4,6-Dimethyl-α-pyrone: a matrix isolation study of the photochemical generation of conjugated ketene, Dewar valence isomer and 1,3-dimethyl-cyclobutadiene

A combined matrix isolation and molecular orbital study of the vibrational spectra and photochemistry of 4,6-dimethyl-α-pyrone (DMAP) was undertaken. Two types of photoreactions: ring opening leading to conjugated ketene and valence isomerization to the Dewar form (1,5-dimethyl-2-oxa-3-oxobicyclo[2.2.0]hex-5-ene; DOOBH), occurred upon UV (λ>315 nm) irradiation. The latter reaction was efficient, whereas aldehyde–ketene was produced only in little amounts. In addition to the IR spectroscopic study of DMAP, the full mid-IR spectrum of the photoproduced DOOBH is reported and interpreted. Observation of 1,3-dimethyl-cyclobutadiene (DMCB), created by shorter wavelength UV irradiation (λ>235 nm) of DOOBH, is reported for the first time. In the matrices, DMCB forms a complex with CO2; the structure and IR absorption features of this cage confined DMCB–CO2 complex are also investigated.