Intermolecular dissociation energies of 1-naphthol· n -alkane complexes

Using the stimulated-emission-pumping/resonant 2-photon ionization (SEP-R2PI) method, we havedetermined accurate intermolecular dissociation energiesD0of supersonic jet-cooled intermolec-ular complexes of 1-naphthol (1NpOH) with alkanes, 1NpOH·S, with S = methane, ethane,propane, andn-butane. Experimentally, the smaller alkanes form a single minimum-energy struc-ture, while 1-naphthol·n-butane forms three different isomers. The ground-state dissociation ener-giesD0(S0) for the complexes with propane andn-butane (isomers A and B) were bracketedwithin±0.5%, being 16.71±0.08 kJ/mol for S = propane and 20.5±0.1 kJ/mol for isomerA and 20.2±0.1 kJ/mol for isomer B ofn-butane. All 1NpOH·S complexes measured previ-ously exhibit a clear dissociation threshold in their hot-band detected SEP-R2PI spectra, but weakSEP-R2PI bands are observed above the putative dissociation onset for the methane and ethanecomplexes. We attribute these bands to long-lived complexes that retain energy in rotation-typeintermolecular vibrations, which couple only weakly to the dissociation coordinates. Accountingfor this, we find dissociation energies ofD0(S0) = 7.98±0.55 kJ/mol (±7%) for S = methane and14.5±0.28 kJ/mol (±2%) for S = ethane. TheD0values increase by only 1% uponS0→S1excitation of1-naphthol. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3, andωB97X-D predict that then-alkanes bind dispersively to the naphthalene “Face.” The assignmentof the complexes to Face structures is supported by the small spectral shifts of theS0→S1electronic origins, which range from +0.5 to−15 cm−1. Agreement with the calculated disso-ciation energiesD0(S0) is quite uneven, the B97-D3 values agree within 5% for propane andn-butane, but differ by up to 20% for methane and ethane. TheωB97X-D method shows goodagreement for methane and ethane but overestimates theD0(S0) values for the largern-alkanesby up to 20%. The agreement of the B3LYP-D3D0values is intermediate between the other twomethods.