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A Computational Investigation of the Stability of Cyclopropyl Carbenes

机译：环丙基卡宾酯稳定性的计算研究

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摘要

The conformations of dicyclopropyl,isopropyl cyclopropyl,and diisopropylcarbenes were optimized using density functional theory(B3LYP/6-31G(d)).We showed that the optimized geometries of carbenes with cyclopropyl groups are fully in accord with those expected for bisected W-shaped conformations,in which the effective hyperconjugation of a cyclopropyl group with singlet carbene can occur.The stabilization energies were evaluated at the B3LYP/6-31l+G(3df,2p)//B3LYP/6-31G(d)+ZPE level using an isodesmic equation.The relative stability of carbenes is in the order (c-Pr)2C:>(i-Pr)(oPr)C:>(i-Pr)_2C:,and a cyclopropyl group stabilizes carbene more than an isopropyl group by nearly 9 kcal/mol.Energies for the decomposition of diazo compounds to carbenes increase in the order(c-Pr)_2<(i-Pr)(c-Pr)<(i-Pr)_2 by approx 9 kcal/mol each.From a singlet-triplet energy gap(Esr)calculation,the singlet level is lower than the triplet level and the EST shows a trend similar to the stabilization energy calculations.For comparison,the optimized geometries and stabilization energies for the corresponding carbocations were also studied at the same level of calculation.The greater changes in geometries and the higher stabilization energies for carbocations compared to carbenes can explain the greater hyperconjugation effect.
机译：使用密度泛函理论（B3LYP / 6-31G（d））优化了二环丙基，异丙基环丙基和二异丙基卡宾的构象。构象，其中可以发生环丙基与单线态卡宾的有效超共轭。使用以下方法评估稳定能：卡宾的相对稳定性按（c-Pr）2C：>（i-Pr）（oPr）C：>（i-Pr）_2C：的顺序排列，环丙基比异丙基更稳定卡宾重氮化合物分解为卡宾的能量大约为9 kcal / mol，按（c-Pr）_2 <（i-Pr）（c-Pr）<（i-Pr）_2的顺序增加约9 kcal /从单重态-三重态能隙（Esr）计算，单重态水平低于三重态水平，并且EST表现出类似于稳定能计算的趋势为了进行比较，还以相同的计算水平研究了相应碳正离子的最佳几何构型和稳定能。与碳烯相比，碳正离子的几何构型变化越大，稳定能越高，可以解释更大的超共轭效应。

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