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Multireference Configuration Interaction Methods – An Application to the Valence Isomerism in Cyclobutadieno-p-benzoquinone and its Diprotonated Form

机译:多参考构型相互作用方法–在环丁二烯对苯醌及其双质子化形式的价异构中的应用

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摘要

Multireference averaged quadratic coupled cluster (MR-AQCC) calculations for cyclobutadieno-p-benzoquinone indicate that valence bond isomers 1a and 1b can exist as distinct species. The energy barrier height for their interconversion are 4.6 and 4.5 kcal mol−1, respectively, what is by ca. 2 kcal mol−1 lower than in the parent cyclobutadiene, implying that they could perhaps exist only under extreme conditions, namely at very low temperatures. For double protonated cyclobutadieno-p-benzoquinone, the CASSCF calculations erroneously predict existence of two valence isomers, 2a and 2b, whereas the MR-AQCC calculations reveal that geometry of the double protonated species could be best described by structure 2b. This nicely illustrates the crucial role of dynamic correlation and the need for using a highly-correlated theoretical method including geometry optimization in studied molecules.
机译:环丁二烯-对-苯醌的多参考平均二次耦合簇(MR-AQCC)计算表明,价键异构体1a和1b可以作为不同的物种存在。它们相互转换的能垒高度分别为4.6 kcal mol-1和4.5 kcal mol-1。比母体环丁二烯低2 kcal mol-1,这表明它们可能仅在极端条件下(即非常低的温度下)存在。对于双质子化的环丁二烯-对苯醌,CASSCF计算错误地预测了两个化合价异构体2a和2b的存在​​,而MR-AQCC计算表明,双质子化物种的几何形状最好由结构2b来描述。这很好地说明了动态相关的关键作用,以及使用高度相关的理论方法(包括研究分子中的几何优化)的需求。

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