Valence Bond Modeling of Barriers in the Nonidentity Hydrogen Abstraction Reactions, X‘• + H−X → X‘−H + X• (X‘ ≠ X = CH3, SiH3, GeH3, SnH3, PbH3)

Song, Lingchun; Wu, Wei; Dong, Kunming; Hiberty, Philippe C.; Shaik, Sason

doi:10.1021/jp026438y

Cited by 46 publications

(74 citation statements)

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“…These are the promotion gap, G, the height of the crossing point, DE c , given as a fraction of the gap, ¦G, and the resonance energy of the transition state, B, which results from avoided crossing and VB mixing. [21,35,39] While full variation is suitable for the adiabatic curve that obeys the variational theorem, this may not necessarily be the case for the diabatic curves. Except for their asymptotic points, the diabatic curves are not physical states, and therefore a straightforward variational procedure may result in a wrong description of the diabatic curve.…”

Section: Methodsmentioning

confidence: 99%

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An Accurate Barrier for the Hydrogen Exchange Reaction from Valence Bond Theory: Is this Theory Coming of Age?

Song

Hiberty

et al. 2003

Chemistry A European J

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One of the landmark achievements of quantum chemistry, specifically of MO-based methods that include electron correlation, was the precise calculation of the barrier for the hydrogen-exchange reaction (B. Liu, J. Chem. Phys. 1973, 58, 1925; P. Siegbahn, B. Liu, J. Chem. Phys. 1978, 68, 2457). This paper reports an accurate calculation of this barrier by two recently developed VB methods that use only the eight classical VB structures. To our knowledge, the present work is the first accurate ab initio VB barrier that matches an experimental value. Along with the accurate barrier, the VB method provides accurate bond energies and diabatic quantities that enable the barrier height to be analyzed by the VB state correlation diagram approach, VBSCD (S. Shaik, A. Shurki, Angew. Chem. 1999, 111, 616; Angew. Chem. Int. Ed. Engl. 1999, 38, 586). This is a proof of principal that VB theory with appropriate account of dynamic electron correlation can achieve quantitative accuracy of reaction barriers, and still retain a compact and interpretable wave function. A sample of S(N)2 barriers and dihalogen bonding energies, which are close to CCSD(T) and G2(+) values, show that the H(3) problem is not an isolated case, and while it is premature to conclude that VB theory has come of age, the occurrence of this event is clearly within sight.

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Section: Methodsmentioning

confidence: 99%

“…(8)]. [35,39] G % 0.75DE ST (8) This equation, the derivation of which was elaborated in previous treatments of the VBSCD, [35,39] is a good approximation to G. However, a precise estimate of G is given in Equation 7, as the average of the singlet-to-triplet excitation and the corresponding bond energy [Eq. (9)] where D is the bond energy of H 2.…”

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confidence: 99%

An Accurate Barrier for the Hydrogen Exchange Reaction from Valence Bond Theory: Is this Theory Coming of Age?

Song

Hiberty

et al. 2003

Chemistry A European J

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“…As in our previous studies, [38][39][40] the reaction coordinate Q is defined as the bond order difference between the bond that is formed and the one that is broken in the reaction [Eq. (10)]:…”

Section: The Reaction Coordinatementioning

confidence: 99%

“…[38][39][40] For the Menshutkin reaction in gas phase, the a value of CH 3 ÀCl is 1.305, and the a value of CH 3 ÀNH 3 is 1.409. With this definition of Q, the path stretches from À1 (reactants) to + 1 (products).…”

Section: The Reaction Coordinatementioning

confidence: 99%

“…[27] The VBSCD model has been described in detail for the general case of non-identity reactions [39] and will be only briefly reiterated here. Such diagrams display, in addition to the ground state potential energy curve, two diabatic curves, one corresponding to the Lewis structure of the reactants, the other to the Lewis structure of the products.…”

Section: Analysis By Use Of the Vbscd Modelmentioning

confidence: 99%

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The Menshutkin Reaction in the Gas Phase and in Aqueous Solution: A Valence Bond Study

