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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >An Accurate Description of the Bergman Reaction Using Restricted and Unrestricted DFT: Stability Test, Spin Density, and On-Top Pair Density
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An Accurate Description of the Bergman Reaction Using Restricted and Unrestricted DFT: Stability Test, Spin Density, and On-Top Pair Density

机译：使用限制和非限制DFT的Bergman反应的准确描述：稳定性测试，自旋密度和顶部对密度

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摘要

DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyne 1 provided the following are considered. (a) Restricted DFT (RDFT) calculations along the reaction path have to be replaced by unrestricted DFT (UDFT) calculations at those locations where the former description becomes unstable. This is the case in hte region of the p-didehydrobenzene biradical 2, which possesses significant multireference charactrer. (b) LSD and pure GGA functionals are more stable than hybrid functionals, which can be directly related to the composition of these functionals. With increasing instability, RDFT calculations lead to increasing errors in the S-T splitting and the geometry of 2 as well as in the energetics of the Bergman kreaction. (c) LSD and GGA functionals underestimate the energy barrier of the Bergman reaction of 1. This becomes obvious when the correct experimental barrier is considered, which was not done in previous DFT investigations. (d) The best description of the Bergman reaction is provided by a mixed RDFT/UDFT description using hte B3LYP functional (average error of 2.7 kcal/mol). Although the B3LYP functional is rather unstable, its semiempirical calibration helps to compensate for the typical underestimation of barriers by GGA functionals, which demonstrates that the performance fo a hybrid functional does not necessarily have to do with its stability. (e) Application of hte sum formula to the UB3LYP energy of biradical 2 improves the description of the Bergman reaction so that the most reliable data are obtained at RB3LYP-UB3LYP(sum)/6-311+G(3df,3pd). Activation enthalpies at 470 K for forward and backward reaction are 29.9 and 21.4 kcal/mol, respectivelyk (exptl values, 28.23 +- 0.5 and 19.75 +- 0.7 kcal/mol), while the calculated reaction enthalpy at 298 K is 8.5 kcal/mol (exptl value, 8.5+-1.0 kcal/mol) in reasonable agreement with experiment. The calculated S-T splitting is 2.6 kcal/mol (after correction, 4.9 kcal/mol; exptl vlaue, 3.8 +- 0.5 kcal/mol at 298 K). It is shown that the UDFT description covers static correlation effects needed for the correct treatment of 2S. Total and on-top pair density reflect this, while Kohn-Sham orbitals and spin density have to be considered as physically not meaningful intermediates in line with the interpretation given by Perdew, Savin, and Burke (Phys. Rev, A 1995, 51, 4531).
机译：如果考虑以下因素，DFT计算可提供（Z）-hex-3-ene-1,5-diyne 1的Bergman反应的可靠描述。 （a）在先前描述变得不稳定的那些位置，必须将沿反应路径的受限DFT（RDFT）计算替换为不受限制的DFT（UDFT）计算。在对-二氢苯双基2的该区域中就是这种情况，其具有显着的多参考特征。 （b）LSD和纯GGA功能比混合功能更稳定，而混合功能可以直接与这些功能的组成相关。随着不稳定性的增加，RDFT计算导致S-T分裂和2的几何以及Bergman反应的能量学中的误差增加。 （c）LSD和GGA功能低估了1的Bergman反应的能垒。当考虑到正确的实验垒时，这变得很明显，而以前的DFT研究并未做到这一点。 （d）使用H3 B3LYP官能团（平均误差为2.7 kcal / mol）的RDFT / UDFT混合描述提供了对Bergman反应的最佳描述。尽管B3LYP功能模块相当不稳定，但其半经验校准有助于补偿GGA功能模块对障碍的典型低估，这表明混合功能模块的性能不一定与其稳定性有关。 （e）将总和公式应用于双自由基2的UB3LYP能量可以改善对Bergman反应的描述，从而获得最可靠的数据，即RB3LYP-UB3LYP（sum）/ 6-311 + G（3df，3pd）。向前和向后反应在470 K下的活化焓分别为29.9 kcal / mol和21.4 kcal / mol（exptl值，分别为28.23±0.5和19.75±0.7 kcal / mol），而在298 K下计算的反应焓为8.5 kcal / mol。 （实验值，8.5±1.0 kcal / mol）与实验合理吻合。计算的S-T分裂为2.6kcal / mol（校正后为4.9kcal / mol; expt vlaue，在298K下为3.8±0.5kcal / mol）。结果表明，UDFT描述涵盖了正确处理2S所需的静态相关效应。总和顶部对的密度反映了这一点，而根据Perdew，Savin和Burke的解释，必须将Kohn-Sham轨道和自旋密度视为在物理上没有意义的中间体（Phys。Rev，A 1995，51， 4531）。

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