Dynamic Electron Correlation: A Fragments-in-Molecules Approach

Malcolm, Nathaniel O. J.; McDouall, Joseph J. W.

doi:10.1021/j100099a020

Cited by 12 publications

(4 citation statements)

References 7 publications

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“…Various approaches are under development to deal with this problem. This activity is marked in part by a rapidly growing body of research167–205 on many issues of combining DFT with multi‐configurational energies or densities and on the closely related subject of orbitally dependent density functionals. One required area of progress is developing correlation functionals that provide accurate results when used with a high percentage of Hartree–Fock exchange, rather than requiring a cancellation of errors between exchange and correlation functionals 206.…”

Section: Quantum Mechanical Description Of Chemical Bondingmentioning

confidence: 99%

Valence bond theory for chemical dynamics

Truhlar

2006

J Comput Chem

108

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This essay provides a perspective on several issues in valence bond theory: the physical significance of semilocal bonding orbitals, the capability of valence bond concepts to explain systems with multireferences character, the use of valence bond theory to provide analytical representations of potential energy surfaces for chemical dynamics by the method of semiempirical valence bond potential energy surfaces (an early example of specific reaction parameters), by multiconfiguration molecular mechanics, by the combined valence bond-molecular mechanics method, and by the use of valence bond states as coupled diabatic states for describing electronically nonadiabatic processes (photochemistry). The essay includes both ab initio and semiempirical approaches.

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Section: Quantum Mechanical Description Of Chemical Bondingmentioning

confidence: 99%

Valence bond theory for chemical dynamics

Truhlar

2006

J Comput Chem

108

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“…Use of larger basis sets lowers the calculated barriers, but the tendency remains the same (see below). The ClHCl system has been much studied at the ab initio level. − A recent accurate ab initio calculation by Schatz et al, at the level of RCCSD(T) with an augmented triple-ζ basis set, reports a barrier of 10.1 kcal/mol for a bent transition state, the collinear transition state lying only 1.3 kcal/mol higher. Another accurate calculation has been reported by Fox and Schlegel for the FHF system, yielding a barrier of 17.5 kcal/mol for reaction 1 via a bent transition state .…”

Section: Introductionmentioning

confidence: 99%

Barriers of Hydrogen Abstraction vs Halogen Exchange: An Experimental Manifestation of Charge-Shift Bonding

et al. 2006

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This paper shows that the differences between the barriers of the halogen exchange reactions, in the H + XH systems, and the hydrogen abstraction reactions, in the X + HX systems (X = F, Cl, Br), measure the covalent-ionic resonance energies of the corresponding X-H bonds. These processes are investigated using CCSD(T) calculations as well as the breathing-orbital valence bond (BOVB) method. Thus, the VB analysis shows that (i) at the level of covalent structures the barriers are the same for the two series and (ii) the higher barriers for halogen exchange processes originate solely from the less efficient mixing of the ionic structures into the respective covalent structures. The barrier differences, in the HXH vs XHX series, which decrease as X is varied from F to I, can be estimated as one-quarter of the covalent-ionic resonance energy of the H-X bond. The largest difference (22 kcal/mol) is calculated for X = F in accord with the finding that the H-F bond possesses the largest covalent-ionic resonance energy, 87 kcal/mol, which constitutes the major part of the bonding energy. The H-F bond belongs to the class of "charge-shift" bonds (Shaik, S.; Danovich, D.; Silvi, B.; Lauvergnat, D. L.; Hiberty, P. C. Chem. Eur. J. 2005, 21, 6358), which are all typified by dominant covalent-ionic resonance energies. Since the barrier difference between the two series is an experimental measure of the resonance energy quantity, in the particular case of X = F, the unusually high barrier for the fluorine exchange reaction emerges as an experimental manifestation of charge-shift bonding.

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“…We expect that this difference in the stability of starting material should significantly affect the energetics of the reaction pathways. In carbon systems we know in detail how dienes and dienophiles behave throughout the reactions in terms of orbital interactions, − and extensive ab initio computational studies − have been carried out so far concerning many types of Diels−Alder reactions of carbon compounds. Digermacyclobutene and digermabutadiene and their interconversion have been also studied from molecular orbital computations .…”

Section: Introductionmentioning

confidence: 99%

Thermal Addition of Disilacyclobutenes and Acetylene: A Theoretical Study on Diels−Alder Type Reactions

et al. 1999

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The reaction pathways for the thermal additions of disilacyclobutenes and acetylene are discussed from B3LYP density-functional-theory computations. Butadiene is more stable in energy than cyclobutene, the corresponding ring compound, whereas disilabutadiene is less stable than disilacyclobutene. From detailed analyses of the potential energy surfaces, disilabutadienes formed by thermal ring opening of disilacyclobutenes are confirmed to play a central role in the addition reactions as intermediates in a manner similar to the Diels−Alder reaction. The activation energies for the symmetry-allowed conrotatory ring opening of 1,2-disilacyclobut-3-ene, 1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene, and 3,4-benzo-1,1,2,2-tetramethyl-1,2-disilacyclobutene are 41.5, 46.7, and 61.9 kcal/mol, respectively, and the activation energies for the Diels−Alder coupling reactions with acetylene are 1−4 kcal/mol when measured from the disilabutadiene intermediates at the B3LYP/6-31G** level of theory. Therefore the ring opening should be the rate-determining step in these reactions; once the four-membered ring of disilacyclobutene is opened by heat treatment, the addition reactions should readily take place, leading to six-membered ring products. The transition state for the addition of 1,4-disila-1,3-butadiene and acetylene is symmetrical with respect to the Si−C bonds being formed, whereas those of 1,1,4,4-tetramethyl-1,4-disila-1,3-butadiene and acetylene and of 1,2-bis(dimethylsilylene)cyclohexa-3,5-diene and acetylene are not symmetrical. There are good reasons that such asymmetrical transition states occur in these Diels−Alder reactions; one is the asymmetrical frontier orbitals in the methyl-substituted disilabutadienes, and the other is just a steric effect of the methyl groups.

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Dynamic Electron Correlation: A Fragments-in-Molecules Approach

Cited by 12 publications

References 7 publications

Valence bond theory for chemical dynamics

Valence bond theory for chemical dynamics

Barriers of Hydrogen Abstraction vs Halogen Exchange: An Experimental Manifestation of Charge-Shift Bonding

Thermal Addition of Disilacyclobutenes and Acetylene: A Theoretical Study on Diels−Alder Type Reactions

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