Smalø Hans S, Astrand Per-Olof, Jensen Lasse
Department of Chemistry, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway.
J Chem Phys. 2009 Jul 28;131(4):044101. doi: 10.1063/1.3166142.
The electronegativity equalization model (EEM) has been combined with a point-dipole interaction model to obtain a molecular mechanics model consisting of atomic charges, atomic dipole moments, and two-atom relay tensors to describe molecular dipole moments and molecular dipole-dipole polarizabilities. The EEM has been phrased as an atom-atom charge-transfer model allowing for a modification of the charge-transfer terms to avoid that the polarizability approaches infinity for two particles at infinite distance and for long chains. In the present work, these shortcomings have been resolved by adding an energy term for transporting charges through individual atoms. A Gaussian distribution is adopted for the atomic charge distributions, resulting in a damping of the electrostatic interactions at short distances. Assuming that an interatomic exchange term may be described as the overlap between two electronic charge distributions, the EEM has also been extended by a short-range exchange term. The result is a molecular mechanics model where the difference of charge transfer in insulating and metallic systems is modeled regarding the difference in bond length between different types of system. For example, the model is capable of modeling charge transfer in both alkanes and alkenes with alternating double bonds with the same set of carbon parameters only relying on the difference in bond length between carbon sigma- and pi-bonds. Analytical results have been obtained for the polarizability of a long linear chain. These results show that the model is capable of describing the polarizability scaling both linearly and nonlinearly with the size of the system. Similarly, a linear chain with an end atom with a high electronegativity has been analyzed analytically. The dipole moment of this model system can either be independent of the length or increase linearly with the length of the chain. In addition, the model has been parametrized for alkane and alkene chains with data from density functional theory calculations, where the polarizability behaves differently with the chain length. For the molecular dipole moment, the same two systems have been studied with an aldehyde end group. Both the molecular polarizability and the dipole moment are well described as a function of the chain length for both alkane and alkene chains demonstrating the power of the presented model.
电负性均衡模型（EEM）已与点偶极相互作用模型相结合，以获得一个分子力学模型，该模型由原子电荷、原子偶极矩和双原子中继张量组成，用于描述分子偶极矩和分子偶极 - 偶极极化率。EEM已被表述为一种原子 - 原子电荷转移模型，允许对电荷转移项进行修正，以避免在无限距离的两个粒子以及长链情况下极化率趋近于无穷大。在本工作中，通过添加一个用于描述电荷在单个原子间传输的能量项，解决了这些缺点。原子电荷分布采用高斯分布，导致短距离处静电相互作用的阻尼。假设原子间交换项可描述为两个电子电荷分布的重叠，EEM还通过一个短程交换项进行了扩展。结果得到一个分子力学模型，其中绝缘和金属系统中电荷转移的差异是根据不同类型系统之间键长的差异来建模的。例如，该模型仅依靠碳σ键和π键之间键长的差异，就能用同一组碳参数对具有交替双键的烷烃和烯烃中的电荷转移进行建模。已获得长线性链极化率的解析结果。这些结果表明，该模型能够描述极化率随系统大小呈线性和非线性缩放。类似地，对具有高电负性端原子的线性链进行了解析分析。该模型系统的偶极矩既可以与链长无关，也可以随链长线性增加。此外，利用密度泛函理论计算的数据对烷烃和烯烃链的模型进行了参数化，其中极化率随链长的变化情况不同。对于分子偶极矩，研究了具有醛端基的相同两个系统。对于烷烃和烯烃链，分子极化率和偶极矩都能很好地描述为链长的函数，这证明了所提出模型的有效性。
