Photochemistry of Cycloalkene*NO2 Collisional Pairs in a Cryogenic Matrix: Chemical Trapping of Cycloalkene Oxirane Biradical Conformers, and Comparison of Product Control for Excitation above and below the NO2 Dissociation Threshold

Article abstract of DOI:10.1021/j100160a007

Oxygen atom transfer from NO2 to cyclohexene and from NO2 to cyclopentene was induced by excitation of reactant pairs isolated in solid Ar by light in the wavelenght range between 610 and 355 nm.Continuous wave dye, Ar ion, and Nd:YAG lasers were used as photolysis sources, and the chemistry was monitored by Fourier transform infrared spectroscopy.The O atom transfer path accessible at wavelenghts longer than the 398-nm NO2 dissociation limit led to cycloalkene oxide as the only final oxidation product.The reaction threshold was at 610 nm.In the case of cyclohexene + NO2, two cyclohexyl nitrite radical diastereomers were produced concurrently with the epoxide.Infrared analysis based on 18O and 15N isotopic substitution, visible light induced wavelength-selective photodissociation, and matrix annealing experiments indicate that the two stereoisomers are cyclohexyl nitrite radical chair conformers with an equatorial and an axial C-O bond, respectively.Since these are transient cyclohexene oxirane biradicals chemically trapped by reaction with NO cage coproduct according to the previous established reaction mechanism, it is concluded that cyclohexene oxidation proceeds along two diastereomeric paths.In the case of the cyclopentene + NO2 reaction only a single cyclopentyl nitrite radical stereoisomer is observed, presumably because of the very low barrier to pseudorotation of the pentyl ring.Loss of product specificity upon photolysis of cyclohexene/NO2/Ar and cyclopentene/NO2/Ar matrices above the NO2 dissociation threshold with 355-nm light is evident from the appearance of two additional products aside from cycloalkene oxide, namely, cyclohexanone (cyclopentanone) and 1-cyclohexen-1-ol (2-cyclopenten-1-ol).Absorbance growth behaviour of these products indicated that the ketone and epoxide originate from the same reservoir of sustained cycloalkene*NO2 collisional pairs that reacts at visible wavelengths by large-amplitude O atom transfer, while cycloalkenol stems from dissociation of NO2, followed by reaction of free O(³P) with cycloalkene.It is proposed that large-amplitude oxygen atom transfer remains the dominant pathway for collisional cycloalkene*NO2 pairs excited above the NO2 dissociation threshold.Reaction by NO2 predissociation is operative for reactant pairs with orientation or distances (including separation by Ar host atoms) that make the large-amplitude reaction inaccessible.