Ying

et al. 2007

ChemPhysChem

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The recently developed (L. Song, W. Wu, Q. Zhang, S. Shaik, J. Phys. Chem. A 2004, 108, 6017-6024) valence bond method coupled to a polarized continuum model (VBPCM) is applied to the Menshutkin reaction, NH3+CH3Cl-->CH3NH3(+)+Cl-, in the gas phase and in aqueous solution. The computed barriers and reaction energies at the level of the breathing orbital VB method (P. C. Hiberty, J. P. Flament, E. Noizet, Chem. Phys. Lett. 1992, 189, 259), BOVB and VBPCM//BOVB, are comparable to CCSD(T) and CCSD(T)//PCM results and to experimental values in solution. The gas-phase reaction is endothermic and leads to an ion-pair complex via a late transition state. By contrast, the reaction in the aqueous phase is exothermic and leads to separate solvated ions as reaction products, via an early transition state. The VB calculations provide also the reactivity parameters needed to apply the valence bond state correlation diagram method, VBSCD (S. Shaik, A. Shurki, Angew. Chem. Int. Ed. 1999, 38, 586). It is shown that the reactivity parameters along with their semiempirical derivations provide together a satisfactory qualitative and quantitative account of the barriers.

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Computational Quantum Chemistry for Free‐Radical Polymerization

Coote

2004

Encyclopedia of Polymer Science and Technology

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COMPUTATIONAL CHEMISTRYVol. 9 efficient algorithms, have made it possible to study the mechanism and kinetics of chemical reactions via computer. In computational quantum chemistry, one can calculate from first principles the barriers, enthalpies, and rates of a given chemical reaction, together with the geometries of the reactants, products, and transition structures. It also provides access to useful related quantities such as the ionization energies, electron affinities, radical stabilization energies, and singlettriplet gaps of the reactants, and the distribution of electrons within the molecule or transition structure. Quantum chemistry can provide a "window" on the reaction mechanism, and assumes only the nonrelativistic Schrödinger equation and values for the fundamental physical constants. Quantum chemistry is particularly useful for studying complex processes such as free-radical polymerization (see RADICAL POLYMERIZATION). In free-radical polymerization, a variety of competing reactions occur and the observable quantities that are accessible by experiment (such as the overall reaction rate, the overall molecular weight distribution of the polymer, and the overall monomer, polymer, and radical concentrations) are a complicated function of the rates of these individual steps. In order to infer the rates of individual reactions from such measurable quantities, one has to assume both a kinetic mechanism and often some additional empirical parameters. Not surprisingly then, depending upon the assumptions, enormous discrepancies in the so-called "measured" values can sometimes arise. Quantum chemistry is able to address this problem by providing direct access to the rates and thermochemistry of the individual steps in the process, without recourse to such model-based assumptions.Of course, quantum chemistry is not without limitations. Since the multielectron Schrödinger equation has no analytical solution, numerical approximations must instead be made. In principle, these approximations can be extremely accurate, but in practice the most accurate methods require inordinate amounts of computing power. Furthermore, the amount of computer power required scales exponentially with the size of the system. The challenge for quantum chemists is thus to design small model reactions that are able to capture the main chemical features of the polymerization systems. It is also necessary to perform careful assessment studies, in order to identify suitable procedures that offer a reasonable compromise between accuracy and computational expense. Nonetheless, with recent advances in computational power, and the development of improved algorithms, accurate studies using reasonable chemical models of free-radical polymerization are now feasible.Quantum chemistry thus provides an invaluable tool for studying the mechanism and kinetics of free-radical polymerization, and should be seen as an important complement to experimental procedures. Already quantum chemical studies have made major contributions to our understanding of fre...

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Valence Bond Modeling of Barriers in the Nonidentity Hydrogen Abstraction Reactions, X‘• + H−X → X‘−H + X• (X‘ ≠ X = CH₃, SiH₃, GeH₃, SnH₃, PbH₃)

Cited by 46 publications

References 58 publications

An Accurate Barrier for the Hydrogen Exchange Reaction from Valence Bond Theory: Is this Theory Coming of Age?

An Accurate Barrier for the Hydrogen Exchange Reaction from Valence Bond Theory: Is this Theory Coming of Age?

The Menshutkin Reaction in the Gas Phase and in Aqueous Solution: A Valence Bond Study

Computational Quantum Chemistry for Free‐Radical Polymerization

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