Dunning Thom H, Xu Lu T
Department of Chemistry, University of Washington, Seattle, Washington 98195, USA.
J Chem Phys. 2022 Aug 28;157(8):084124. doi: 10.1063/5.0104693.
We extended our studies of the effect of dynamical electron correlation on the covalent bonds in the AH and AF series (A = B-F) to the recoupled pair bonds in the excited aΣ states of the CH and CF molecules. Dynamical correlation is energetically less important in the aΣ states than in the corresponding XΠ states for both molecules, which is reflected in smaller changes in bond energies (D). Changes in the equilibrium bond distance (R) and vibrational frequency (ω), on the other hand, are influenced by the changes in the slope and curvature of the dynamical electron correlation energy as a function of the internuclear distance R, E(R). In the CH(aΣ) state, these changes are much smaller than in the CH(XΠ) state, but in the CF(aΣ) state, they are larger, reflecting a significant difference in the shapes of E(R) curves. At large R, the shape of E(R) curves for covalent and recoupled pair bonds is similar although different in magnitude. For the CH(aΣ) state, E(R) has a minimum at R = R + 0.72 Å as the orbitals associated with the formation of the recoupled pair bond switch places. E(R) for the CF(aΣ) state decreases continuously throughout the bound region of the potential energy curve because the dynamical correlation energy associated with the electrons in the lone pair orbitals is increasing. These results support our earlier conclusion that we still have much to learn about the nature of dynamical electron correlation in molecules.
我们将对AH和AF系列（A = B - F）中共价键上动态电子关联效应的研究扩展到了CH和CF分子激发态aΣ中的重耦合对键。对于这两种分子，动态关联在aΣ态中的能量重要性低于相应的XΠ态，这反映在键能（D）的较小变化上。另一方面，平衡键长（R）和振动频率（ω）的变化受动态电子关联能随核间距R变化的斜率和曲率变化的影响，即E（R）。在CH（aΣ）态中，这些变化比在CH（XΠ）态中小得多，但在CF（aΣ）态中，它们更大，这反映了E（R）曲线形状的显著差异。在大R时，共价键和重耦合对键的E（R）曲线形状相似，尽管幅度不同。对于CH（aΣ）态，随着与重耦合对键形成相关的轨道交换位置，E（R）在R = R + 0.72 Å处有一个最小值。CF（aΣ）态的E（R）在势能曲线的束缚区域内持续下降，因为与孤对轨道中电子相关的动态关联能在增加。这些结果支持了我们早期的结论，即我们对分子中动态电子关联的本质仍有很多需要了解的地方。
