The Bergman cyclization is an important reaction in which an enediyne cyclizes to produce a highly reactive diradical species, p-benzyne. Enediyne motifs are found in natural antitumor antibiotic compounds, such as calicheammicin and dynemicin. Understanding the energetics of cyclization is required to better control the initiation of the cyclization, which induces cell death. We computed the singlet and triplet potential energy surfaces for the Bergman cyclization of (Z)-hex-3-ene-1,5-diyne using the CCSD and EOM-SF-CCSD methods. The triplet enediyne and transition state were found to have C₂ symmetry, which contrasts with the singlet reactant and transition state that possess C_2v symmetry. We analyzed the frontier orbitals of both cyclization pathways to explain the large energetic barrier of the triplet cyclization. Reaction energies were calculated using CCSD(T)/cc-pVTZ single-point calculations on structures optimized with CCSD/cc-pVDZ. The singlet reaction was found to be slightly endothermic (ΔH_rxn = 13.76 kcal/mol) and the triplet reaction was found to be highly exothermic (ΔH_rxn = −33.29 kcal/mol). The adiabatic singlet−triplet gap of p-benzyne, computed with EOM-SF-CCSD/cc-pVTZ, was found to be 3.56 kcal/mol, indicating a singlet ground state.

中文翻译：

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一个从头计算的环化伯格曼的探索

Bergman环化是重要的反应，其中烯二炔环化以产生高反应性双自由基物质，对-苯并ben。在天然抗肿瘤抗生素化合物（例如加利车霉素和达尼米星）中发现了烯对炔基序。需要了解环化的能量学以更好地控制环化的引发，这会诱导细胞死亡。我们使用CCSD和EOM-SF-CCSD方法计算了（Z）-hex-3-ene-1,5-diyne的Bergman环化的单重态和三重态势能面。发现三重态烯二炔和过渡态具有C ₂对称性，这与具有C _{2 v}的单重态反应物和过渡态形成对比对称。我们分析了两个环化途径的前沿轨道，以解释三重态环化的高能垒。在使用CCSD / cc-pVDZ优化的结构上，使用CCSD（T）/ cc-pVTZ单点计算来计算反应能。发现单重态反应略微吸热（ΔH _rxn = 13.76 kcal / mol），发现三重态反应高度放热（ΔH _rxn = -33.29 kcal / mol）。用EOM-SF-CCSD / cc-pVTZ计算得出的对苯甲酰的绝热单重态-三重态间隙为3.56 kcal / mol，表明是单重态基态。