Photoinduced dissociation of methylnitrite on Ag( [formula omitted

Abstract

Electronic excitations and photoinduced electron attachment states of methylnitrite (CH 3ONO) adsorbed on Ag(1 1 1) are investigated by ab initio methods. The calculated ground state energies of the cis and trans isomers of the methylnitrite molecule are nearly degenerate, with the trans structure lower than the cis by 0.03 eV at the CI level. Experimentally, the cis isomer is estimated to be more stable by 0.03 eV. The energy of methylnitrite in its ground electronic state on the silver surface is nearly the same for both isomer configurations and changes only slightly with isomer orientation and adsorption site. In both cis and trans forms, structures in which the internal oxygen points towards the surface and the O(internal)N internuclear axis lies between 30° and 45° from the surface are favored. The calculated adsorption energy is 0.60 eV for the cis isomer. Ground and excited state potential energy curves for dissociation to CH 3O/Ag and NO (gas phase) are calculated at the CI level for both internal electronic excitations and for electron attachment excited states. The calculated ground state dissociation energy is 1.5 eV, a value 0.20 eV less than calculated for the gas phase dissociation. The metal-mediated excitation (electron attachment to the adsorbate and hole in the metal) leads to a repulsive triplet state with a vertical excitation energy of 4.7 eV and negligible singlet–triplet splitting. A second excited state produced by electron excitation within methylnitrite produces distinct singlet and triplet states. The vertical excitation occurs at 2.4 eV for the triplet state and at 3.4 eV for the singlet state. In comparing the methylnitrite/Ag(1 1 1) potential energy surface to that of gas phase methylnitrite, a shallow minimum in the first excited singlet state for the molecule appears only as a plateau in the adsorbate/metal system when the excitation is from within methylnitrite. There is a 10-fold increase in methylnitrite/silver mixing in the metal-mediated excitation as compared to the internal excitation, which may explain the surprising experimental result that dissociation does not follow metal-mediated excitation.