Institute of Chemistry, Edmond J. Safara Campus at Givat Ram, The Hebrew University, Jerusalem 9190401, Israel.
J Am Chem Soc. 2020 Nov 25;142(47):20002-20013. doi: 10.1021/jacs.0c09041. Epub 2020 Nov 12.
In this study, we address the long-standing issue-arising prominently from conceptual density functional theory (CDFT)-of the relative importance of electrostatic, i.e., "hard-hard", versus spin-pairing, i.e., "soft-soft", interactions in determining regiochemical preferences. We do so from a valence bond (VB) perspective and demonstrate that VB theory readily enables a clear-cut resolution of both of these contributions to the bond formation/breaking process. Our calculations indicate that appropriate local reactivity descriptors can be used to gauge the magnitude of both interactions individually, e.g., Fukui functions or HOMO/LUMO orbitals for the spin-pairing/(frontier) orbital interactions and molecular electrostatic potentials (and/or partial charges) for the electrostatic interactions. In contrast to previous reports, we find that protonation reactions cannot generally be classified as either charge- or frontier orbital-controlled; instead, our results indicate that these two bonding contributions generally interplay in more subtle patterns, only giving the impression of a clear-cut dichotomy. Finally, we demonstrate that important covalent, i.e., spin pairing, reactivity modes can be missed when only a single spin-pairing/orbital interaction descriptor is considered. This study constitutes an important step in the unification of CDFT and VB theory.
在这项研究中，我们解决了一个长期存在的问题，即概念密度泛函理论（CDFT）中静电相互作用（即“硬-硬”相互作用）和自旋配对相互作用（即“软-软”相互作用）对确定区域化学偏好的相对重要性。我们从价键（VB）的角度来解决这个问题，并证明 VB 理论可以很容易地清楚地区分这两种相互作用对成键/断键过程的贡献。我们的计算表明，可以使用适当的局部反应性描述符来单独衡量这两种相互作用的大小，例如，对于自旋配对/（前沿）轨道相互作用，可以使用福井函数或 HOMO/LUMO 轨道，对于静电相互作用，可以使用分子静电势（和/或部分电荷）。与以前的报告不同，我们发现质子化反应通常不能被归类为电荷控制或前沿轨道控制；相反，我们的结果表明，这两种键合贡献通常以更微妙的模式相互作用，只是给人以明显的二分法的印象。最后，我们证明，当只考虑单个自旋配对/轨道相互作用描述符时，可能会错过重要的共价，即自旋配对反应模式。这项研究是 CDFT 和 VB 理论统一的重要步骤。
