Mahata Prabhash, Maiti Biswajit
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005, India.
J Phys Chem A. 2021 Dec 9;125(48):10321-10329. doi: 10.1021/acs.jpca.1c07785. Epub 2021 Nov 22.
The photodissociation of methyl hydroperoxide (CHOOH) at 193 nm has been studied using a direct dynamics trajectory surface-hopping (TSH) method. The potential energies, energy gradients, and nonadiabatic couplings are calculated on the fly at the MRCIS(6,7)/aug-cc-pVDZ level of theory. The hopping of a trajectory from one electronic state to another is decided on the basis of Tully's fewest switches algorithm. An analysis of the trajectories reveals that the cleavage of the weakest O-O bond leads to major products CHO(E) + OH(Π), contributing about 72.7% of the overall product formation. This OH elimination was completed in the ground degenerate product state where both the ground singlet (S) and first excited singlet (S) states become degenerate. The O-H bond dissociation of CHOOH is a minor channel contributing about 27.3% to product formation, resulting in products CHOO + H. An inspection of the trajectories indicates that unlike the major channel OH elimination, the H-atom elimination channel makes a significant contribution (∼3% of the overall product formation) through the nonadiabatic pathway via conical intersection S/S leading to ground-state products CHOO(X A″) + H(S) in addition to adiabatic dissociation in the first excited singlet state, S, correlating to products CHOO(1 A') + H(S). The computed translational energy of the majority of the OH products is found to be high, distributed in the range of 70 to 100 kcal/mol, indicating that the dissociation takes place on a strong repulsive potential energy surface. This finding is consistent with the nature of the experimentally derived translational energy distribution of OH with an average translational energy of 67 kcal/mol after the excitation of CHOOH at 193 nm.
采用直接动力学轨迹表面跳跃（TSH）方法研究了193nm波长下过氧化氢甲基（CHOOH）的光解离。在MRCIS(6,7)/aug-cc-pVDZ理论水平上实时计算势能、能量梯度和非绝热耦合。轨迹从一个电子态到另一个电子态的跳跃是根据塔利的最少开关算法确定的。对轨迹的分析表明，最弱的O-O键的断裂产生主要产物CHO(E)+OH(Π)，约占总产物形成的72.7%。这种OH消除在基态简并产物态中完成，此时基态单重态（S）和第一激发单重态（S）都变成简并态。CHOOH的O-H键解离是一个次要通道，对产物形成的贡献约为27.3%，产生产物CHOO+H。对轨迹的检查表明，与主要通道OH消除不同，H原子消除通道除了在第一激发单重态S中绝热解离产生产物CHOO(1 A')+H(S)外，还通过锥形交叉S/S的非绝热途径对总产物形成做出了显著贡献（约占总产物形成的3%），导致基态产物CHOO(X A″)+H(S)。发现大多数OH产物的计算平动能很高，分布在70至100kcal/mol范围内，这表明解离发生在强排斥势能面上。这一发现与实验得出的CHOOH在193nm激发后OH平动能分布的性质一致，其平均平动能为67kcal/mol。
