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WIREs Comput Mol Sci
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Correlation diagram approach as a tool for interpreting chemistry: an introductory overview

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Abstract In the frameworks of both molecular orbital (MO) and valence bond (VB) theories, the correlation diagram approach aids understanding of a wide range of chemical phenomena. MO‐based correlation diagrams are based upon one‐electron MOs or electronic configuration functions and have been broadly applied to both thermal and photochemical reactions. VB‐based correlation diagrams utilize VB structures to express diabatic states and are especially powerful in analyzing chemical bonding and chemical reactivity. These approaches help us to understand the electronic behavior of molecules and fill the gap between intricate wave functions and the chemists' viewpoint. This paper provides an introductory overview of the correlation diagram approach, with a greater emphasis placed on VB state correlation diagrams. © 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 337–349 DOI: 10.1002/wcms.20 This article is categorized under: Structure and Mechanism > Molecular Structures

(a) Orbital correlation diagram for He (united atom) and H (separated atom). (b) Walsh diagram for H2O plotting orbital energy levels of valence electrons (solid line), as calculated at the HF/6–31G* level. The OH distances were fixed at 0.956 Å. To facilitate understanding of the stability, the average level is also indicated by a dashed line. (c) Orbital correlation diagrams for the interconversion between cyclobutene and butadiene.4 S and A denote symmetric and antisymmetric relation of orbitals, respectively, with respect to the σv plane (for the disrotatory mode) or the C2 axis (for the conrotatory mode).

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(a) Orbital interactions in the Diels–Alder reaction. (b) Key electron configurations of RBO correlation diagram analysis. Arrows in the figure represent electron‐shift modes, and RBO stands for a transformed localized orbitals. (Reprinted with permission from Ref 53. Copyright 2008 Wiley‐VCH GmbH & Co KG).

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Dissociation energy profile of LiF (a) in vacuum and (b) in solution. Note that ΦCOV and ΦION represent the VB structures [Li••F] and [Li+ F], respectively. (Reprinted with permission from Ref 49. Copyright 2005 American Chemical Society).

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VBSCD for the identity SN2 reaction of Cl exchange. The Lewis curves are shown with thin lines and the adiabatic curves with bold lines. (Reprinted with permission from Ref 39. Copyright 2004 American Chemical Society).

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VB correlation diagrams for the rebound processes of the (a) quartet and (b) doublet spin states of the PorFeIVOH/R intermediate. Thin lines represent putative diabatic states that correspond to VB structures alongside the curves, whereas thick lines indicate adiabatic states. (Reprinted with permission from Ref 37. Copyright 2008 American Chemical Society).

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Plots of the estimated barriers [Eq. (5)] (a) against the VB calculated barriers and (b) against the Marcus equation for the entire set of identity and nonidentity reactions. (Reprinted with permission from Ref 32. Copyright 2004 American Chemical Society).

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Dissociation energy curves for (a) H2 and (b) F2. Open circles represent the purely covalent VB structure. Filled circles represent the optimized covalent + ionic ‘exact’ground state. (Reprinted with permission from Ref 27. Copyright 2009 Wiley‐VCH GmbH & Co KG).

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Correlation diagrams for (a) CH5 and (b) SiH5 along the SN2 reaction coordinate, and (c) key VB structures 1–4. (Reprinted with permission from Ref 20. Copyright 1990 American Chemical Society).

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VB correlation diagrams for benzene along the b2u coordinate. (a) Diagram including the σ and π contributions. (b) Diagram including the π contributions only. (Reprinted with permission from Ref 16. Copyright 1996 American Chemical Society).

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Schematic illustrations of two different types of correlation diagrams: (a) VBSCD and (b) VBCMD. Thick lines represent adiabatic curves, whereas thin solid and broken lines represent diabatic curves.

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State correlation diagrams for the interconversion between cyclobutene and butadiene. (Reprinted with permission from Ref 6. Copyright 1965 American Chemical Society).

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