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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Computational investigation of the conrotatory and disrotatory isomerization channels of bicyclo[1.1.0]butane to buta-1,3-diene: A completely renormalized coupled-cluster study
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Computational investigation of the conrotatory and disrotatory isomerization channels of bicyclo[1.1.0]butane to buta-1,3-diene: A completely renormalized coupled-cluster study

机译：双环[1.1.0]丁烷向buta-1,3-diene的旋转和旋转异构化通道的计算研究：完全归一化的偶合簇研究

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摘要

The conrotatory and disrotatory mechanisms of the isomerization of bicyclo[1.1.0]butane to trans-buta-1,3-diene have been computationally investigated with the CASSCF, MCQDPT2, (U)B3LYP, CCSD(T), CR-CCSD(T), and CR-CC(2,3) approaches. The coupled-cluster (CC) methods, including the CC approach with singles, doubles, and noniterative triples (CCSD(T)), and its completely renormalized (CR) extensions called CR-CCSD(T) and CR-CC(2,3), and the density functional theory B3LYP approach do an excellent job of correctly predicting the activation barrier for the conrotatory pathway, which corresponds to a weakly biradical transition state (TS), producing values within experimental error bars. In particular, the recently developed CR-CC(2,3) method gives 40.8 or 41.1 kcal/mol, in perfect agreement with the experimental value of 40.6 +/- 2.5 kcal/mol. The complete-active-space self-consistent-field (CASSCF) approach and the second-order multireference perturbation theory (MCQDPT2) are less accurate in describing the conrotatory barrier than CR-CC(2,3). The higher energy disrotatory pathway, which has not been characterized experimentally and which involves a strongly biradical TS, poses a great challenge for many methods. CCSD(T) fails, predicting the activation barrier for the disrotatory pathway significantly below the conrotatory barrier, contradicting the experimental result that the conrotatory pathway describes the mechanism. The strongly biradical character of the disrotatory TS, spin contamination, and the proximity of singlet and triplet potential energy surfaces cause difficulties for B3LYP, which does not link this TS with gauche-buta-1,3-diene. No such difficulties occur in the CASSCF calculations, which offer a proper description of the structure of the disrotatory TS that links it with the reactant and product molecules. The CR-CC(2,3) approach, which accurately balances dynamical and nondynamical correlations in systems containing closed-shell and biradical structures, predicts the activation enthalpy for the disrotatory mechanism of similar to 66 kcal/mol. CR-CCSD(T) gives similar to 69 kcal/mol. In agreement with experiment and earlier multireference configuration interaction calculations of Nguyen and Gordon, CR-CCSD(T) and CR-CC(2,3) favor the conrotatory mechanism. The CASSCF, MCQDPT2, and B3LYP methods correctly place the disrotatory barrier above the conrotatory one, but, on the basis of a comparison with the accurate CR-CC(2,3) results, they underestimate the activation energy for the disrotatory pathway. All CC approaches employed in this study produce very good estimates of the enthalpy of isomerization of bicyclo[1.1.0]butane into buta-1,3-diene, the experimental value of which is -25.9 +/- 0.4 kcal/mol, giving about -28 kcal/mol, when trans-buta-1,3-diene is used as a product, and -25 kcal/mol, when the nearly isoenergetic gauche-buta-1,3-diene rotamer is used as a product. The CC reaction enthalpies are more accurate than those obtained with CASSCF, MCQDPT2, and B3LYP.
机译：使用CASSCF，MCQDPT2，（U）B3LYP，CCSD（T），CR-CCSD（），通过计算研究了双环[1.1.0]丁烷异构化为反式-1,3-二烯的顺反旋转机理。 T）和CR-CC（2,3）方法。耦合群集（CC）方法，包括具有单，双和非迭代三元组（CCSD（T））的CC方法，以及称为CR-CCSD（T）和CR-CC（2）的完全重新规范化（CR）扩展， 3），并且密度泛函理论B3LYP方法在正确预测旋转路径的激活障碍方面做得非常出色，该旋转障碍对应于弱双基过渡态（TS），在实验误差条内产生值。特别是，最近开发的CR-CC（2,3）方法得到40.8或41.1 kcal / mol，与实验值40.6 +/- 2.5 kcal / mol完全吻合。完全活动空间自洽场（CASSCF）方法和二阶多参考扰动理论（MCQDPT2）在描述旋转障碍方面不如CR-CC（2,3）准确。更高的能量分解途径尚未进行实验表征，涉及强双自由基TS，对许多方法提出了巨大挑战。 CCSD（T）失败，预测旋转路径的激活障碍明显低于旋转障碍，这与旋转路径描述该机理的实验结果相矛盾。可旋转的TS的强双自由基特性，自旋污染以及单重态和三重态势能面的接近性给B3LYP带来了困难，而B3LYP并未将此TS与gauche-buta-1,3-diene连接。在CASSCF计算中不会出现此类困难，该计算提供了将旋转TS与反应物和产物分子连接的TS结构的正确描述。 CR-CC（2,3）方法可以精确地平衡包含闭壳和双自由基结构的系统中的动力学和非动力学相关性，并预测旋转机理的活化焓接近66 kcal / mol。 CR-CCSD（T）给出的近似值为69 kcal / mol。与Nguyen和Gordon的实验和较早的多参考配置相互作用计算一致，CR-CCSD（T）和CR-CC（2,3）支持旋转机制。 CASSCF，MCQDPT2和B3LYP方法正确地将旋转障碍置于旋转障碍之上，但是，在与准确的CR-CC（2,3）结果进行比较的基础上，它们低估了旋转路径的活化能。本研究中使用的所有CC方法均能很好地估计双环[1.1.0]丁烷异构化为1,3-二丁烯的焓，其实验值为-25.9 +/- 0.4 kcal / mol，得出当使用反丁-1,3-二烯作为产物时约为-28 kcal / mol，当使用几乎等能量的gauche-buta-1,3-二烯旋转异构体时为-25 kcal / mol。 CC反应焓比CASSCF，MCQDPT2和B3LYP获得的精确。

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