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 models toward diazirine (c-CH2N2) war made using the LCGTO-MCP-LSD method. Adsorption with the nitrogen pair directly over surface atoms (the mu-top site) is clearly more stable than when the NN pair is perpendicular to the rows of the (110) surface (the mu-bridge site). The NN bond is strongly affected by adsorption, a significant decrease of its bond order is observed for both palladium and copper. One main difference between palladium and copper with regards to the adsorption of c-CH2N2 is the magnitude of the M-N bond order; palladium tends to form a stronger chemisorption bond than copper. A second difference is that partial occupation of the LUMO of diazirine only occurs for the copper cluster model systems. The concerted dissociation of CN bonds is energetically demanding but appears to be easier on Pd than on Cu by around 28 kcal mol(-1). The study of electronically perturbed diazirine (excited, ionized or isomerized) provides insight on how chemisorption induces 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 result is obtained when the first excited states of the chemisorbed complexes are studied. A 0.4 eV electron excitation in the c-CH2N2/Cu-4 complex (mu-top) leads to a significant decrease of the bond order of the NN bond but does not induce even a small change for the c-CH2N2/Pd-4 complex. The calculations provide some insights on the markedly different bond scission selectivity observed in experimental studies of the thermal decomposition of diazirine on Pd and Cu surfaces. Experiments show that NN bond scission occurs with essentially 100% selectivity on copper, whereas NN bond retention as well as NN bond scission occurs on Pd(110).