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.

Torsional Barriers in Aromatic Molecular Clusters as Probe of the Electronic Properties of the Chromophore

“…The change in barrier height upon electronic excitation is slightly different from the respective values determined by Plusquellic et al [18] by using the approximation of equal F for both electronic states (F = 6.58 cm…”

“…If the hole-burning spectrum shown in ref. [19] was analyzed at the high-frequency side of the origin band, [a] AE splitting, DB g , and D g constants from Humphrey et al [16] [b] Vibronic transitions from Henseler et al [15] [c] DF data from Henseler et al [15] [a] AE splitting, DB g , and D g constants from Plusquellic et al [18] [b] Vibronic transitions from Droz et al [17] [c] DF data from Droz et al [17] [a] Vibronic bands of phenol(NH 3 ) 1 from Schiefke et al [19] [b] Vibronic bands of phenol(ND 3 ) 1 from Jacoby. [20] the small intensity of the E bands in the hole-burning spectrum could easily be explained.…”

“…The phenol-ammonia cluster shows in some respect a quite different behavior compared to that of other hydroxyaromatics-ammonia clusters. The lifetimes of trans-1-naphthol(NH 3 ) 1 and trans-2-naphthol(NH 3 ) 1 have been determined to be 40 ns and 31 ns, respectively, [16,18] while the lifetime of phenol(NH 3 ) 1 is only about 1 ns. [4] This short lifetime has been attributed to the excited-state photoreaction, Equation (19),…”

“…Plusquellic et al determined the torsion-rotation perturbation terms and the AE splitting from the rotationally resolved spectrum of the electronic origin of trans-1-naphthol(NH 3 ) 1 . [18] As in the case of ). Barriers of V 3 '' = 33.9 cm À1 and V 3 ' = 57.9 cm À1 were obtained ( Table 2).…”

“…The integrals which are used to calculate the perturbation coefficients in Equation (15) and Equation (17) can easily be determined from the coefficients of the torsional Eigenfunctions given in Equation (18).…”