Xu Lu T, Dunning Thom H
Department of Chemistry, University of Illinois at Urbana-Champaign , 600 South Mathews Avenue, Urbana, Illinois 61801, United States.
J Chem Theory Comput. 2015 Jun 9;11(6):2496-507. doi: 10.1021/acs.jctc.5b00104.
The ground state, X1Σg+, of N2 is a textbook example of a molecule with a triple bond consisting of one σ and two π bonds. This assignment, which is usually rationalized using molecular orbital (MO) theory, implicitly assumes that the spins of the three pairs of electrons involved in the bonds are singlet-coupled (perfect pairing). However, for a six-electron singlet state, there are five distinct ways to couple the electron spins. The generalized valence bond (GVB) wave function lifts this restriction, including all of the five spin functions for the six electrons involved in the bond. For N2, we find that the perfect pairing spin function is indeed dominant at Re but that it becomes progressively less so from N2 to P2 and As2. Although the perfect pairing spin function is still the most important spin function in P2, the importance of a quasi-atomic spin function, which singlet couples the spins of the electrons in the σ orbitals while high spin coupling those of the electrons in the π orbitals on each center, has significantly increased relative to N2 and, in As2, the perfect pairing and quasi-atomic spin couplings are on essentially the same footing. This change in the spin coupling of the electrons in the bonding orbitals down the periodic table may contribute to the rather dramatic decrease in the strengths of the Pn2 bonds from N2 to As2 as well as in the increase in their chemical reactivity and should be taken into account in more detailed analyses of the bond energies in these species. We also compare the spin coupling in N2 with that in C2, where the quasi-atomic spin coupling dominants around Re.
N₂的基态X¹Σg⁺是一个典型的教科书示例，展示了一个具有三键的分子，该三键由一个σ键和两个π键组成。这种归属通常使用分子轨道（MO）理论进行合理化解释，隐含地假设参与键合的三对电子的自旋是单重态耦合（完美配对）。然而，对于一个六电子单重态，存在五种不同的方式来耦合电子自旋。广义价键（GVB）波函数消除了这一限制，包括了参与键合的六个电子的所有五个自旋函数。对于N₂，我们发现完美配对自旋函数在平衡核间距（Re）处确实占主导地位，但从N₂到P₂和As₂，其主导程度逐渐降低。尽管完美配对自旋函数在P₂中仍然是最重要的自旋函数，但一种准原子自旋函数的重要性相对于N₂显著增加，该准原子自旋函数使每个中心上σ轨道中的电子自旋单重态耦合，而使π轨道中的电子自旋高自旋耦合，并且在As₂中，完美配对和准原子自旋耦合基本上处于相同地位。沿着元素周期表，键合轨道中电子自旋耦合的这种变化可能导致Pn₂键强度从N₂到As₂相当显著的下降以及它们化学反应性的增加，并且在对这些物种键能的更详细分析中应予以考虑。我们还比较了N₂和C₂中的自旋耦合，在C₂中，准原子自旋耦合在平衡核间距（Re）附近占主导地位。
