Effect of Methyl Substitution on the Interaction of Benzene with Ni(100): An Experimental and Theoretical Study

The adsorption of benzene, toluene, o-, m-, and p-xylene, and mesitylene on the Ni(100) crystal face was studied in order to elucidate the role of methyl substitution on the interaction of the aromatic ring with the surface. Temperature-programmed reaction (TPR) and reflection absorption infrared spectroscopy (RAIS) were used to characterize adsorption bond strengths and modes of bonding to the surface. All molecules appear to initially adsorb with the ring parallel to the surface. Methyl substituents were found to decrease the binding energy of the ring to the surface by about 65 kJ/mol independent of the number of substituents. Whereas benzene and mesitylene desorption occurred in single peaks, toluene and xylenes exhibited several additional lesser peaks or shoulders, suggesting the possibilty of more than one type of bonding configuration. The placement of the methyl groups on the ring was found to influence reactivity with the Ni(100) surface: m- and p-xylene decomposed to give only H₂and adsorbed carbon, whereas o-xylene evolved various hydrocarbon fragments as well. Studies with partially deuteriated toluene showed that the methyl group hydrogens are lost before most of the ring hydrogens. The chemisorption of benzene and the methyl-substituted benzenes was also modeled by using a semiempirical molecular orbital method, extended Hückel theory (EHT). EH calculations show that the bonding configuration of the molecules is with the ring nearly parallel to the surface. Methyl substituents were found to weaken the chemisorption bond by destabilizing the π-bond to the surface; the repulsive interaction of the methyl group with the nickel atoms was much less significant. Although EHT was in qualitative agreement with the experiments, it overestimated the effect of the methyl groups on binding energy for xylenes and mesitylene. EHT was incorrect in predicting the direction of charge transfer, a consequence of the underestimation of the benzene π-π* energy gap by EHT.