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WIREs Comput Mol Sci
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A primer on qualitative valence bond theory – a theory coming of age

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Abstract The use of valence bond (VB) theory as a general and wide‐ranging qualitative matrix of ideas and predictions is highlighted. Applications are demonstrated for chemical reactivity, properties of polyradicals, excited states, and chemical bonding. Highlighted subtopics include estimations of reaction barriers from simple properties of reactants and products; predictions of stable intermediates in organic, inorganic, and organometallic reactions; and predictions of reaction stereoselectivity. Additional coverage includes a pictorial model for ground and excited states of polyenes and polyradicals, predictions of Hund's rule violations, spin density polarizations and shifts, and a bonding rule in excited states of conjugated molecules. Discussion of new bonding ideas shows the far‐reaching prospects of qualitative VB theory. © 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 18‐29 DOI: 10.1002/wcms.7 This article is categorized under: Structure and Mechanism > Molecular Structures

(a) Covalent and ionic structures of a bond. (b) Mixing of valence bond (VB) structures. (c) Repulsive interactions in VB theory.

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(a) The covalent ground state and π‐excited states of allyl radical. (b) Spin‐density distribution in the two states. (Reprinted with permission from Ref 5. Copyright 2007 Wiley).

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Diradicals 1 and 2 and their most spin alternant determinants.

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Barriers, for H‐abstraction reactions, calculated semiempirically using the valence bond state correlation diagram (VBSCD) parameters and Eq. (2), plotted against VB ab initio computed barriers.

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Predictions of stereochemistry of electron transfer (ET) and substitution (SUB) processes, based on FO‐VB considerations, in reactions of nucleophiles (Nu:) and cation radicals.

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The effect on an intermediate state: a low triple ionic structure in the left leads to a stable minimum (FHF) in left‐hand side. Removal of just one electron results in the transition state [(FHF) in the right‐hand side. (Reprinted with permission from Ref 5. Copyright 2007 Wiley).

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Free energy barriers for nucleophilic cleavage reaction of an ester (shown) plotted against the vertical ionization energy of the nucleophile; the localized and delocalized nucleophiles generate two separate family lines with different slopes corresponding to different f values in Eq. (6). (Reprinted with permission from Ref 5. Copyright 2007 Wiley).

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The promotion gap in the nucleophilic‐attack step of a nucleophile X: on the carbonyl group of an ester. (Reprinted with permission from Ref 5. Copyright 2007 Wiley).

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The ground and promoted states for the cycloaddition of two π‐bonds.

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A valence bond state correlation diagram (VBSCD) with a large promotion gap leads to a transition state (TS) (left‐hand side), whereas one with a very small gap will lead to a stable delocalized cluster (right‐hand side).

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Valence bond (VB) diagram models, R and P are ground states of reactants and products, R* and P* are promoted excited states: (a) Valence bond state correlation diagram (VBSCD) for an elementary step, and (b) Valence bond configuration mixing diagram (VBCMD) describing a stepwise mechanism. (Reprinted with permission from Ref 5. Copyright 2007 Wiley).

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