CarbonAb initio quantum mechanical calculations using triple-5-plus double polarization quality basis sets in conjunction with self-consistent-field, two-configuration SCF, and configuration interaction methods show that the three halogenocarbenes, CHF, CHCI, and CHBr, all have singlet ground states with singlet-triplet separations predicted to be 13.2, 5.4, and 4.1 kcal mol-', respectively.' Two methods are available for the generation of carbenes: the decomposition of the appropriate diazo or diazirine derivatives. The diazirine method has been employed-for the generation of phenylchlorocarbene,' methoxycarbene,3 and phen~xycarbene~ in argon or nitrogen matrices at 10 K. Warming phenylchlorocarbene in an argon matrix containing oxygen to 35 K resulted in the formation of the corresponding yellow-green carbonyl oxide, photolysis of which with visible light gave the corresponding dioxirane, benzoyl chloride, and ozone (Scheme 1 ) . Irradiation of the dioxirane gave mainly phenyl chloroformate together with a small +o/O N=N hv. 320-380 nm, 24 K