Sun Hongyan, Bozzelli Joseph W, Law Chung K
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, USA.
J Phys Chem A. 2007 Jun 14;111(23):4974-86. doi: 10.1021/jp070072d. Epub 2007 May 19.
Unimolecular dissociation of a neopentyl radical to isobutene and methyl radical is competitive with the neopentyl association with O2 ((3)Sigma(g)-) in thermal oxidative systems. Furthermore, both isobutene and the OH radical are important primary products from the reactions of neopentyl with O2. Consequently, the reactions of O2 with the 2-hydroxy-1,1-dimethylethyl and 2-hydroxy-2-methylpropyl radicals resulting from the OH addition to isobutene are important to understanding the oxidation of neopentane and other branched hydrocarbons. Reactions that correspond to the association of radical adducts with O2((3)Sigma(g)-) involve chemically activated peroxy intermediates, which can isomerize and react to form one of several products before stabilization. The above reaction systems were analyzed with ab initio and density functional calculations to evaluate the thermochemistry, reaction paths, and kinetics that are important in neopentyl radical oxidation. The stationary points of potential energy surfaces were analyzed based on the enthalpies calculated at the CBS-Q level. The entropies, S(degrees)298, and heat capacities, C(p)(T), (0 <or= T/K <or= 1500), from vibration, translation, and external rotation contributions were calculated using statistical mechanics based on the vibrational frequencies and structures obtained from the density functional study. The hindered internal rotor contributions to S(degrees)298 and C(p)(T) were calculated by solving the Schrödinger equation with free rotor wave functions, and the partition coefficients were treated by direct integration over energy levels of the internal rotation potentials. Enthalpies of formation (DeltaH(f)(degrees)298) were determined using isodesmic reaction analysis. The DeltaH(f)(degrees)298 values of (CH3)2CCH(2)OH, (CH3)2C(OO)CH(2)OH, (CH3)2C(OH)CH2, and (CH3)2C(OH)CH(2)OO radicals were determined to be -23.3, -62.2, -24.2, and -61.8 kcal mol(-1), respectively. Elementary rate constants were calculated from canonical transition state theory, and pressure-dependent rate constants for multichannel reaction systems were calculated as functions of pressure and temperature using multifrequency quantum Rice-Ramsperger-Kassel (QRRK) analysis for k(E) and a master equation for pressure falloff. Kinetic parameters for intermediate and product formation channels of the above reaction systems are presented as functions of temperature and pressure.
在热氧化体系中，新戊基自由基单分子解离生成异丁烯和甲基自由基的反应与新戊基与O₂（³Σg⁻）的缔合反应相互竞争。此外，异丁烯和羟基自由基都是新戊基与O₂反应的重要初级产物。因此，O₂与因羟基加成到异丁烯上而产生的2-羟基-1,1-二甲基乙基自由基和2-羟基-2-甲基丙基自由基的反应，对于理解新戊烷和其他支链烃的氧化过程至关重要。与自由基加合物和O₂（³Σg⁻）缔合相对应的反应涉及化学活化的过氧中间体，这些中间体在稳定之前可以异构化并反应形成几种产物之一。通过从头算和密度泛函计算对上述反应体系进行了分析，以评估新戊基自由基氧化过程中重要的热化学、反应路径和动力学。基于CBS-Q水平计算的焓值，对势能面的驻点进行了分析。利用统计力学，根据密度泛函研究得到的振动频率和结构，计算了振动、平动和外转动贡献的熵S°₂₉₈和热容Cₚ(T)（0≤T/K≤1500）。通过用自由转子波函数求解薛定谔方程，计算受阻内转子对S°₂₉₈和Cₚ(T)的贡献，并通过对内转动势的能级进行直接积分来处理配分系数。利用等键反应分析确定了生成焓（ΔHf°₂₉₈）。(CH₃)₂CCH(₂)OH、(CH₃)₂C(OO)CH(₂)OH、(CH₃)₂C(OH)CH₂和(CH₃)₂C(OH)CH(₂)OO自由基的ΔHf°₂₉₈值分别确定为-23.3、-62.2、-24.2和-61.8 kcal mol⁻¹。根据正则过渡态理论计算基元速率常数，并使用多频量子 Rice-Ramsperger-Kassel（QRRK）分析k(E)和压力下降主方程，计算多通道反应体系的压力依赖速率常数作为压力和温度的函数。上述反应体系中中间体和产物生成通道的动力学参数表示为温度和压力的函数。
