N-甲基羟胺盐酸盐的热分解机理的理论研究

为深入研究N-甲基羟胺盐酸盐(NMHH)热分解的反应特征,应用密度泛函理论,在w B97xd/6-311++g(2df,2pd)水平下,对NMHH的初步分解产物N-甲基羟胺(NMHA)及质子化的NMHA热分解反应中涉及的反应物、过渡态、中间体、产物的几何结构和能量情况进行了优化计算,提出了可能的热分解路径。结果表明,NMHA与质子化NMHA均存在两种热分解路径。NMHA在两条路径下分解需越过的能垒为250.75 k J/mol与428.64 k J/mol,而质子化NMHA在对应分解路径下需要越过的能垒分别为217.19 k J/mol与286.77k J/mol。在对应路径下质子化NMHA分解需越过的能垒明显低于NMHA的直接分解,并且随着反应的持续进行,质子化的NMHA的形成和分解将越来越容易发生。这表明NMHH的分解具有自催化特征,与试验结果吻合。

Theoretical approach to the thermal decomposition mechanism of the NMethylhydroxylamine Hydrochloride

This article intends to make an investigation of the mechanism of the thermal decomposition for the N-Methylhydroxylamine hydrochloride (NMHH) so as to identify and determine its decomposition characteristic features via the density functional theory (DFT). In the paper,we have also accomplished the calculation at the theoretical wB97xd level in combination with the 6-311 + + g (2df,2pd) basis sets. At this level,we have managed to propose the protonated NMHA through the optimized calculation of all the related reactants,the geometrical structure and the energetics of N-methylhydroxylamine (NMHA) by clarifying the related states,the intermediates,the complexes and the products via the likely pathways. Furthermore,we have managed to analyze,perform and verify all the stationary points and the normal mode vibration frequencies that the stable minima are of all the positive vibration frequencies while the transition states enjoy an imaginary frequency merely. And,next,we have done intrinsic reaction coordinate (IRC) calculations at the same theoretical level so as to verify that the transition states are obviously connected with the desired reactants and products. The optimization results show that the newly formulated N-H bonds in the protonated NMHA,which tends to become a more symmetrical structure than that of NMHA,and the protonation process may release a great deal of energy of 864. 16 kJ /mol. Thus,on the basis of the calculations we would like to propose respectively two pathways for NMHA and the other two pathways for the protonated NMHA. The energy barrier between the zero-point vibration corrected energy of the transition state and the energy of the starting reactants of the two pathways for the NMHA presented here should be equal to 250. 75 kJ /mol and 428. 64 kJ /mol,while those of the similar pathways for the protonated NMHA are 217. 19 kJ /mol and 286. 77 kJ /mol,which seems to be much lower. In other words,the thermo-decomposition of the protonated NMHA turns out to be more likely to take place than that of NMHA. What is more,the energy release in the protonation process and the increasing content of H3O + can facilitate the formation and decomposition of the protonated NMHA while the reaction is going on. Thus,the results of our investigation and experiment help us to deduce an autocatalysis results that may take place in the decomposition, which is well in accord with the actual experimental status-in-situ.