Comprehensive Study of Methylation on the Silicon(100)-2 × 1 Surface: A Density Functional Approach.

A detailedmechanistic investigation of Si–Me formationover the silicon (100)-2 × 1 surface using the Si9H12cluster model has been performed using various reagents,based on two basic mechanisms: dissociation and substitution. Thereagents CH4, CH3Cl for dissociation and CH3Li, CH3MgBr for substitution mechanism are usedto explore the methylation process on the silicon surface at the M062X/6-311+G(2d,p) level of theory. The associated potential energy surfaces exploredhere are aimed to unveil the most favored pathway of methylation withappropriate reagents. Dissociation of methane forms a monomethylatedproduct (D1) through an energetically unfavorable pathway. All theadsorption modes of CH3Cl over the silicon surface arealso detected and analyzed. Methyl chloride dissociates to form anothermonomethylated product D2 and its derivative D3 in the entrance channel,while, in the next step, bridged compounds I1 (Cl-bridged) and I2(H-bridged) are produced from them, respectively. The C–Cldissociation leads to the formation of D2 having a lower activationbarrier. With a comparably high activation barrier in the C–Hdissociation, producing D3, very interestingly carbene intermediatehas been detected in the reaction pathway. Detection of energeticallyunfavored conversions from D2 to I1 and D3 to I2 ensured that themethylation process will not be hampered through these interconversions.For substitution, HCl- and Cl2-passivated Si surfaces aretaken, where chlorine is to be substituted by the methyl group ofboth of the methylating agents. With both substituents, HCl-passivatedSi9H12gives D1. The substitution process onCl2-passivated Si9H12leads to theformation of D2 in the first step and dimethylated product (S1) inthe final step. In all the above substitution processes, methyl lithiumproved to be the better substituent for the formations of D1, D2,and S1 on HCl- or Cl2-passivated surfaces. The presentwork not only demonstrated methyl lithium as one of the best methylatingagents but also revealed the interrelation among the dissociativeadsorption modes of CH3Cl, reported earlier, in a singlepotential energy surface with a remarkable detection of carbene intermediateformed in the pathway of C–H dissociation. [ABSTRACT FROM AUTHOR]