol⋅cyclopropane intermolecular complex is formed in a supersonic jet and investigated by resonant two-photon ionization (R2PI) spectroscopy, UV holeburning, and stimulated emission pumping (SEP)-R2PI spectroscopy. Two very different structure types are inferred from the vibronic spectra and calculations. In the “edge” isomer, the OH group of 1-naphthol is directed towards a C—C bond of cyclopropane, the two ring planes are perpendicular. In the “face” isomer, the cyclopropane is adsorbed on one of the π-aromatic faces of the 1-naphthol moiety, the ring planes are nearly parallel. Accurate ground-state intermolecular dissociation energies D₀ were measured with the SEP-R2PI technique. The D₀(S₀) of the edge isomer is bracketed as 15.35 ± 0.03 kJ/mol, while that of the face isomer is 16.96 ± 0.12 kJ/mol. The corresponding excited-state dissociation energies D₀(S₁) were evaluated using the respective electronic spectral shifts. Despite the D₀(S₀) difference of 1.6 kJ/mol, both isomers are observed in the jet in similar concentrations, so they must be separated by substantial potential energy barriers. Intermolecular binding energies, D_e, and dissociation energies, D₀, calculated with correlated wave function methods and two dispersion-corrected density-functional methods are evaluated in the context of these results. The density functional calculations suggest that the face isomer is bound solely by dispersion interactions. Binding of the edge isomer is also dominated by dispersion, which makes up two thirds of the total binding energy.