Proton transfer in neutral gas-phase clusters: α-Naphthol⋅(NH3)n

Abstract

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.