Photophysics of intramolecularly hydrogen-bonded aromatic systems: Ab initio exploration of the excited-state deactivation mechanisms of salicylic acid

Andrzej L. Sobolewski, Wolfgang Domcke

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Abstract

Excited state reaction paths and the corresponding energy profiles of salicylic acid have been determined with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory. At crucial points of the potential energy hypersurfaces, single-point energy calculations have been performed with the CASPT2 method (second-order perturbation theory based on the complete active space self-consistent field reference). Hydrogen transfer along the intramolecular hydrogen bond as well as torsion and pyramidization of the carboxy group have been identified as the most relevant photochemical reaction coordinates. The keto-type planar S1 state reached by barrierless intramolecular hydrogen transfer represents a local minimum of the S1 energy surface, which is separated by a very low barrier from a reaction path leading to a low-lying S1-S0 conical intersection via torsion and pyramidization of the carboxy group. The S1-S0 conical intersection, which occurs for perpendicular geometry of the carboxy group, is a pure biradical. From the conical intersection, a barrierless reaction path steers the system back to the two known minima of the S 0 potential energy surface (rotamer I, rotamer II). A novel structure, 7-oxa-bicyclo[4.2.0]octa-1(6),2,4-triene-8,8-diol, has been identified as a possible transient intermediate in the photophysics of salicylic acid.

Original languageEnglish
Pages (from-to)3410-3417
Number of pages8
JournalPhysical Chemistry Chemical Physics
Volume8
Issue number29
DOIs
StatePublished - 2006

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