The Chemistry of Carbenes: New Insights from the Gas Phase

This review describes insights obtained from recent studies of unimolecular and bimolecular reactions of small carbenes in the gas phase and cryogenic environments. Following a description of what determines the singlet-triplet splitting in carbenes, we discuss the challenges involved in producing carbenes in concentrations sufficient for studying their reactions. We document the methods developed for their preparation and the array of spectroscopic techniques available for their characterization. The review emphasizes recent progress in studies of hydroxycarbenes and small alkyl carbenes that easily isomerize to more stable isomers. The studies of unimolecular reactions of hydroxycarbenes show how quantum mechanical tunneling determines their lifetimes. A new carbonyl-ene mechanism has been demonstrated in the biomolecular reactions of hydroxymethylene and methylhydroxycarbenes. We evaluate the impact of these new results on chemical processes relevant to atmospheric, planetary, and interstellar environments and highlight the importance of collaboration between theory and experiment in interpreting mechanisms.