Photoinduced Bimolecular Reactions in Homogeneous [CH₃ONO]_nClusters

Abstract

The photodissociation of homogeneous methyl nitrite clusters, [CH3ONO] n with n ≈ 400−1000, was investigated in a supersonic jet using excitation mainly at 365 nm, which corresponds to S 0 → S 1 (nπ*) excitation in the monomer. Besides the two types of NO(X̃2II) photofragment distributions, a rotationally relaxed one (T rot ∼ 250 K) and a nonthermally “hot” one (〈J‘‘〉 = 35.5) which result from the primary dissociation step CH3ONO → CH3O + NO of cluster-bound CH3ONO, we observed the products HNO(X̃A‘) and H2CO(X̃1A1) by state-selected LIF spectroscopy. Their product−yield excitation spectra and their formation dependence on the backing pressure revealed that HNO and H2CO originate exclusively from cluster photodissociation and not from primary photodissociation of the monomer. The mechanism of their formation was found to be the disproportionation reaction of the primary photofragments, CH3O + NO → HNO + H2CO, mediated by caging of the cluster environment. The fragments collide with, and recoil at, the solvent shell followed by subsequent recombination, disproportionation, or escape from the evaporating solvent cage. The present results are consistent with previous findings on the photolysis of isolated CH3ONO molecules in solid noble gas matrices where exclusively the products HNO and H2CO were found.

Dissociation Pathways of the CH2CH2ONO Radical: NO2+ Ethene, NO + Oxirane, and a Non-Intrinsic Reaction Coordinate HNO + Vinoxy Pathway

“…There has been some debate in the literature regarding whether a monomeric nitrite can evolve HNO under collision-free conditions. Most relevant to the present study is that Bergmann and Huber, 22 in a laser-induced fluorescence experiment, observed HNO production from a molecular beam of methyl nitrite clusters ([CH 3 ONO] n , n ≈ 400−1000) photoexcited at 365 nm, which they attributed entirely to disproportionation within the clusters. The HNO signal in this work, however, cannot be assigned to photodissociation within nitrite clusters.…”

“…2,3 In studies of the bulk kinetics of alkyl nitrites, nitrosoalkyl products (R− NO) are also observed, and the primary and secondary nitrites moreover evolve HNO + aldehyde or ketone. 19−21 A disproportionation mechanism for these pathways has been proposed in which nascent NO radicals recombine with nascent alkoxy radicals, leading either to H atom abstraction or C−C bond cleavage: 3,22,23…”

“…Gaseous alkyl nitrites (R–ONO, where R is an alkyl group) are important intermediates in the formation of chemical smog and in the regulation of ozone levels. − Both photolysis and thermal decomposition of these species are rapid; photolysis under ultraviolet light occurs readily via photoabsorption in the nitrite moiety, which has absorption bands near 350 and 200 nm. The atmospheric impact of alkyl nitrite chemistry and photochemistry has motivated studies on nitrous acid (HONO), − methyl nitrite (CH 3 ONO), − and a number of higher nitrites as well. − …”

“…A common reaction pathway is dissociation to an alkoxy radical and nitric oxide, RONO → RO + NO. , In studies of the bulk kinetics of alkyl nitrites, nitrosoalkyl products (R–NO) are also observed, and the primary and secondary nitrites moreover evolve HNO + aldehyde or ketone. − A disproportionation mechanism for these pathways has been proposed in which nascent NO radicals recombine with nascent alkoxy radicals, leading either to H atom abstraction or C–C bond cleavage: ,, There is also recent evidence of a unimolecular mechanism for HNO elimination. Zhu et al have reported a roaming-mediated transition state for this process in methyl nitrite, and Liu et al did likewise for ethyl nitrite .…”

Quantum chemical molecular dynamical investigation of alkyl nitrite photo-dissociated on copper surfaces

“…Alkyl nitrites in the form R-O-N O have many features such as (1) well-defined chromophore, (2) highly sensitive to light, and (3) relatively simple model heteroatom species [11][12][13] and therefore are taken as the ideal probe to the photolysis reactions. For gas-phase photolysis [14][15][16][17][18][19][20][21][22][23], alkyl nitrite can be dissociated into an alkoxy fragment and NO under UV radiation in the following reaction sequence:…”