Liu Shubin, Liu Lianghong, Yu Donghai, Rong Chunying, Lu Tian
Research Computing Center, University of North Carolina, Chapel Hill, North Carolina 27599-3420, USA.
Phys Chem Chem Phys. 2018 Jan 17;20(3):1408-1420. doi: 10.1039/c7cp07678a.
The steric effect is one of the most widely used chemical concepts in chemistry, yet a generally accepted implementation of its formulation and quantification from a theoretical viewpoint, if even possible, is still an unaccomplished task. Based on the energetics viewpoint of our earlier proposal using the Weizsäcker kinetic energy as a quantification of the energy contribution from the steric effect as well as our recent work using steric force as a reliable local descriptor to account for the origin of the stereoselective propensity in chemical reactions, in this work, we systematically examine the local behavior and general applicability of another closely related quantity, steric charge. To this end, its physical origin and physiochemical properties are formalized and highlighted, and are shown with numerical illustrations and verifications. To showcase its usefulness in appreciating chemical reactivity, we present three case studies of steric charge in this work. In the case of ethane flexible rotation, we found that the eclipsed conformer is more sterically charged than the staggered conformer, leading to the reduced stability and higher energy of the former. For the case of S2 reactions, a remarkable linear correlation has been obtained between the steric charge on the central atom in the transition state and the reaction barrier height of S2 reactions, in good agreement with experimental findings. In the case of stereoselectivity properties for the nucleophilic addition of carbonyl compounds, we found that steric charge is equally applicable as steric force to justify the stereoselective origin for the nucleophilic attack to the carbonyl carbon atom with different substituent groups. Put together, our results from the present study should pave the way towards the general use of steric charge, together with steric energy and steric force, as an insightful global and local descriptor to appreciate and quantify chirality and stereoselectivity related phenomena in chemical processes and transformations.
空间效应是化学中应用最广泛的化学概念之一，但从理论角度对其进行公式化和量化的普遍接受的方法，即便有可能，仍是一项未完成的任务。基于我们早期提出的以魏茨泽克动能作为空间效应能量贡献量化的能量学观点，以及我们最近将空间力作为可靠的局部描述符来解释化学反应中立体选择性倾向起源的工作，在本研究中，我们系统地研究了另一个密切相关的量——空间电荷的局部行为和普遍适用性。为此，我们对其物理起源和物理化学性质进行了形式化和突出展示，并通过数值示例和验证进行了说明。为了展示其在理解化学反应性方面的有用性，我们在本研究中给出了三个空间电荷的案例研究。在乙烷的柔性旋转案例中，我们发现重叠构象比交叉构象具有更高的空间电荷，导致前者稳定性降低且能量更高。对于S2反应的案例，在过渡态中心原子上的空间电荷与S2反应的反应势垒高度之间获得了显著的线性相关性，与实验结果高度吻合。在羰基化合物亲核加成的立体选择性性质案例中，我们发现空间电荷与空间力同样适用于解释不同取代基对羰基碳原子亲核进攻的立体选择性起源。综上所述，我们本研究的结果应为空间电荷与空间能和空间力一起作为有洞察力的全局和局部描述符，用于理解和量化化学过程及转化中与手性和立体选择性相关的现象的普遍应用铺平道路。
