Acid-base chemistry in the gas phase. The c i s- and t r a n s-2-naphthol⋅NH3 complexes in their S and S1 states

Abstract

A unique view of the nascent acid‐base reaction between 2‐naphthol and ammonia along the proton transfer coordinate is provided by analyses of the rotationally resolved S₁←S₀ electronic spectra of their hydrogen bonded complexes cis‐ and trans‐2HNA in the gas phase. Both complexes, in both electronic states, have structures in which ammonia, acting as a base, forms an in‐plane hydrogen bond with the hydroxy hydrogen atom of 2‐naphthol. The ground state O–H⋅⋅⋅N heavy atom separations are R=2.77 Å in cis‐2HNA and R=2.79 Å in trans‐2HNA. Electronic excitation of the significantly more acidic S₁ state of 2‐naphthol produces large decreases in R in both complexes. S₁ cis‐2HNA has R=2.62 Å and S₁ trans‐2HNA has R=2.57 Å. Comparing these results to the Lippincott–Schroeder potential for the hydrogen bond shows that there is little change in the vibrationally averaged position of the hydroxy hydrogen atom. But decreasing R produces significant decreases in the barrier to proton transfer, in the distance from reactant to product along the reaction coordinate, and in the energy difference between them. We thus conclude that whether or not such transfer occurs is primarily dependent on the ability of the two heavy atoms to come into close proximity during the early stages of the reaction, a condition that is not satisfied in either cis‐ or trans‐2HNA, in either electronic state. This view is supported by observed changes in the shapes of the potential surfaces along the NH₃ torsional coordinate that occur on S₁←S₀ excitation of the two complexes.