Matrix-isolated-diazoketones (DZ) RCOCN2R (R = CH3 or CD3) were photolyzed and their reactions were monitored by FT-IR spectroscopy. The ketene (KE) products from the Wolff rearrangement were always more abundant than the α,β-unsaturated ketones (ON); this selectivity increased when a broad band source (λ > 230 nm) was used instead of a monochromatic laser source (457.9 nm). Oxirenes (OX) were detected as minor, but well-identified reaction products, stable at temperatures less than 25 K, even under the monochromatic irradiation; they isomerized to KE when irradiated at λ > 230 nm. With matrices doped with carbon monoxide, the reaction diverted toward ketoketene, which certainly resulted from ketocarbene (KC) trapped by CO molecules. The kinetic data showed that the rate constants of DZ → KC, KC → KE, KC → ON, and KC → OX processes have the same order of magnitude. After complete DZ photolysis, the KE concentration still increased under extended irradiation; therefore, KC(T0) is suspected to be another reaction product that slowly photoisomerized to KE. Some IR absorption bands might correspond to that intermediate, but unambiguous assignments could not be made. On the other hand, our previous oxirene identification has been supported by ab initio calculations at the SCF level, which justified the high-frequency value of oxirene v(C=C). To account for the results of matrix isolation experiments and for those previously recorded during the gas-phase photolysis of several DZ [R = CH3, (CH3)3C; R-R = -(CH2)4-], the relative stabilities of different isomers (KC, OX, and KE) were computed, and the assumed reaction paths from KC to reaction products were studied, in each series, through the MNDOC semiempirical method. Substituent effects and ring strain deeply influence the oxirene stability. Considering that the photolysis and photoisomerization processes occur on the singlet potential energy surface, we finally established a unified energy diagram, approximately scaled, that gathers together the different species in different electronic states and allows the interpretation of the main reaction features. © 1990, American Chemical Society. All rights reserved.
