Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, Switzerland.
J Phys Chem A. 2012 Apr 5;116(13):3436-47. doi: 10.1021/jp212209q. Epub 2012 Mar 20.
Detailed knowledge of hydrocarbon radical thermochemistry is critical for understanding diverse chemical phenomena, ranging from combustion processes to organic reaction mechanisms. Unfortunately, experimental thermochemical data for many radical species tend to have large errors or are lacking entirely. Here we develop procedures for deriving high-quality thermochemical data for hydrocarbon radicals by extending Wheeler et al.'s "generalized bond separation reaction" (GBSR) scheme (J. Am. Chem. Soc., 2009, 131, 2547). Moreover, we show that the existing definition of hyperhomodesmotic reactions is flawed. This is because transformation reactions, in which one molecule each from the predefined sets of products and reactants can be converted to a different product and reactant molecule, are currently allowed. This problem is corrected via a refined definition of hyperhomodesmotic reactions in which there are equal numbers of carbon-carbon bond types inclusive of carbon hybridization and number of hydrogens attached. Ab initio and density functional theory (DFT) computations using the expanded GBSRs are applied to a newly derived test set of 27 hydrocarbon radicals (HCR27). Greatly reduced errors in computed reaction enthalpies are seen for hyperhomodesmotic and other highly balanced reactions classes, which benefit from increased matching of hybridization and bonding requirements. The best performing DFT methods for hyperhomodesmotic reactions, M06-2X and B97-dDsC, give average deviations from benchmark computations of only 0.31 and 0.44 (±0.90 and ±1.56 at the 95% confidence level) kcal/mol, respectively, over the test set. By exploiting the high degree of error cancellation provided by hyperhomodesmotic reactions, accurate thermochemical data for hydrocarbon radicals (e.g., enthalpies of formation) can be computed using relatively inexpensive computational methods.
详细了解烃自由基热化学对于理解从燃烧过程到有机反应机制等各种化学现象至关重要。不幸的是，许多自由基的实验热化学数据往往存在较大误差或完全缺失。在这里，我们通过扩展 Wheeler 等人的“广义键分离反应”（GBSR）方案（J. Am. Chem. Soc.，2009，131，2547）来开发为烃自由基生成高质量热化学数据的程序。此外，我们表明现有的超等焓反应定义存在缺陷。这是因为在转换反应中，目前允许从预定义的产物和反应物集合中各取一个分子转化为不同的产物和反应物分子。通过对超等焓反应的精确定义来纠正这个问题，其中包括包含碳杂化和连接氢原子数量的碳-碳键类型的数量相等。使用扩展的 GBSR 进行从头算和密度泛函理论（DFT）计算应用于新衍生的 27 个烃自由基（HCR27）测试集。对于超等焓和其他高度平衡的反应类别，计算得到的反应焓的误差大大降低，这得益于杂化和键合要求的匹配度增加。对于超等焓反应表现最佳的 DFT 方法，M06-2X 和 B97-dDsC，在测试集上，相对于基准计算的平均偏差分别仅为 0.31 和 0.44（置信水平为 95%时为±0.90 和±1.56）kcal/mol。通过利用超等焓反应提供的高度误差抵消，可以使用相对廉价的计算方法计算烃自由基的准确热化学数据（例如生成焓）。
