Effects of Anharmonicity, Recrossing, Tunneling, and Pressure on the H‑Abstractions from Dimethylamine by Triplets O and O2

Rate constants of the H-abstraction reactions from dimethylamine (DMA) by triplets O and O2 are theoretically determined with the canonical variational transition-state theory (CVT). By comparing the barrier heights and reaction energies obtained from different density-functional theory methods to those computed from the gold-standard method CCSD­(T)/CBS­(T-Q), we identify the M08-HX/ma-TZVP method as the best with a mean unsigned deviation of 1.0 kcal mol–1. On the basis of the optimized geometries and frequencies with the selected method, the rate constants are calculated using the CVT method combined with the multistructural torsional anharmonicity and small-curvature tunnelling (MS-CVT/SCT) options in the temperature range 200–2000 K. The calculations show that OH and HO2 are mainly produced from the direct abstraction from the C–H bond. The multistructural torsional anharmonicity has a large contribution to the rate constants, and the effects of recrossing and tunneling at the N-site are more important than those at C-site. Additionally, given the formation of reactant complex between DMA and triplet O, the H-abstraction channel is not favored at high pressure. Our calculations with both the Polyrate and MESS codes agree with the reported data within the uncertainty.本研究采用正则变分过渡态理论（canonical variational transition-state theory, CVT），理论计算了三重态氧（O）与氧气（O₂）夺取二甲胺（dimethylamine, DMA）中氢原子的反应速率常数。我们将不同密度泛函理论方法得到的势垒高度与反应能，与金标准方法CCSD(T)/CBS(T-Q)的计算结果进行对比，确定M08-HX/ma-TZVP方法为最优方法，其平均无符号偏差为1.0 kcal mol⁻¹。基于该优选方法得到的优化几何结构与振动频率，我们在200~2000 K的温度范围内，结合CVT方法与多结构扭转非简谐效应、小曲率隧穿（multistructural torsional anharmonicity and small-curvature tunnelling, MS-CVT/SCT）选项，计算了上述反应的速率常数。计算结果表明，羟基（OH）与过氧羟基（HO₂）主要通过直接夺取C–H键的路径生成。多结构扭转非简谐效应对速率常数具有显著贡献，且氮位点的折返效应与隧穿效应的影响强于碳位点。此外，由于二甲胺与三重态氧之间会形成反应物络合物，高压条件下氢原子夺取反应并不占优。本研究通过Polyrate与MESS程序得到的计算结果，与已报道的数据在不确定度范围内保持一致。