A. Rochefort et al., BOND SELECTIVITY IN THE DISSOCIATIVE ADSORPTION OF C-CH2N2 ON SINGLE-CRYSTALS - A COMPARATIVE DFT-LSD INVESTIGATION FOR PD(110) AND CU(110), Surface science, 347(1-2), 1996, pp. 11-24
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).