Ayarde-Henríquez Leandro, Guerra Cristian, Pérez Patricia, Chamorro Eduardo
School of Physics, Trinity College Dublin, Dublin, Dublin 2, Ireland.
AMBER, Advanced Materials and BioEngineering Research Centre, Dublin, Dublin2, D02 CP49, Ireland.
J Mol Model. 2025 Jan 13;31(2):45. doi: 10.1007/s00894-024-06229-z.
This study meticulously examines the criteria for assigning electron rearrangements along the intrinsic reaction coordinate (IRC) leading to bond formation and breaking processes during the pyrolytic isomerization of cubane (CUB) to 1,3,5,7-cyclooctatetraene (COT) from both thermochemical and bonding perspectives. Notably, no cusp-type function was detected in the initial thermal conversion step of CUB to bicyclo[4.2.0]octa-2,4,7-triene (BOT). Contrary to previous reports, all relevant fluxes of the pairing density must be described in terms of fold unfolding. The transannular ring opening in the second step highlights characteristics indicative of a cusp-type catastrophe, facilitating a direct comparison with fold features. This fact underscores the critical role of density symmetry persistence near topographical events in determining the type of bifurcation. A fold-cusp unified model for scaling the polarity of chemical bonds is proposed, integrating ubiquitous reaction classes such as isomerization, bimolecular nucleophilic substitution, and cycloaddition. The analysis reveals that bond polarity index (BPI) values within the [0, 10] au interval correlate with cusp unfolding, whereas fold spans over a broader [10, ∞) au spectrum. These insights emphasize that the cusp polynomial is suitable for describing chemical processes involving symmetric electron density distributions, particularly those involving homolytic bond cleavages; in contrast, fold characterizes most chemical events.
Geometry optimization and frequency calculations were conducted using various DFT functionals. In line with recent findings concerning the rigorous application of BET, the characterization of bond formations and scissions via unfoldings was carried out by carefully monitoring the determinant of the Hessian matrix at all potentially degenerate CPs and their relative distance. The computed gas-phase activation enthalpies strongly align with experimental values, stressing the adequacy of the chosen levels of theory in describing the ELF topography along the IRC. The BPI was determined using the methodology proposed by Allen and collaborators.
本研究从热化学和键合的角度，细致地研究了在立方烷（CUB）热解异构化为1,3,5,7-环辛四烯（COT）过程中，沿着导致键形成和断裂过程的内禀反应坐标（IRC）分配电子重排的标准。值得注意的是，在CUB热转化为双环[4.2.0]辛-2,4,7-三烯（BOT）的初始步骤中未检测到尖点型函数。与先前的报道相反，配对密度的所有相关通量必须用折叠展开来描述。第二步中的跨环开环突出了指示尖点型突变的特征，便于与折叠特征进行直接比较。这一事实强调了在地形事件附近密度对称性持久性在确定分叉类型中的关键作用。提出了一个用于缩放化学键极性的折叠-尖点统一模型，该模型整合了异构化、双分子亲核取代和环加成等常见反应类型。分析表明，在[0, 10]原子单位区间内的键极性指数（BPI）值与尖点展开相关，而折叠跨越更宽的[10, ∞）原子单位谱。这些见解强调，尖点多项式适用于描述涉及对称电子密度分布的化学过程，特别是那些涉及均裂键断裂的过程；相比之下，折叠表征了大多数化学事件。
使用各种DFT泛函进行几何优化和频率计算。根据最近关于BET严格应用的研究结果，通过仔细监测所有潜在简并临界点（CP）处的海森矩阵行列式及其相对距离，通过展开来表征键的形成和断裂。计算得到的气相活化焓与实验值高度吻合，强调了所选理论水平在描述沿IRC的ELF地形方面的充分性。BPI是使用Allen及其合作者提出的方法确定的。
