Computational study of the perhalogenated methyl nitrates CX3ONO2, CX x Y3−x ONO2 (X, Y = F, Cl)

Abstract

Ab initio and density functional theory methods are employed to study the structures, harmonic frequencies, energetics and thermodynamic properties of the perhalogenated methyl nitrates, CX3ONO2, CX x Y3−x ONO2 (X, Y = F, Cl), which may be formed as secondary reactive intermediates in the coupling of the halogenated methylperoxy radicals with NO. Reaction energies are computed with respect to CX3O2 + NO, CX x Y3−x O2 + NO and CX3O + NO2, CX x Y3−x O + NO2 radical pair decomposition species. The large heat of formation values calculated indicate the high stabilization achieved upon halogenation of the methyl radical, particularly for the fluorinated compounds. Thus, the halogenated methyl nitrates which appear in the oxidation chain of halomethanes can be thermally deactivated under suitable temperature and pressure conditions and act as reservoir compounds for the halogenated methylperoxy radicals, CX3O2, CX x Y3−x O2 and NO, in the troposphere. The computational investigation also demonstrates the significant structural changes caused by the halogen electron withdrawing effect, compared with the methyl analogue, CH3ONO2.