Transition states, avoided crossing states and valence-bond mixing: fundamental reactivity paradigms

Shaik, Sason; Reddy, A. Chandrasekhar

doi:10.1039/ft9949001631

Cited by 53 publications

(52 citation statements)

References 56 publications

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“…Similar applications of VB theory to deduce the structure of conical intersections in photoreactions were done by other groups. 25,104,141,142 VB theory enables a very straightforward account of environmental effects, such as those imparted by solvents and/or protein pockets. The solvent effect is exerted through the ionic VB configurations and leads to polarization of charge and to changes of potential energy surface via covalent-ionic VB curve crossings.…”

Section: Concluding Remarks and Prospectivementioning

confidence: 99%

A survey of recent developments in ab initio valence bond theory

Hiberty

Shaik

2006

J Comput Chem

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Starting from the 1980s and onwards, Valence Bond theory has been enjoying renaissance that is characterized by the development of a growing number of ab initio methods, and by many applications to chemical reactivity and to the central paradigms of chemistry. Owing the increase of computational power of modern computers and to significant advances in the methodology, valence bond theory begins to offer a sound and attractive alternative to Molecular Orbital theory. This review aims at summarizing the most important developments of ab initio valence bond methods during the last two or three decades, and is primarily devoted to a description of what the various methods can actually achieve within their specific scopes and limitations. Key available softwares are surveyed.

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Section: Concluding Remarks and Prospectivementioning

confidence: 99%

A survey of recent developments in ab initio valence bond theory

Hiberty

Shaik

2006

J Comput Chem

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“…However, the contention that every TS must have such a bound excited state cannot be justified in general. Thus, in cases that are not geometrically constrained, or those with high ionicity, [15] the twin-excited state will find alternative geometries in which it is more stable (e.g., H 3 , H À 3 , the S N 2 TS [15] ). Similarly, it does not apply to conformational interconversion, as we recently verified for the conformational isomerization of cycloheptatriene: An exhaustive search for an excited state with B 1 symmetry and a planar structure did not reveal any bound state.…”

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confidence: 97%

“…This mode corresponds to motion along the coordinate that interchanges the two VB structures used to construct the twin pair. Figure 1 represents an ideal situation; it must be recognized that the minimum of the twin state may collapse [15] as a result of mixing with other states and distortion along the other degrees of freedom. It is therefore essential to ascertain first the conditions for survival of the energy minimum of the twin state.…”

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confidence: 98%

“…It is therefore essential to ascertain first the conditions for survival of the energy minimum of the twin state. From previous experience, avoidance of situations typified by strong mixing with ionic structures, [2] while conferring some structural rigidity on the system, [15] is likely to lead to survival of the twin-state minimum. This is the reason for our choice of the particular target molecule in question, which seems to meet the basic requirements.…”

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confidence: 99%

“…[15] We set out to increase the complexity of such systems by designing an artificial regulatory unit in which several molecular components are coupled in a well-adjusted manner. [16,17] Here, we demonstrate that the interaction of a redox-responsive chelating aminoferrocene, a redox-switchable oxaferrocene cryptand, a Zn 2 and a Na salt, and a polyazamacrocycle leads to a system in which the availability of sodium ions can be controlled.…”

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confidence: 99%

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The Twin-Excited State as a Probe for the Transition State in Concerted Unimolecular Reactions: The Semibullvalene Rearrangement

Zilberg

Haas

Danovich

1998

Angewandte Chemie International Edition

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A twin of the transition state, which can be investigated spectroscopically and can thus supply information about the structure of the transition state, has now been characterized for the Cope rearrangement of semibullvalene (shown below). It involves an excited state with B2 symmetry and results from a linear combination of the ground‐state wave functions of (mirror‐image) reactant and product.

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Increased-valence structures and hypervalent molecules

Harcourt

1996

Int. J. Quantum Chem.

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rnHypervalent molecules may involve the use of increased-valence structures to provide valence bond descriptions of their electronic structure. For electron-rich molecules with four electrons distributed among three overlapping (nuclear-centered) atomic orbitals, the increased-valence structures are Y-A * B and ? -A--B. Each structure involves a fractional electron-pair bond and a one-electron bond. It is deduced that the Armstrong-Perkins-Stewart valence of the A atom is able to exceed unity in each of these structures when the three bonding electrons occupy nonorthogonal localized molecular orbitals. It is also shown that increased valence for the A atom does not occur when the four electrons occupy localized molecular orbitals to give the valence-bond structure Y-A-B with three overlapping atomic orbitals, and the same number of orbital variational parameters as occurs in the wave functions for either of the increasedvalence structures. The results of ab initio valence bond calculations with minimal basis sets are reported for HT1, CH;, HF,, F,, CIF,, and FF,, and the resulting wave functions for resonance between six canonical Lewis structures are related to those for resonance between the two increased-valence structures. The use of the latter structures to indicate how electronic reorganization proceeds via one-electron delocalizations for S,Z reactions is redescribed, and an elementary argument is presented to deduce that this class of reactions cannot involve the delocalization of a pair of electrons in concert from the nucleophile. Increased-valence wave functions are used to deduce an expression for the avoided crossing for the transition state of the identity S , Z reaction.

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Transition states, avoided crossing states and valence-bond mixing: fundamental reactivity paradigms

Cited by 53 publications

References 56 publications

A survey of recent developments in ab initio valence bond theory

A survey of recent developments in ab initio valence bond theory

The Twin-Excited State as a Probe for the Transition State in Concerted Unimolecular Reactions: The Semibullvalene Rearrangement

Increased-valence structures and hypervalent molecules

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