Acid–base chemistry in the gas phase: The trans-1-naphthol⋅NH3 complex in its S and S1 electronic states

We deduce information about the dynamics of a proton transfer reaction between an acid and a base. Our probe is the fully resolved S1←S0 fluorescence excitation spectrum of the 1:1 complex of 1-naphthol and ammonia in the gas phase. Analysis of this spectrum shows that the complex is planar in both electronic states, with the NH3 forming a nearly linear hydrogen bond to the hydroxy hydrogen atom of 1-naphthol. The O–H...N heavy atom separation is R=2.86 Å and the barrier to rotation of the NH3 group about its C3 axis is V3=39.9 cm−1 in the S0 state. Excitation of the complex to its S1 state increases the acidity of 1-naphthol, decreases the heavy atom separation to R=2.72 Å, and increases the torsional barrier to V3=46.5 cm−1. Modeling these changes using the Lippincott–Schroeder potential for the hydrogen bond shows that the photoinitiated heavy atom motion produces a significant decrease in the barrier to proton transfer in the S1 state.