Intermediates Involved in the Oxidation of Nitrogen Monoxide: Photochemistry of the cis-N2O2•O2 complex and of sym-N2O4 in Solid Ne Matrices

Pure sym-N2O4 isolated in solid Ne was obtained by passing cold neon gas over solid N2O4 at -115 degrees C and quenching the resulting gaseous mixture at 6.3 K. Filtered UV irradiation (260-400 nm) converts sym-N2O4 into trans-ONONO2 a weakly interacting (NO2)(2) radical pair, and traces of the cis-N2O2 center dot O-2 complex. Besides the weakly bound ON-O-2, complex, cis-N2O2 center dot O-2 was also obtained by co-deposition of NO and O-2 in solid Ne at 6.3 K, and both complexes were characterised by their matrix IR spectra. Concomitantly formed cis-N2O2 dissociated on exposure to filtered IR irradiation (400-8000 cm(-1)), and the cis-N2O2 center dot O-2 complex rearranged to sym-N2O4 and trans-ONONO2. Experiments using O-18(2), in place of O-16(2), revealed a non-concerted conversion of cis-N2O2 center dot O-2 into these species, and gave access to four selectively di-O-18-substituted trans-ONONO2 isotopomers. No isotopic scrambling occurred. The IR spectra of sym-N2O4 and of trans-ONONO2 in solid Ne were recorded. IR fundamentals of trans-ONONO2 were assigned based on experimental O-16/18 isotopic shifts and guided by DFT calculations. Previously reported contradictory measurements on cis- and trans-ONONO2 are discussed. Dinitroso peroxide, ONOONO, a proposed intermediate in the IR photoinduced rearrangement of cis-N2O2 center dot O-2 to the various N2O4 species, was not detected. Its absence in the photolysis products indicates a low barrier (<= 10 kJ mol(-1)) for its exothermic O-O bond homolysis into a (NO2)(2) radical pair.