Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic.
Acc Chem Res. 2013 Apr 16;46(4):927-36. doi: 10.1021/ar300083h. Epub 2012 Aug 8.
Aromatic systems contain both σ- and π-electrons, which in turn constitute σ- and π-molecular orbitals (MOs). In discussing the properties of these systems, researchers typically refer to the highest occupied and lowest unoccupied MOs, which are π MOs. The characteristic properties of aromatic systems, such as their low ionization potentials and electron affinities, high polarizabilities and stabilities, and small band gaps (in spectroscopy called the N → V1 space), can easily be explained based on their electronic structure. These one-electron properties point to characteristic features of how aromatic systems interact with each other. Unlike hydrogen bonding systems, which primarily interact through electrostatic forces, complexes containing aromatic systems, especially aromatic stacked pairs, are predominantly stabilized by dispersion attraction. The stabilization energy in the benzene dimer is rather small (2.5 kcal/mol) but strengthens with heteroatom substitution. The stacked interaction of aromatic nucleic acid bases is greater than 10 kcal/mol, and for the most stable stacked pair, guanine and cytosine, it reaches approximately 17 kcal/mol. Although these values do not equal the planar H-bonded interactions of these bases (29 kcal/mol), stacking in DNA is more frequent than H-bonding and, unlike H-bonding, is not significantly weakened when passing from the gas phase to a water environment. Consequently, the stacking of aromatic systems represents the leading stabilization energy contribution in biomacromolecules and in related nanosystems. Therefore stacking (dispersion) interactions predominantly determine the double helical structure of DNA, which underlies its storage and transfer of genetic information. Similarly, dispersion is the dominant contributor to attractive interactions involving aromatic amino acids within the hydrophobic core of a protein, which is critical for folding. Therefore, understanding the nature of aromatic interactions, which depend greatly on quantum mechanical (QM) calculations, is of key importance in biomolecular science. This Account shows that accurate binding energies for aromatic complexes should be based on computations made at the (estimated) CCSD(T)/complete basis set limit (CBS) level of theory. This method is the least computationally intensive one that can give accurate stabilization energies for all common classes of noncovalent interactions (aromatic-aromatic, H-bonding, ionic, halogen bonding, charge-transfer, etc.). These results allow for direct comparison of binding energies between different interaction types. Conclusions based on lower-level QM calculations should be considered with care.
芳香体系既包含σ 电子又包含π 电子，进而构成σ 键和π 分子轨道（MO）。在讨论这些体系的性质时，研究人员通常会参考最高占据和最低未占据的 MO，也就是π MO。芳香体系的特征性质，如低电离能和电子亲和能、高极化率和稳定性以及小的能带隙（在光谱学中称为 N→V1 空间），很容易根据它们的电子结构来解释。这些单电子性质指出了芳香体系相互作用的特征。与主要通过静电力相互作用的氢键体系不同，包含芳香体系的配合物，特别是芳香堆积对，主要通过色散吸引来稳定。苯二聚体的稳定能相当小（2.5 kcal/mol），但随着杂原子取代而增强。芳香核酸碱基的堆积相互作用大于 10 kcal/mol，对于最稳定的堆积对，鸟嘌呤和胞嘧啶，其稳定能达到约 17 kcal/mol。尽管这些值与这些碱基的平面氢键相互作用（29 kcal/mol）不相等，但在 DNA 中堆积比氢键更常见，而且与氢键不同，从气相到水环境传递时不会显著减弱。因此，芳香体系的堆积是生物大分子及其相关纳米体系中主要的稳定能贡献。因此，堆积（色散）相互作用主要决定了 DNA 的双螺旋结构，这是其存储和遗传信息传递的基础。同样，色散是蛋白质疏水性核心中芳香族氨基酸之间吸引相互作用的主要贡献者，这对折叠至关重要。因此，了解芳香相互作用的性质对于生物分子科学至关重要，而芳香相互作用在很大程度上取决于量子力学（QM）计算。本文表明，对于芳香配合物的准确结合能，应该基于在（估计的）CCSD(T)/完全基组极限（CBS）理论水平上进行的计算。这种方法是计算量最小的方法，可以为所有常见类型的非共价相互作用（芳香-芳香、氢键、离子、卤键、电荷转移等）提供准确的稳定能。这些结果允许直接比较不同相互作用类型之间的结合能。基于较低 QM 计算的结论应谨慎考虑。
