"In Situ" Orbital Correlations.

ConspectusOrbital correlation diagrams are central to chemistry. Based on the symmetry compatibility and orbital overlap amplitude, they link the energy-ordered frontier molecular orbitals (MOs) of reactants and products and have long been a powerful and essential tool for understanding chemical interactions (reactions) and molecular properties. The frontier MOs typically include the highest occupied MOs (HOMOs) and the lowest unoccupied MOs (LUMOs), along with a few nearby orbitals of the reactants. However, it is also known that some reactions cannot be well explained with a few frontier MOs. The main drawback of traditional orbital correlation diagrams is that the orbital energies of the reactants shown in the diagram are calculated assuming they are in free, isolated states. But orbital energy levels can be significantly shifted by external fields and the existence of neighboring molecules. In other words, orbital energy levels can be notably reshuffled when we put reactants "physically" (via electrostatic interactions, Pauli repulsion, and van der Waals interactions) together, even without "chemical" interactions (via orbital mixtures or electron transfers).Here, we introduce a novel concept, "in situ" orbital correlation, and demonstrate its applications. This concept is based on our developed block-localized wave function (BLW), which is the simplest variant of ab initio valence bond (VB) theory. The uniqueness of the BLW method lies in its ability to derive orbital energies of a molecule self-consistently in the presence of other species or external fields, as a BLW solution essentially corresponds to a hypothetical diabatic (or resonance) state, a mathematical construct in which all electron transfers between interacting species are "disabled". In such a way, we can correlate orbitals by considering the field (physical) effects from neighboring species even without any orbital (chemical) interactions.This "in situ" orbital correlation concept was first proposed in the study of the activation mechanism of CO by the diboryne compound B2(NHCR)2, where we demonstrated that when CO approaches B2(NHCR)2, there is a HOMO-LUMO swap in B2(NHCR)2 primarily due to the Pauli repulsion from the carbon lone pair of CO, leading to the compatibility of HOMO and HOMO-1 of B2(NHCR)2 with both π* orbitals of CO. Since then, this concept has been adopted in much of our research. For instance, in our most recent study of NCCL- anions (L = N2, CO, CS), which exhibit notable geometric differences, "in situ" orbital correlation diagrams reveal an orbital swap in the fragment NCC- with the approach of the ligand L and subsequently confirm the C(0) theory proposed by the Frenking group. Previously, we explored the "anti-electrostatic" nature of the Al-Mg bond and confirmed that the bond is purely ionic. This contradicts the view from frontier orbitals of Al(I) and Mg compounds, which exhibit a perfect match for a dative covalent bond between them. Now, with the help of the "in situ" orbital correlation diagram, it becomes obvious that the metal-metal bond is a typical ionic bond, because when the Mg compound is brought close, the energy level of the HOMO of Al(I) compound decreases significantly, leading to a reversal of the HOMO-LUMO energy level order and the extension of the HOMO-LUMO band gap and subsequently minimal probability of any electron transfer. We expect that the novel concept of "in situ" orbital correlation will fundamentally enrich our understanding of chemical reactions, electron transfer pathways, and molecular bonding.