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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >The Isomerization Barrier in Cyanocyclobutadienes: An ab Initio Multireference Average Quadratic Coupled Cluster Study
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The Isomerization Barrier in Cyanocyclobutadienes: An ab Initio Multireference Average Quadratic Coupled Cluster Study

机译：氰基环丁二烯中的异构化障碍：从头开始多参考平均二次耦合簇研究

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摘要

The energy profiles of the isomerization of mono, di-, and tetracyano-substituted cyclobutadienes (CBDs) are computed at the multireference average quadratic coupled cluster/complete active space self-consistent field level of theory. It was found that the energy barrier heights for the automerization reaction are 2.6 (tetracyano_CBD), 5.1 (1,3-dicyano-CBD), and 6.4 (cyano-CBD) kcal mol~(-1), implying that they are lowered relative to that in the parent CBD (6.4 kcal mol~(-1), the monosubstituted derivative being an exception. Since the free CBD shuttles between two equivalent structures even at low temperature of 10 K, it follows that bond-stretch isomerism does not take place in cyanocyclobutadienes. Instead, these compounds exhibit rapid fluxional interconversion at room temperature between two bond-stretch isomers by the double bond flipping mechanism. The reason behind the decrease in the barrier heights is identified as a slightly enhanced resonance effect at the saddle points separating two (equivalent) bond-stretch isomers, compared to that in the equilibrium structures, predominantly due to the diradical character of the former. It is also shown that the energy gap between the singlet ground state saddle point structure and the first triplet equilibrium geometry decreases upon multiple substitution by the cyano groups. The splitting of the S and T energy is small being within the range of 6.5-8.2 kcal mol~(-1).
机译：单，二和四氰基取代的环丁二烯（CBD）异构化的能量分布是在理论的多参考平均二次耦合簇/完全活性空间自洽场水平下计算的。结果表明，该自聚合反应的能垒高度为2.6 kcal mol〜（-1），为2.6（tetracyano_CBD），5.1（1,3-dicyano-CBD）和6.4（cyano-CBD）kcal mol〜（-1），表明它们相对降低了。相对于母体CBD（6.4 kcal mol〜（-1）），单取代的衍生物是一个例外，因为即使在10 K的低温下，游离的CBD仍在两个等效结构之间穿梭，因此不存在键-拉伸异构现象取而代之的是，这些化合物在室温下通过双键翻转机制在两个键-拉伸异构体之间表现出快速的通量互变，其势垒高度降低的原因被认为是在鞍点分离时共振效应略有增强。与平衡结构中的两个（等价）键-拉伸异构体相比，主要是由于前者的双自由基特性，还表明单重态基态鞍形位点之间的能隙int结构和第一三重态平衡几何结构在被氰基多次取代后降低。 S和T能的分裂很小，在6.5-8.2kcal mol·（-1）的范围内。

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