Proton transfer in neutral gas-phase clusters: α- naphthol·NH_3n

Efficient excited-state proton transfer in neutral acid-base clusters α-naphthol·B_n has been detected and studied by a combination of laser spectroscopic techniques (resonant two-photon ionization, fluorescence excitation, and emission spectroscopy). S₁ state proton transfer was observed for B = NH₃ and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α-naphthol and smaller clusters; (b) very large Stokes shift (∼8000 cm^-1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α-naphtholate anion in basic aqueous solution. No proton-transfer reaction was observed for small solvent clusters with B = NH₃ and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B = triethylamine, 3-dimethylamino-1- aminopropane, 1,4-bis(dimethylamino)butane] which we have studied so far. This behavior illustrates the difficulty of achieving charge separation in neutral gas-phase complexes or clusters. A critical gas-phase proton affinity PA _crit = 248 ± 3 kcal/mol was determined for proton transfer to take place in the α-naphthol·B_n (or base B) system.