Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, USA.
J Am Chem Soc. 2012 Oct 24;134(42):17520-5. doi: 10.1021/ja303676q. Epub 2012 Oct 9.
The stacking interaction between π systems is a well-recognized structural motif, but stacking between σ systems was long considered of secondary importance. A recent paper points out that σ stacking can reach the energy of chemical bonds and concludes that "σ/σ and π/π interactions are equally important" (Fokin, A. F.; Gerbig, D.; Schreiner, P. R. J. Am. Chem. Soc. 2011, 133, 20036). Our analysis shows that strong dispersion interaction requires rigid subsystems and good fits of their repulsive potential walls, conditions which are satisfied for both graphenes and larger graphanes (perhydrographenes). Comparison of the dimerization energies of decalin and perhydrocoronene with those of the naphthalene and coronene dimers at the coupled cluster (CC) CCSD(T) level confirms the substantial σ-stacking energies in graphanes but shows lower binding energies than do the B97D calculations of Fokin et al. Graphane dimerization energies are substantially lower at the CC level than the corresponding π-stacking energies: the value for perhydrocoronene is only 67% of the value for coronene, and the difference increases with system size. Our best estimate for the dimerization energy of naphthalene is 6.1 kcal/mol. Spin-component scaled MP2 is unbalanced: it gives only 70% of the CCSD(T) binding energy in σ dimers. The B3LYP-D3 method compares very well with CC for both σ and π dimers at the van der Waals minimum but underestimates the binding at larger distances. We used the largest possible atomic basis for these systems with 64-bit arithmetic, half-augmented-pVDZ, and the results were corrected for basis set incompleteness at the MP2 level.
π 体系之间的堆积相互作用是一种公认的结构模式，但 σ 体系之间的堆积作用长期以来被认为是次要的。最近的一篇论文指出，σ 堆积可以达到化学键的能量，并得出结论，“σ/σ 和 π/π 相互作用同样重要”（Fokin，A. F.；Gerbig，D.；Schreiner，P. R. J. Am. Chem. Soc. 2011, 133, 20036）。我们的分析表明，强色散相互作用需要刚性子系统和其排斥势能壁的良好拟合，这些条件对于石墨烯和更大的石墨烷（全氢化石墨烯）都满足。将十氢萘和全氢化 coronene 的二聚化能与萘和 coronene 二聚体在耦合簇（CC）CCSD（T）水平的二聚化能进行比较，证实了石墨烷中存在大量的 σ 堆积能，但与 Fokin 等人的 B97D 计算相比，结合能较低。在 CC 水平上，石墨烷的二聚化能远低于相应的 π 堆积能：全氢化 coronene 的二聚化能仅为 coronene 的 67%，并且随着体系尺寸的增加，差异增大。我们对萘二聚化能的最佳估计值为 6.1 kcal/mol。自旋分量标度 MP2 是不平衡的：它只给出了 σ 二聚体中 CCSD（T）结合能的 70%。B3LYP-D3 方法在范德华最低点对 σ 和 π 二聚体与 CC 相比非常好，但在较大距离处低估了结合能。我们对这些体系使用了尽可能大的原子基组，使用 64 位算术、半增 pVDZ，并在 MP2 水平上对基组不完备性进行了校正。
