Parent- and daughter-mediated halogenation reactions modeled for 1,2- and 1,4-Dibromobenzene (cas 106-37-6) at Si(1 1 1)-7 × 7

Add time:08/19/2019    Source:sciencedirect.com

We report a first attempt, using AM1, to model the thermal reaction of 1,2- and 1,4-Dibromobenzene (cas 106-37-6) with a Si(1 1 1)-7 × 7 surface, a reaction recently studied experimentally by scanning tunneling microscopy (STM) [S. Dobrin et al., Surf. Sci. 561 (2004) 11]. As postulated on the basis of the experimental findings, two generalizable types of reaction pathways are found to be open. The indirect, ‘Parent-mediated’, dynamics involves prior attachment of the parent molecule by (in the present case) its benzene ring to the surface, followed by detachment of one or two bromine atoms from the ring, R, which most often remains attached to the surface. This results in formation of a single Br–Si or a pair or Br–Si with, in each case, an adjacent feature, R–Si. The alternate ‘Daughter-mediated’ pathway involves the direct approach of the reactive daughter Br-atoms to the Si surface to form the same products without, generally, leaving an organic residue, R–Si, at the surface. The calculated activation energies for successive bromination of the surface by either the Parent-mediated (P) or Daughter-mediated (D) pathways were found to lie in the range 0.7–1.27 eV, consistent with the observation of thermal reaction of the title molecules to give both single Br–Si and pairs of Br–Si at room temperature by either mechanism. Activation energies for bromination by P or D are less than the bond-dissociation energies for the C–Br bond being broken, since Br–Si bond-formation occurs concurrently with bond breaking. The old C–Br bond extends to form the new Br–Si, so that Br–Si imprints at the surface have a wider mean separation than do the Br substituents in the parent, as well as a wider mean separation for Br–Si when they come from 1,4- than from 1,2 dibromobenzene. This agrees with observation. In Daughter-mediated reaction the adsorbate is postulated to approach laterally, ‘stamping’ a characteristic narrow Br⋯Br separation or ‘rocking’ to imprint the wider separations observed.

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