title: Theoretical study on the reaction mechanisms of CH3O- with O2(X3Σ(g)-) and O2(a1Δ(g)).
abstract: The detailed potential energy surfaces (PESs) of poorly understood  ion-molecule reactions of CH(3)O(-) with O(2)(X(3)Σ(g)(-)) and O(2)(a(1)Δ(g)) are accounted for by the density functional theory and ab initio of QCISD and CCSD(T) (single-point) theoretical levels with 6-311++G(d,p) and 6-311++G(3df,2pd) basis sets for the first time. For the reaction of CH(3)O(-) with O(2)(X(3)Σ(g)(-)) ((3)R), it is shown that a hydrogen-bonded complex (3)1 is initially formed on the triplet PES, which is 1.8 kcal/mol above reactants (3)R at the CCSD(T)//QCISD level, from which all the products P(1)-P(8) can be generated. As to the reaction of CH(3)O(-) with O(2)(a(1)Δ(g)) ((1)R), it is found that the two energetically low-lying complexes of (1)1(-31.5 kcal/mol) and (1)2(-24.1 kcal/mol) are initiated on the singlet PES. Starting from them, a total of seven products may be possible, that is, besides P(1), P(2), P(3), P(4), and P(8), which are the same as on the triplet PES, there exist also another two products, P(9) and P(10). For both reactions, taking the thermodynamics and kinetics into consideration, the hydride-transfer species P(1)(CH(2)O + HO(2)(-)) should be the most favorable product followed by P(8)(e + CH(2)O + HO(2)), which is a secondary product of electron-detachment from P(1), and the generation of endothermic P(7)(17.7 kcal/mol) for the reaction of CH(3)O(-) with O(2)(X(3)Σ(g)(-)) is also possible at high temperature, whereas the remaining products are negligible. The measured branching ratio of products for CH(3)O(-) with O(2)(X(3)Σ(g)(-)) by Midey et al. is 0.85:0.15 for P(1) and P(8), and that of CH(3)O(-) with O(2)(a(1)Δ(g)) is 0.52:0.48 with more P(8), which can be rationalized by our theoretical results that P(8) on the triplet PES is 4.9 kcal/mol above (3)R, whereas both P(1) and P(8) on the singlet PES are very low-lying at 45.6 and 25.2 kcal/mol below (1)R energetically. The measured total reaction rate constant of CH(3)O(-) with O(2)(a(1)Δ(g)) is k = 6.9 × 10(-10) cm(3) s(-1) at 300 K, which is larger than that of k = 1.1 × 10(-12) cm(3) s(-1) for the reaction of CH(3)O(-) with O(2)(X(3)Σ(g)(-)). This is understandable because both P(1) and P(8) on the singlet PES can be generated barrierlessly, whereas to give all the products on the triplet PES has to pass the barrier of (3)1(1.8 kcal/mol) at the CCSD(T)//QCISD level. It is expected that the present theoretical study may be helpful for understanding the reaction mechanisms related to CH(3)O(-) and even CH(3)S(-).
authors.affiliation: Department of Chemistry, Shantou University, Guangdong 515063, People's Republic of China.
authors.forename: Hai-Xia
authors.initials: HX
authors.lastname: Lin
authors.forename: Hai-Long
authors.initials: HL
authors.lastname: Liang
authors.forename: Guang-Hui
authors.initials: GH
authors.lastname: Chen
authors.forename: Feng-Long
authors.initials: FL
authors.lastname: Gu
authors.forename: Wen-Guang
authors.initials: WG
authors.lastname: Liu
authors.forename: Shao-Fei
authors.initials: SF
authors.lastname: Ni