Multi-path variational transition state theory for chiral molecules: the site-dependent kinetics for abstraction of hydrogen from 2-butanol by hydroperoxyl radical, analysis of hydrogen bonding in the transition state, and dramatic temperature dependence of the activation energy
Multi-path variational transition state theory for chiral molecules: the site-dependent kinetics for abstraction of hydrogen from 2-butanol by hydroperoxyl radical, analysis of hydrogen bonding in the transition state, and dramatic temperature dependence of the activation energy
复制标题
DOI：
10.1039/c5sc01848j
复制
发表时间：
2015-01-01
期刊：
影响因子：
8.4
通讯作者：
Truhlar, Donald G.
中科院分区：
文献类型：
--
作者：
Bao, Junwei Lucas;Meana-Paneda, Ruben;Truhlar, Donald G.
本工作的目标是对生物燃料异丁醇燃烧过程中的一个关键反应的动力学进行建模。为此，我们在小曲率隧道(SCT)近似下扩展了多路径变分过渡态理论(MP-VTST)，考虑了手性碳分子的多结构非谐因子。我们用得到的理论预测了过氧化氢自由基从2-丁醇中抽氢的位置相关的速率常数。在四条能量最低的反应路径上对广义传输系数进行了平均。计算的正向反应速率常数表明，在200~2400K温度范围内，C-2位的吸氢反应对整个反应的贡献最大，在200K时为99.9988%，800K时为88.9%，3000K时为21.2%，而氧位上的吸氢反应在所有温度下的贡献最小，在200K时为2.5×10(-9)%，800K时为0.65%，3000K时为18%。我们还分析了氢键在过渡态中所起的作用，并说明了(A)在计算速率常数时只考虑最低能构象或(B)忽略活化能的非线性温度依赖性的风险。过渡态的氢键降低了活化热，但提高了活化能。我们发现，对于这种与生物燃料分子的自由基反应，活化能从200K的11kcal摩尔(-1)增加到高温下的36kcal摩尔(-1)以上。
The goal of the present work is modeling the kinetics of a key reaction involved in the combustion of the biofuel 2-butanol. To accomplish this we extended multi-path variational transition state theory (MP-VTST) with the small curvature tunneling (SCT) approximation to include multistructural anharmonicity factors for molecules with chiral carbons. We use the resulting theory to predict the site-dependent rate constants of the hydrogen abstraction from 2-butanol by hydroperoxyl radical. The generalized transmission coefficients were averaged over the four lowest-energy reaction paths. The computed forward reaction rate constants indicate that hydrogen abstraction from the C-2 site has the largest contribution to the overall reaction from 200 K to 2400 K, with a contribution ranging from 99.9988% at 200 K to 88.9% at 800 K to 21.2% at 3000 K, while hydrogen abstraction from the oxygen site makes the lowest contribution at all temperatures, ranging from 2.5 x 10(-9)% at 200 K to 0.65% at 800 K to 18% at 3000 K. This work highlights the importance of including the multiple-structure and torsional potential anharmonicity in the computation of the thermal rate constants. We also analyzed the role played by the hydrogen bond at the transition state, and we illustrated the risks of (a) considering only the lowest-energy conformations in the calculations of the rate constants or (b) ignoring the nonlinear temperature dependence of the activation energies. A hydrogen bond at the transition state can lower the enthalpy of activation, but raise the free energy of activation. We find an energy of activation that increases from 11 kcal mol(-1) at 200 K to more than 36 kcal mol(-1) at high temperature for this radical reaction with a biofuel molecule.