π bond versus radical character of the diamond (1 0 0)-2×1 surface

The dimers on clean diamond (1 0 0)-2×1 are linked by a σ bond and a highly strained π bond. The weakness of the surface π bond causes the reactivity of this surface to be intermediate between that of an alkene and a bi-radical. We illustrate this behavior by investigating two prototypical cycloaddition surface reactions using multiple-internal-reflection infrared spectroscopy. Adsorption of 1,3-butadiene occurs via [4+2] addition to surface dimers, analogous to Diels–Alder chemistry of alkenes. However, [2+2] cycloaddition of cyclopentene, a reaction that is symmetry-forbidden with alkenes, occurred with a sticking coefficient of ∼10−3. These reactions illustrate novel approaches to the functionalization of diamond surfaces.