Theoretical Studies On The Microcosmic Mechanism Of Reaction CH₃O+NO₂ And [CH₃,N,C,S] System

In this paper, quantum chemistry methods were used to investigate the microcosmicreaction mechanisms and dynamics of radical-radical reaction of CH₃O with NO₂ and theglobal potential energy surface of [CH₃,N,C,S] system in singlet and triplet statestheoretically. The study of their microcosmic reaction mechanisms has always been theleading subjects in theoretical chemistry, in terms of their high reaction temperature, highreaction rate, trace quantity products, especially some reactions can not proceed in laboratoryand the information of microcosmic reaction mechanisms are still lacking, so many attentionhas been paid to the theoretical research on those reactions.CH₃O, the simplest alkoxy radical, is a common intermediate in both combustion andatmospheric systems. The reaction of CH₃O+NO₂ is potentially important at highconcentrations of NO₂ in lower stratosphere, particularly for the formation of methylnitrate(CH₃ONO₂). Although previous experimental and theoretical studies have reported possiblechannels and thermal rate constants for CH₃O+NO₂ reaction and confirmed that CH₃ONO₂ isthe main product, the reaction mechanism CH₃O+NO₂(?) HONO+CH2O is still not clear.Methylisothiocyanate (CH₃NCS, MITC) is the sulfur analogue of methylisocyanicCH₃NCO, which is the primary breakdown product of metamsodium, and a potentialreplacement fumigant pesticide for methyl bromide. Due to its toxicity and high potential forvolatilization, scientists have paid great efforts these years to investigate the structures ofCH₃NCS and its isomers CH₃SCN, CH₃CNS, and their photochemical behaviors includingthe mechanism and the rate for their removal mechanism. However, there is no high levelcomputational study to explore the potential energy surface of [CH₃,N,C,S] system up to now.The results of this work show that:1. The mechanism for reaction of CH₃O with NO₂ is investigated at DFT (B3LYP), MP2and QCISD levels with 6-311++G(d,p) basis set. The geometries and vibrational frequencies ofstationary points on potential energy surface are obtained. There are 6 possible reaction pathsthat involve 8 conformers, 15 transition structures and four possible products for the titlereaction. The recombination channel for the product CH₃ONO₂ is dominant. At the adoptedlevels of theory, the disproportionation channel, which is thought to be possible by simpleintermolecular H-atom abstraction conjectured experimentally, has not been found. Instead, itshould be completed through the formation of intermediate [CH₃ONO₂]^# and then dissociateinto products via a transition structure. The aim of this part is to perfect and predict themicrocosmic mechanism of reaction CH₃O+NO₂.2. The global potential energy surface (PES) of the [CH₃,N,C,S] system in singlet andtriplet states, involving 16 isomers and 15 transition structures, is studied at DFT(B3LYP),MP2 and QCISD levels with 6-311+G(d,p) baisi set. It is shown that the chainlike singletisomer CH₃NCS is the most stable species among all the isomers and the 3branched-C(CH₃)NShas the lowest energy among the triplet species. The investigation on the dissociation pathsshows that the singlet isomers' dissociations are barrierless whereas those of triplet ones arefinished through transition states. At same time, the potential energy surface of [CH3,N,C,S]system are also compared with [H,N,C,S] system.