Rearrangement as a probe for radical formation: bromomethylcyclopropane on oxygen-covered Mo(110)

Abstract The reactions of bromomethylcyclopropane on oxygen-covered Mo(1 1 0) were studied in order to investigate the lifetimes of radical intermediates, which are important in heterogeneous oxidation catalysis. The methylcyclopropyl radical is known to rearrange on the nanosecond time scale, providing us with a means of probing for radical formation. Surprisingly, no rearrangement occurs subsequent to C–Br bond dissociation, which commences at ∼220 K. Instead, displacement of bromine by oxygen occurs to yield adsorbed methylcyclopropoxide, which is identified using infrared spectroscopy. The C–O bond of methylcyclopropoxide is cleaved at ∼400 K to yield a transient methylcyclopropyl radical. As shown previously, the methylcyclopropyl radical rearranges and the ring-opened butenyl species is trapped on the surface. Addition to oxygen yields 3-buten-1-oxy and addition to the metal affords the butenyl–Mo moiety. Infrared spectroscopy is used to identify these intermediates. The same linear species are formed from the reaction of 4-bromo-1-butene. The 3-buten-1-oxy species is also formed from reactions of 3-buten-1-ol on O-covered Mo(1 1 0). Upon further heating, the 3-buten-1-oxy reacts to form 1,3-butadiene, 1-butene, water, and dihydrogen between 450 and 600 K. Ethene is also evolved at ∼560 K. The primary mechanism for ethene evolution is elimination from metal-bound butenyl. Carbon monoxide is also formed above 900 K, due to reaction of surface carbon and oxygen. The implications of our results for studies where alkyl halides are used as models for radical reactions on surfaces are discussed.