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Cycloaddition reactions between 1,3-butadiene and the C-terminated SiC(1 0 0)-c(2x2) surface have been addressed using quantum-chemical methods. The c(2x2) structure consists of -C(tripple bar)C- bridges between underlayer Si atoms which themselves form Si-Si bonds. Of various possible reaction products, the one formed by a [2+ 4] reaction with the -C(tripple bar)C- bridge (giving a species resembling 1,4-cyclohexadiene) is the lowest in energy. Density functional calculations for the bare c(2x2) 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.