Advances in Charge Displacement Analysis

We
define new general density-based descriptors for the quantification
of charge transfer and polarization effects associated with the interaction
between two fragments and the formation of a chemical bond. Our aim
is to provide a simple yet accurate picture of a chemical interaction
by condensing the information on the charge rearrangement accompanying
it into a few chemically meaningful parameters. These charge displacement
(CD) parameters quantify the total charge displaced upon bond formation
and decompose it into a charge transfer component between the fragments
and charge rearrangements taking place within the fragments. We then
show how the new parameters can be easily calculated using the well-known
CD function, which describes the charge flow along a chosen axis accompanying
the formation of a bond. The approach presented here can be useful
in a wide variety of contexts, ranging from weak interactions to electronic
excitations to coordination chemistry. In particular, we discuss here
how the scheme can be used for the characterization of the donation
and back-donation components of metal–ligand bonds, in combination
with the natural orbitals for chemical valence (NOCV) theory. In doing
so, we discuss the interesting relationship between the proposed parameters
and the corresponding NOCV eigenvalues, commonly used as a measure
of the electron charge displacement associated with a given bonding
contribution. As a prototype case study, we investigate the bond between
a N-heterocyclic carbene and different metallic fragments. Finally,
we show that our approach can be used in combination with the energy
decomposition of the extended transition state method, providing an
estimate of both charge transfer and polarization contributions to
the interaction energy.

本研究旨在提出一种新颖的基于密度的通用描述符，以量化两个片段相互作用及化学键形成过程中伴随的电荷转移和极化效应。我们的目标是提供一个简洁而精确的化学相互作用图景，通过将与之相关的电荷重排信息浓缩为几个具有化学意义的参数。这些电荷位移（CD）参数量化了键形成过程中总电荷的位移，并将其分解为片段间的电荷转移以及片段内部的电荷重排。随后，我们展示了如何利用已知的CD函数，该函数描述了伴随键形成的电荷沿选定轴的流动，轻松计算出新的参数。该方法在多种情境下均具有实用价值，从弱相互作用到电子激发，直至配位化学领域。特别是，我们讨论了如何将此方案应用于金属-配体键的供电子和反供电子成分的表征，并结合化学价自然轨道（NOCV）理论。在此过程中，我们探讨了所提议参数与相应NOCV本征值之间的有趣关系，后者通常作为与特定键合贡献相关的电子电荷位移的度量标准。作为原型案例研究，我们调查了N-杂环卡宾与不同金属片段之间的键合。最终，我们展示了一种方法，即结合扩展过渡态方法的能量分解，可以估算电荷转移和极化对相互作用能的贡献。