Silvestrelli Pier Luigi, Tessarolo Matteo, Seif Abdolvahab, Ambrosetti Alberto
Dipartimento di Fisica e Astronomia "G. Galilei", Università degli Studi di Padova, Via Marzolo 8, I-35131 Padova, Italy.
J Phys Chem A. 2024 Aug 1;128(30):6229-6239. doi: 10.1021/acs.jpca.4c03494. Epub 2024 Jul 20.
Recently we investigated from first-principles screening properties in systems where small molecules, characterized by a finite electronic dipole moment, are encapsulated in different nanocages. The most relevant result was the observation of an antiscreening effect in alkali-halide nanocages characterized by ionic bonds: in fact, due to the relative displacement of positive and negative ions, induced by the dipole moment of the encapsulated molecule, these cages act as dipole-field amplifiers, different from what is observed in carbon fullerene nanocages, which exhibit instead a pronounced screening effect. Here we extend the study to another class of nanostructures: the nanotubes. Using first-principles techniques based on density functional theory, we studied the properties of endohedral nanotubes obtained by encapsulation of a water molecule or a linear HF molecule. A detailed analysis of the effective dipole moment of the complexes and of the electronic charge distribution suggests that screening effects crucially depend not only on the nature of the intramolecular bonds but also on the size and the shape of the nanotubes and on the specific encapsulated molecule. As observed in endohedral nanocages, screening is maximum in covalent-bond carbon nanotubes, while it is reduced in partially ionic nanotubes, and an antiscreening effect is observed in some ionic nanotubes. However, in this case, the scenario is more complex than in corresponding ionic nanocages. In fact the specific geometric structure of alkali-halide nanotubes turns out to be crucial for determining the screening/antiscreening behavior: while nanotubes with transversal section can exhibit an antiscreening effect, which quantitatively depends on the number of layers in the longitudinal direction, instead nanotubes with section are always characterized by a reduction of the total dipole moment so that a screening behavior is observed. Our results show that, even in nanotube structures, in principle one can tune the dipole moment and generate electrostatic fields at the nanoscale without the aid of external potentials.
最近，我们从第一性原理出发，研究了小分子（其特征为具有有限的电子偶极矩）被封装在不同纳米笼中的系统的筛选特性。最相关的结果是在以离子键为特征的碱金属卤化物纳米笼中观察到了反屏蔽效应：事实上，由于被封装分子的偶极矩引起的正负离子的相对位移，这些笼子起到了偶极场放大器的作用，这与在碳富勒烯纳米笼中观察到的情况不同，后者表现出明显的屏蔽效应。在这里，我们将研究扩展到另一类纳米结构：纳米管。使用基于密度泛函理论的第一性原理技术，我们研究了通过封装水分子或线性HF分子获得的内包层纳米管的特性。对复合物的有效偶极矩和电子电荷分布的详细分析表明，屏蔽效应不仅关键取决于分子内键的性质，还取决于纳米管的尺寸和形状以及特定的封装分子。正如在内包层纳米笼中观察到的那样，共价键碳纳米管中的屏蔽作用最大，而在部分离子型纳米管中屏蔽作用减弱，并且在一些离子型纳米管中观察到了反屏蔽效应。然而，在这种情况下，情况比相应的离子型纳米笼更为复杂。事实上，碱金属卤化物纳米管的特定几何结构对于确定屏蔽/反屏蔽行为至关重要：具有横截面的纳米管可以表现出反屏蔽效应，其定量取决于纵向的层数，而具有截面的纳米管总是以总偶极矩的减小为特征，从而观察到屏蔽行为。我们的结果表明，即使在纳米管结构中，原则上也可以在不借助外部势的情况下调节偶极矩并在纳米尺度上产生静电场。
