Energy Localization and Infrared-Induced Proton Transfer Reactions in Solution

Damien Laage (a), Hagop Demirdjian (a) and James T. Hynes (a,b)

(a)CNRS UMR 8640 PASTEUR Departement de Chimie Ecole Normale Superieure 24, rue Lhomond, 75231 Paris, FR
(b) Dept. of Chemistry and Biochemistry University of Colorado Boulder, CO 80309-0215 USA


We will discuss the present status of our theoretical and computational work addressed to the question of whether a quantum of vibrational energy can be retained in a hydrogen stretching mode sufficiently long to induce, by infrared excitation, an acid-base proton transfer reaction in solution, schematically AH + B A- + HB+, where AH is the acid and B is the base. Ordinarily, one would view the solvent exclusively as a source of thwarting such a selective process, i.e. causing vibrational de-excitation of the excited H stretch before the excess energy could induce the reaction to occur; vibrational de-excitation can be quite rapid in hydrogen-bonded liquids of main interest as solvents for proton transfer reactions [1]. A non-traditional view of proton transfer reactions indicates, however, that the solvent can actually assist the IR-induced reaction [2]. After reviewing the concepts involved and initial calculations supporting them, we will discuss recent results [3,4] addressed to the efficacy of competing processes which could 'short-circuit' the desired reaction, focussing on the model system of hydrofluoric acid (HF) in water [5].

References

[1] R. Rey, K. B. Moller and J.T. Hynes, Ultrafast Vibrational Population Dynamics of Water and Related Systems : A Theoretical Perspective', Chem. Rev., 104, 1915-1928 (2004).
[2] H. J. Kim, A. Staib, and J. T. Hynes, `Ultrafast Dynamics at the Transition State inSolution', Ultrafast Reaction Dynamics at Atomic-Scale Resolution Femtochemistry and Femtobiology, Nobel Symposium 101, Villy Sundstrom, Ed., (Imperial College Press, London, 1998) pp. 510-527.
[3] D. Laage and J.T. Hynes, unpublished.
[4] H. Demirdjian, These, ENS-Paris VI (2004).
[5] K. Ando and J.T. Hynes, "Acid Ionization of HF in Water, an Electronic Structure and Monte Carlo Study", J. Phys. Chem. A, 103, 10398-10408 (1999).