Computational study of cycloaddition reactions

Cycloaddition reactions between 1,3-butadiene and the C-terminated SiC(1 0 0)-c(2 · 2) surface have been addressed using quantum-chemical methods. The c(2 · 2) structure consists of ACBCA bridges between underlayer Si atoms which themselves form SiASi bonds. Of various possible reaction products, the one formed by a [2+ 4] reaction with the ACBCA bridge (giving a species resembling 1,4-cyclohexadiene) is the lowest in energy. Density functional calculations for the bare c(2· 2) surface, using a cluster model with mechanical embedding, gave good agreement with structural parameters obtained in previous fully ab initio studies. Similar calculations for the cycloaddition product and for the transition state gave a reaction energy of )50.3 kcal/mol and an activation energy of +6.1 kcal/mol to form a planar ring structure lying normal to the surface. Detailed results for the frequency and infrared polarization behavior of adsorbate vibrational modes have also been obtained. 2003 Elsevier B.V. All rights reserved.