Intermediates involved in the oxidation of nitrogen monoxide: photochemistry of the cis-N(2)O(2)O(2) complex and of sym-N(2)O(4) in solid Ne matrices.

Pure sym-N(2)O(4) isolated in solid Ne was obtained by passing cold neon gas over solid N(2)O(4) at -115 degrees C and quenching the resulting gaseous mixture at 6.3 K. Filtered UV irradiation (260-400 nm) converts sym-N(2)O(4) into trans-ONONO(2), a weakly interacting (NO(2))(2) radical pair, and traces of the cis-N(2)O(2)O(2) complex. Besides the weakly bound ONO(2) complex, cis-N(2)O(2)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-N(2)O(2) dissociated on exposure to filtered IR irradiation (400-8000 cm(-1)), and the cis-N(2)O(2)O(2) complex rearranged to sym-N(2)O(4) and trans-ONONO(2). Experiments using (18)O(2) in place of (16)O(2) revealed a non-concerted conversion of cis-N(2)O(2)O(2) into these species, and gave access to four selectively di-(18)O-substituted trans-ONONO(2) isotopomers. No isotopic scrambling occurred. The IR spectra of sym-N(2)O(4) and of trans-ONONO(2) in solid Ne were recorded. IR fundamentals of trans-ONONO(2) were assigned based on experimental (16/18)O isotopic shifts and guided by DFT calculations. Previously reported contradictory measurements on cis- and trans-ONONO(2) are discussed. Dinitroso peroxide, ONOONO, a proposed intermediate in the IR photoinduced rearrangement of cis-N(2)O(2)O(2) to the various N(2)O(4) species, was not detected. Its absence in the photolysis products indicates a low barrier (<or=10 kJ mol(-1)) for its exothermic O-O bond homolysis into a (NO(2))(2) radical pair.