Effects Of Intermolecular Interaction On Proton Tunneling: Theoretical Study On Two-Dimensional Potential Energy Surfaces For 9-Hydroxyphenalenone-Co2/H2o Complexes

The effects of binding of CO2 or H2O with 9-hydroxyphenalenone (9HPO) on proton tunneling in the S-0 state have been theoretically investigated. High-level ab initio calculations predict that CO2 is van der Waals-bonded to the C=O.OH moiety of 9HPO in the most stable structure. This planar structure is more stable than the nonplanar structure where CO2 is bonded above the aromatic rings of 9HPO. In the 9HPO-H2O complex, H2O is hydrogen-bonded to the carbonyl group in the most stable structure. Two-dimensional potential energy surfaces (PESs) for 9HPO-CO2 and 9HPO-H2O have been calculated with the reaction surface method, and the contour plots of PESs for the complexes are compared with those for the 9HPO monomer. The binding of CO2 with 9HPO induces slight asymmetry in the double-minimum potential well, whereas the asymmetry of the PES is very large for the binding of H2O. The transition state energy for 9HPO-CO2 drastically decreases to be about a half that of 9HPO, while that for 9HPO-H2O is only slightly smaller than the transition energy for 9HPO. The vibrational wave function for in 9HPO-CO2 is substantially delocalized over two potential minima, but that for 9HPO-H2O is completely localized around a single potential minimum. The calculated tunneling splitting of the zero-point level in 9HPO-CO2 is only 10% smaller than the corresponding splitting of 9HPO, whereas proton tunneling is quenched in 9HPO-H2O. The calculated results are consistent with the prediction from the electronic spectra measured in a supersonic free jet. (C) 2003 American Institute of Physics.