Automated Reaction Mechanisms and Kinetics based transition state search process AMK-gau_xtb and its application to the substitution reaction of the nitroso group in 2,4,6-trinitrotoluene by hydroxide anion in the aqueous phase

The automated reaction mechanisms and kinetics (AutoMeKin) program evolved from a transition state search using chemical dynamics simulations (TSSCDS). It combines a series of empirical, semi-empirical and ab initio calculation methods to provide a two-step transition state search process from low-level calculation to high-level calculation. However, in this process, with the lack of solution keywords, low-level calculation has the problem of low accuracy or high computational cost. To address this problem, the gau_xtb interface that combines the high efficiency of xTB and the comprehensiveness of Gaussian09 was incorporated into the AutoMeKin2020 in this work and after adding some keywords, the AMK-gau_xtb software was obtained. Meanwhile, to adapt to the interface, the MD sampling results used Quadratic Synchronous Transit 3 (QST3) for the low-level transition state search. As an application, the reaction in which the nitroso group is replaced by hydroxide anion during the alkaline hydrolysis of 2,4,6-trinitrotoluene (TNT) in the water phase was studied with AMK-gau_xtb. The results of Intrinsic Reaction Coordinate (IRC) calculations revealed that the reactions on the front side and back side are different, with higher energy barriers obtained for the reactions on the front side. In addition, the hydrogen atom of the hydroxide anion has a slightly higher energy barrier for motion toward the inside of the benzene ring than for motion out of the benzene ring. Examination of the transition state structures of the low-level and high-level results showed that all reactions involve the stretching and restoration of the benzene ring. This process will lead to the incorrect identification of several transition states by the gau_xtb-based low-level calculation, while high-level calculation eliminates these incorrect results. The results of this research showed that AMK-gau_xtb has high efficiency and high fault tolerance, and has potential for use in large-scale system transition state searches.