The halocarbenes: model systems for understanding the spectroscopy, dynamics and chemistry of carbenes

We review recent studies of the spectroscopy and dynamics of halocarbenes :CXY (X = H, F, Cl, Br, I; Y = F, Cl, Br, I), which are set forth as model systems for understanding the spectroscopy, photochemistry and photophysics of carbenes, and benchmarks for comparing experiment and theory concerning carbene singlet-triplet gaps and spin-orbit coupling from the limit of mixing of isolated rovibronic levels to energy perturbations involving entire vibronic levels. Following a historical overview of halocarbene chemistry, which spans more than 50 years, we discuss experimental methods for generating and spectroscopically detecting these elusive species. Subsequent sections focus on: (a) the spectroscopy of the halocarbenes, where a variety of non-adiabatic interactions are operative, (b) the singlet-triplet gap in the halocarbenes, where experimental methods for measuring this quantity are outlined and their results are compared with benchmark theoretical calculations and (c) the photochemistry and photodissociation dynamics of the halocarbenes, which emphasises recent observations of the hitherto unobserved quasilinear B state.