THEORETICAL-STUDIES OF CHEMISORPTION AND DIMER MODEL SYSTEMS - MOLLER-PLESSET AND CONFIGURATION-INTERACTION CALCULATIONS ON PDH, PDC, PDO, PDF, PD2, AND PDCO

Ab initio SCF studies have been performed to study the molecular properties of several single-bonded palladium compounds, PdH, PdC, PdO, PdF, Pd2, and PdCO, which are important in surface and materials science. Electron correlation effects were evaluated by a second- and third-order Moller-Plesset (MP) perturbation theory and a size-consistency-corrected configuration interaction with single and double substitutions (CISC). Relativistic effects were investigated for PdH and PdF. The ground state of PdC has been calculated at the CISC level to be a 3-PI state which is only 0.26 eV below the 3-SIGMA-state (previously assigned ground state) and 0.51 eV below the 1-SIGMA+ state. PdC is predicted to be stable in the gas phase, and the possibility of preparing this compound is investigated. The bonding in CO chemisorbed on palladium is studied by using the model Pd-CO system. The effect of d-pi-pi* back-bonding, discussed at the Hartree-Fock and CI level, is compared with results from multiple-scattering X-alpha calculations. The C-O stretching frequency shift for CO on palladium was analyzed at various levels of theory, and the results indicated that the decrease in the CO force constant associated with chemisorption is not solely the result of d-pi-pi* back-bonding. At the CISC level, the calculated chemisorption shift of DELTA-omega = -72 cm-1 in the C-O stretching frequency compared well with the infrared experimental value (-95 cm-1) for PdCO monomer in a Kr matrix and with terminal CO on the surface of palladium hydrosols (-78 cm-1), while the calculated MP2 value of DELTA-omega = -122 cm-1 overestimated the experimental result. The calculations indicated that the CO bond distance decreases only slightly (approximately 0.007 angstrom) as a result of chemisorption on single palladium atoms.