We predict the structures and energies for the dissociative adsorption of pyrrole producing C4H4N radicals and H atoms on the Si(100)-2 x 1 surface by density functional cluster model calculations. The dissociative adsorption process was found to occur primarily by barrierless adsorption at an alpha -C atom followed by N-H dissociation and the isomerization of the radical, leading to the formation of aligned C4H4N species on the surface. Additionally, molecular adsorption reactions by [2+2] and [4+2] cycloaddition mechanisms are discussed. These cyclic adducts were predicted to be short-lived at room temperature and should not affect the formation of the aligned C4H4N radical layer.