Ventura Elizete, Rodrigues Gessenildo Pereira, Leitão Ezequiel Fragoso Vieira, do Monte Silmar Andrade
Departamento de Química, CCEN, Universidade Federal da Paraíba, 58059-900 João Pessoa, Brazil.
Unidade Acadêmica de Ciências Exatas e da Natureza, Universidade Federal de Campina Grande, Cajazeiras, PB 58900-000, Brazil.
ACS Omega. 2024 Aug 2;9(32):34981-34989. doi: 10.1021/acsomega.4c04914. eCollection 2024 Aug 13.
For many years researchers believed that hydrocarbons only contain covalent bonds. However, since 1985 Okamoto et al. demonstrated the formation of hydrocarbon salts in several systems, demolishing the structural principle that hydrocarbons only contain covalent bonds. Despite the great importance of this outcome to the study of chemical bonds, quantum chemical calculations on these systems are essentially nonexistent. The stability of the hydrocarbon ions along with the steric hindrance associated with the formation of the covalent bond contribute to their occurrence either in solution (dissociated) or in the solid state. These facts along with the common formation of ion pairs in solvents of low polarity motivated us to search for hydrocarbon ion pairs in the gas phase. Its energetics has also been studied in four nonprotic solvents, through a continuum solvation model (CPCM). DFT and CASSCF calculations indicate a metastable and highly polar ion pair between the tricyclopropylcyclopropenylium cation and a simplified Kuhn's anion. The barrier to the covalent structure varies from ∼4.8 to 14.4 kcal/mol, while the energy difference between the ion pair and the covalent form varies from ∼4.3 to 25.4 kcal/mol. The obtained theoretical results along with previous experimental results suggest the following strategy to obtain kinetically and thermodynamically stable hydrocarbon ion pairs: choose very stable hydrocarbon ions and systematically increase the steric hindrance between them.
多年来，研究人员一直认为碳氢化合物只含有共价键。然而，自1985年冈本等人在多个体系中证明了碳氢化合物盐的形成以来，打破了碳氢化合物只含有共价键的结构原理。尽管这一结果对化学键研究非常重要，但对这些体系的量子化学计算基本上不存在。碳氢离子的稳定性以及与共价键形成相关的空间位阻导致它们在溶液（解离状态）或固态中出现。这些事实以及在低极性溶剂中常见的离子对形成促使我们在气相中寻找碳氢离子对。通过连续溶剂化模型（CPCM），还在四种非质子溶剂中研究了其能量学。密度泛函理论（DFT）和完全活性空间自洽场（CASSCF）计算表明，三环丙基环丙烯鎓阳离子与简化的库恩阴离子之间存在亚稳且高度极化的离子对。共价结构的势垒在4.8至14.4千卡/摩尔之间变化，而离子对与共价形式之间的能量差在4.3至25.4千卡/摩尔之间变化。获得的理论结果与先前的实验结果表明，获得动力学和热力学稳定的碳氢离子对的策略如下：选择非常稳定的碳氢离子，并系统地增加它们之间的空间位阻。
