BOND SELECTIVITY IN THE DISSOCIATIVE ADSORPTION OF C-CH2N2 ON SINGLE-CRYSTALS - A COMPARATIVE DFT-LSD INVESTIGATION FOR PD(110) AND CU(110)

A comparison between the reactivity of palladium and copper cluster mo dels toward diazirine (c-CH2N2) war made using the LCGTO-MCP-LSD metho d. Adsorption with the nitrogen pair directly over surface atoms (the mu-top site) is clearly more stable than when the NN pair is perpendic ular to the rows of the (110) surface (the mu-bridge site). The NN bon d is strongly affected by adsorption, a significant decrease of its bo nd order is observed for both palladium and copper. One main differenc e between palladium and copper with regards to the adsorption of c-CH2 N2 is the magnitude of the M-N bond order; palladium tends to form a s tronger chemisorption bond than copper. A second difference is that pa rtial occupation of the LUMO of diazirine only occurs for the copper c luster model systems. The concerted dissociation of CN bonds is energe tically demanding but appears to be easier on Pd than on Cu by around 28 kcal mol(-1). The study of electronically perturbed diazirine (exci ted, ionized or isomerized) provides insight on how chemisorption indu ces variations in bond lengths and vibrational frequencies as a result of charge transfer. The results of the calculations show that the mu- top adsorbed state is more similar to the n_-->pi first excited state of the free molecule than to the ionized state. A more striking resul t is obtained when the first excited states of the chemisorbed complex es are studied. A 0.4 eV electron excitation in the c-CH2N2/Cu-4 compl ex (mu-top) leads to a significant decrease of the bond order of the N N bond but does not induce even a small change for the c-CH2N2/Pd-4 co mplex. The calculations provide some insights on the markedly differen t bond scission selectivity observed in experimental studies of the th ermal decomposition of diazirine on Pd and Cu surfaces. Experiments sh ow that NN bond scission occurs with essentially 100% selectivity on c opper, whereas NN bond retention as well as NN bond scission occurs on Pd(110).