Ab initio potential-energy functions for excited state intramolecular proton transfer: A comparative study of o-hydroxybenzaldehyde, salicylic acid and 7-hydroxy-1-indanone

Potential-energy profiles along the minimum-energy reaction path for intramolecular proton transfer in the ¹ππ* excited state have been calculated for the title compounds. The CASSCF and CIS electronic-structure methods have been employed for excited-state geometry optimization. Single- point energy calculations along the reaction path have been performed using the CASPT2 and TDDFT methods. The TDDFT method has been tested against accurate CASSCF and CASPT2 data for malonaldehyde. CASPT2 yields transition energies for photon absorption and emission which are in excellent agreement with experimental data (within 0.2 eV). The CASPT2 potential energy functions exhibit, however, artifactual kinks (on a scale of a single kcal mol^-1) which reflect inherent limitations of the CASSCF-based perturbation approach. TDDFT yields potential-energy functions which are essentially parallel to the CASPT2 functions and free of artifacts. Transition energies for absorption and emission are systematically overestimated, however, by about 0.5 eV in TDDFT. For all three title compounds, a barrierless ¹ππ* potential- energy function is predicted. The location of the ¹ππ* minimum varies from near-enol in salicylic acid to near-keto in 7-hydroxy-1-indanone.