Excited state proton transfer in the S1 state of 2-allylphenol, 2-propenylphenol, and 2-propylphenol and their van der Waals clusters with water and ammonia

Abstract

Excited state intramolecular and intermolecular proton transfer reactions in cold, isolated 2-allylphenol, 2-propenylphenol and 2-propylphenol, and their clusters with water and ammonia are investigated employing a combination of spectroscopic techniques (mass resolved excitation, threshold photoionization, dispersed emission), a semiempirical calculation (MOPAC 5) and a potential energy calculation of cluster structure. Threshold photoionization spectroscopy proves to be useful for the identification of molecular conformers in these systems but has mixed results for the identification of proton transfer in their clusters. The total collection of generated data suggests the following conclusions: (1) isolated, cold 2-allylphenol displays only one conformation which appears to have a significant stabilizing intramolecular interaction between the allyl group double bond and the hydroxyl group hydrogen atom; (2) 2-propenylphenol displays only one conformer; (3) 2-propylphenol has many conformations—probably more than five under the experimental conditions; (4) no evidence of intramolecular proton transfer can be found for these three isolated cold molecules; (5) no evidence for intermolecular proton transfer in water clusters has been found by any of the above techniques; and (6) evidence is found for intermolecular proton transfer in 2-allyl- and 2-propenylphenol(NH3)n, n≥3, in dispersed emission spectra. Dispersed emission spectra of 2-propylphenol(NH3)n n≥3 are too weak to yield conclusive evidence for intermolecular excited state proton transfer. Potential energy minimization calculations of cluster geometry suggest that the difference between water and ammonia cluster behavior with regard to proton transfer arises because water molecules hydrogen bond with the hydroxyl group (both OH⋅⋅⋅OH2 and HO⋅⋅⋅HOH) and each other while ammonia molecules are more evenly distributed over the entire molecular structure of the phenol moiety. Apparently, for efficient proton transfer to occur in clusters, the proton affinity of the solvent must be large and both the anion and the proton must be well solvated (stabilized) by the solvent.