Laboratory of Quantum Chemistry, Faculty of Chemistry, University of Gdańsk, Wita Stwosza 63, 80-308 Gdańsk, Poland.
Henry Eyring Center for Theoretical Chemistry, Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States.
J Am Chem Soc. 2022 Jun 29;144(25):11348-11363. doi: 10.1021/jacs.2c03422. Epub 2022 Jun 14.
Electronic structure methods are now widely used to assist in the interpretation of many varieties of experimental data. The energies and physical characteristics (e.g., sizes, shapes, and spatial localization) of valence antibonding π* and σ* orbitals play key roles in a variety of chemical processes including photochemical reactions and electron attachment reductions and are used in Woodward-Hoffmann-type analyses to probe reaction energy barriers and energy surface intersections leading to internal conversion or intersystem crossings. One's ability to properly populate such valence antibonding orbitals within electronic structure calculations is often hindered by the presence of other molecular orbitals having similar energies. These intruding orbitals can be of Rydberg, pseudo-continuum, or dipole-bound characteristic. This article shows how, within the most widely available electronic structure codes, one can avoid the pitfalls presented by these intruding orbitals to properly populate a valence π* or σ* orbital and how to subsequently use that orbital in a calculation that includes electron correlation effects and thereby offers the possibility of chemically useful precision. Special emphasis is given to cases in which the electronic state is metastable.
电子结构方法现在被广泛用于帮助解释许多不同类型的实验数据。价反键π和σ轨道的能量和物理特性（例如大小、形状和空间定位）在各种化学过程中起着关键作用，包括光化学反应、电子附加还原反应，并在伍德沃德-霍夫曼型分析中用于探测反应能垒和导致内部转换或系间窜跃的能量表面交叉。在电子结构计算中正确填充这些价反键轨道的能力常常受到具有相似能量的其他分子轨道的存在的阻碍。这些侵入轨道可以是里德伯、伪连续或偶极束缚的特征。本文展示了在最广泛使用的电子结构代码中，如何避免这些侵入轨道带来的陷阱，正确填充价π或σ轨道，以及如何随后在包含电子相关效应的计算中使用该轨道，从而提供具有化学实用性的精度的可能性。特别强调了电子态是亚稳态的情况。
