Accurate Full-Dimensional Potential Energy Surfaces And Dynamics Study Of F/Cl/H+CH₃OH Multiple-Channel Reaction Systems

Over the past few decades,our knowledge of how chemical reactions take place has reached an unprecedentedly high level,both theoretically and experimentally.Detailed measurements and sophisticated theoretical investigations have led to a thorough understanding of the dynamics for many prototypical reactions involving three or four atoms,such as the H/F/Cl/O/N/C+H₂ and H/F/O/Cl+H₂O reactions.Recent focus has been shifted to more complex ones,such as the H/F/Cl/O/OH+CH₄ reactions.These studies have played a pivotal role in advancing our understanding of fundamental mechanisms and dynamics in chemical reactions,and have shed valuable light on a wide array of important dynamical issues.However,the aforementioned reactions with only a single type of reaction channel are not representative of most chemical reactions of larger molecules.many reactions involving organic molecules often have multiple reaction pathways,and one product may be highly desired.It is thus of great importance to understand product selectivity among competing reaction pathways.Due to the difficulty of constructing full dimensional and multi-channel potential energy surfaces,it is very challenging to study the reaction dynamics of these systems.In order to better understand the complex reaction system,we have studied the reaction dynamics of the hydrogen abstraction of methanol by F/Cl/H atoms,F/Cl/H+CH₃OH.Hydrogen abstraction from methanol(CH₃OH)by F atoms presents an ideal proving ground to investigate dynamics of multi-channel reactions,because two types of hydrogen can be abstracted from the methanol molecule leading to the HF+CH₃O and HF+CH₂OH products.Using the quasi-classical trajectory approach on a globally accurate potential energy surface based on high-level ab initio calculations,we report a comprehensive dynamical investigation of this multi-channel reaction,yielding measurable attributes including integral and differential cross sections,as well as branching ratios.It is shown that while complex-forming and direct mechanisms coexist at low collision energies,these barrierless reaction channels are dominated at high energies by the direct mechanism,in which the reaction is only possible for trajectories entering into the respective dynamical cones of acceptance.In addition,the non-statistical product branching is found to be dictated by unique stereodynamics in the entrance channels.Cl+CH₃OH?HCl+CH₃O/CH₂OH is a prototypical multiple-channel reaction.Experimentally,ample dynamical and kinetic information is available,but there are still many uncertainties concerning the reaction mechanism.Theoretical investigations are rare due to the absence of a potential energy surface(PES),which has greatly hindered our understanding of the reaction dynamics.Using a machine-learning approach,an accurate full-dimensional PES for the title reaction based on tens of thousands of high-level ab initio data points is reported.Comprehensive dynamical calculations are performed on the PES using quasi-classical trajectories,and the results provide insights into the reaction kinetics and dynamics.The calculated non-Arrhenius rate coefficients are consistent with the experimental data,attributable to a complex-forming mechanism at low temperatures.At high energies,the reaction is dominated by a direct mechanism,which results in dominant forward scattering via a stripping mechanism augmented by less prominent sideways and backward scattering via a rebound mechanism.At collision energies of 5.6 and 8.7 kcal/mol,the measured product translational are well reproduced.In addition,mode specificity is revealed and rationalized by the sudden vector projection model.This work sheds valuable light on the microscopic mechanism and dynamics of this prototypical multichannel reaction.The H+CH₃OH reaction,which plays an important role in combustion and the interstellar medium,presents a prototypical system with multi channels and tight transition states.However,no globally reliable potential energy surface(PES)has been available to date.Here we develop global analytical PESs for this system using the permutation-invariant polynomial neural network(PIP-NN)and the high-dimensional neural network(HD-NN)methods based on a large number of data points calculated at the level of the explicitly correlated unrestricted coupled cluster single,double,and perturbative triple level with the augmented correlation corrected valence triple-?basis set(UCCSD(T)-F12a/AVTZ).We demonstrate that both machine learning PESs are able to accurately describe all dynamically relevant reaction channels.At a collision energy of20 kcal/mol,quasi-classical trajectory calculations reveal that the dominant channel is the hydrogen abstraction from the methyl site,yielding H₂+CH₂OH.The reaction of this major channel takes place mainly via the direct rebound mechanism.Both the vibrational and rotational states of the H₂ product are relatively cold,and large portions of the available energy are converted into the product translational motion.