The cyclopropylmethyl-3-butenyl rearrangement on Mo(110): A radical clock on a surface?

The mechanism for the transformation of adsorbed cyclopropylmethyloxide into its ring-opened form, 3-butenyloxide, on the Mo(110) surface is explored theoretically. An alternative emerges to the radical clock mechanism that involves the cleavage of the C-O bond in the adsorbate as the critical reaction step. The alternative pathway involves the cleavage of a C-C bond in the three-membered ring leading to a diradical, which could transform via a 1,2-H shift to the same reaction product. Density functional theory (DFT) calculations for relevant reaction intermediates in molecular model systems show an energetic preference for the C-C cleavage in the initial step. The barrier of the subsequent 1,2-H shift of the singlet diradical is slightly lower than the barrier for the radical clock rearrangement, rendering the diradical pathway a possible alternative. Adsorbate structures for the reactant and the product were obtained by DFT slab calculations. We carry out a MO analysis of the bonding in the adsorbate, comparing also C-C and C-O bonding.