Lau K-C, Ng C Y
Department of Chemistry, University of California, Davis, California 95616, USA.
J Chem Phys. 2006 Jan 28;124(4):044323. doi: 10.1063/1.2148950.
The ionization energies (IEs) for the 2-propyl (2-C(3)H(7)), phenyl (C(6)H(5)), and benzyl (C(6)H(5)CH(2)) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasiperturbative triple excitation [CCSD(T)]. The zero-point vibrational energy correction, the core-valence electronic correction, and the scalar relativistic effect correction have been also made in these calculations. Although a precise IE value for the 2-C(3)H(7) radical has not been directly determined before due to the poor Franck-Condon factor for the photoionization transition at the ionization threshold, the experimental value deduced indirectly using other known energetic data is found to be in good accord with the present CCSD(T)/CBS prediction. The comparison between the predicted value through the focal-point analysis and the highly precise experimental value for the IE(C(6)H(5)CH(2)) determined in the previous pulsed field ionization photoelectron (PFI-PE) study shows that the CCSD(T)/CBS method is capable of providing an accurate IE prediction for C(6)H(5)CH(2), achieving an error limit of 35 meV. The benchmarking of the CCSD(T)/CBS IE(C(6)H(5)CH(2)) prediction suggests that the CCSD(T)/CBS IE(C(6)H(5)) prediction obtained here has a similar accuracy of 35 meV. Taking into account this error limit for the CCSD(T)/CBS prediction and the experimental uncertainty, the CCSD(T)/CBS IE(C(6)H(5)) value is also consistent with the IE(C(6)H(5)) reported in the previous HeI photoelectron measurement. Furthermore, the present study provides support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can be used to provide reliable IE predictions for C(3)-C(7) hydrocarbon radicals with an uncertainty of +/-35 meV. Employing the atomization scheme, we have also computed the 0 K (298 K) heats of formation in kJ/mol at the CCSD(T)/CBS level for 2-C(3)H(7)/2-C(3)H(7) (+) ,C(6)H(5)/C(6)H(5) (+), and C(6)H(5)CH(2)/C(6)H(5)CH(2) (+) to be 105.2/822.7 (90.0/806.4), 351.4/1148.5 (340.4/1138.8), and 226.2/929.0 (210.3/912.7), respectively. Comparing these values with the available experimental values, we find that the discrepancies for the 0 and 298 K heats of formation values are < or =2.6 kJ/mol for 2-C(3)H(7)/2-C(3)H(7) (+),< or =4.1 kJ/mol for C(6)H(5)/C(6)H(5) (+), and < or =3.2 kJ//mol for C(6)H(5)CH(2)C(6)H(5)CH(2) (+).
采用基于波函数的从头算CCSD(T)/CBS方法计算了2-丙基（2-C(3)H(7)）、苯基（C(6)H(5)）和苄基（C(6)H(5)CH(2)）自由基的电离能，该方法涉及在单双激发加准微扰三重激发的耦合簇水平上对完全基组（CBS）极限进行近似[CCSD(T)]。在这些计算中还进行了零点振动能校正、芯价电子校正和标量相对论效应校正。尽管由于电离阈值处光电离跃迁的弗兰克-康登因子较差，之前尚未直接确定2-C(3)H(7)自由基的精确电离能值，但发现使用其他已知能量数据间接推导的实验值与当前的CCSD(T)/CBS预测结果吻合良好。通过焦点分析预测的值与先前脉冲场电离光电子（PFI-PE）研究中确定的IE(C(6)H(5)CH(2))的高精度实验值之间的比较表明，CCSD(T)/CBS方法能够为C(6)H(5)CH(2)提供准确的电离能预测，误差限为35 meV。CCSD(T)/CBS对IE(C(6)H(5)CH(2))的预测基准表明，此处获得的CCSD(T)/CBS对IE(C(6)H(5))的预测具有相似的35 meV精度。考虑到CCSD(T)/CBS预测的这一误差限和实验不确定性，CCSD(T)/CBS的IE(C(6)H(5))值也与先前HeI光电子测量中报告的IE(C(6)H(5))一致。此外，本研究支持以下结论：具有高级能量校正的CCSD(T)/CBS方法可用于为C(3)-C(7)烃基自由基提供可靠的电离能预测，不确定性为±35 meV。采用原子化方案，我们还在CCSD(T)/CBS水平上计算了2-C(3)H(7)/2-C(3)H(7) (+)、C(6)H(5)/C(6)H(5) (+)和C(6)H(5)CH(2)/C(6)H(5)CH(2) (+)在0 K（298 K）时以kJ/mol为单位的生成热，分别为105.2/822.7（90.0/806.4）、351.4/1148.5（340.4/1138.8）和226.2/929.0（210.3/912.7）。将这些值与可用的实验值进行比较，我们发现2-C(3)H(7)/2-C(3)H(7) (+)的0 K和298 K生成热值的差异≤2.6 kJ/mol，C(6)H(5)/C(6)H(5) (+)的差异≤4.1 kJ/mol，C(6)H(5)CH(2)/C(6)H(5)CH(2) (+)的差异≤3.2 kJ/mol。
