Instituto de Química, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos 149, 21941-909, Rio de Janeiro, Brazil.
Instituto de Física, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos 149, 21941-972, Rio de Janeiro, Brazil.
Phys Chem Chem Phys. 2019 Dec 7;21(45):24984-24992. doi: 10.1039/c9cp04964a. Epub 2019 Nov 11.
Ring strain energy is a very well documented feature of neutral cycloalkanes, and influences their structural, thermochemical and reactivity properties. In this work, we apply density functional theory and high-level coupled cluster calculations to describe the geometry and relative stability of CH˙ radical cations, whose cyclic isomers are prototypes of singly-charged cycloalkanes. Molecular ions with the mentioned stoichiometry were produced via electron impact experiments using a gaseous cyclohexane sample (20-2000 eV). From our calculations, in addition to structures that resemble linear and branched alkenes as well as distinct conformers of cyclohexane, we have found low-lying species containing three-, four- and five-membered rings with the presence of an elongated C-C bond. Remarkably, the stability trend of these ring-bearing radical cations is anomalous, and the three-membered species are up to 11.3 kcal mol more stable than the six-membered chair structure. Generalized Valence Bond calculations and the Spin Coupled theory with N electrons and M orbitals were used in conjunction with the Generalized Product Function Energy Partitioning (GPF-EP) method and Interference Energy Analysis (IEA) to describe the chemical bonding in such moieties. Our results confirm that these elongated C-C motifs are one-electron sigma bonds. Our calculations also reveal the effects that drive thermochemical preference of strained systems over their strained-free isomers, and the origin of the unusual stability trend observed for cycloalkane radical cations.
环应变能是中性环烷烃的一个非常有文献记载的特征，它影响它们的结构、热化学和反应性性质。在这项工作中，我们应用密度泛函理论和高级耦合簇计算来描述 CH˙自由基阳离子的几何形状和相对稳定性，其环状异构体是单电荷环烷烃的原型。通过使用气态环己烷样品（20-2000 eV）进行电子冲击实验，产生了具有所述化学计量的分子离子。除了类似于线性和支链烯烃以及环己烷的不同构象的结构外，我们还从我们的计算中发现了含有三、四和五元环的低能物种，其中存在伸长的 C-C 键。值得注意的是，这些带环的自由基阳离子的稳定性趋势是异常的，并且三员物种比六员椅构象稳定高达 11.3 kcal/mol。广义价键计算和具有 N 个电子和 M 个轨道的自旋耦合理论与广义产物函数能量分割（GPF-EP）方法和干涉能分析（IEA）一起用于描述这些部分的化学键。我们的结果证实，这些伸长的 C-C 基序是单电子 sigma 键。我们的计算还揭示了驱动热化学应变体系相对于其无应变异构体的热力学偏好的影响，以及观察到的环烷烃自由基阳离子异常稳定性趋势的起源。
