Valence bond curve crossing diagram

Fig. 13. Valence bond curve-crossing diagram for substrate sulfoxidation by [Fe =0(Por+ )SH].

FIGURE 2.1 Valence bond curve-crossing diagram, for the Jt-component, interconversion of two Kekule structural forms along the bond alternation coordinate (bj mode). VG and QMRE correspond to vertical gap and quantum mechanical resonance energy, respectively. The two putative possibilities represent that in one case (a) the vertical gap is high and therefore a localized structure is preferred in the other ease (b) a small vertical gap results in bringing the stability to a fully delocalized symmetric structure. 

The concept of diabatic states dates back to the beginnings of molecular quantum mechanics, since it is implicit in the work of Landau and Zener on the electronic transitions occurring at a curve crossing (see Valence Bond Curve Crossing Models). Similar concepts underlie the drawing of correlation diagrams, first introduced by Hund and Mulliken around 1930, and subsequently exploited, among others, by Woodward and Hoffmann, to formulate their famous rules (see Photochemistry). 

S. Shaik, P. C. Hiberty, Adv. Quant. Chem. 26, 100 (1995). Valence Bond Mixing and Curve Crossing Diagrams in Chemical Reactivity and Bonding. 

G. Sini, S. S. Shaik, J.-M. Lefour, G. Ohanessian, P. C. Hiberty, J. Phys. Chem. 93, 5661 (1989). Quantitative Valence Bond Computation of a Curve Crossing Diagram for a Model SN2 reaction H + CH3H HCH3 + H . 

VBSCD Valence bond state correlation diagram. A VB diagram that views the barrier formation as a result of avoided crossing between two state curves that are anchored in the ground and two excited states of reactants and products. The VBSCD is a paradigm for the barrier in chemical reactions (see Chapter 6). 

Shaik S, Hiberty PC (1995) Valence bond mixing and curve crossing diagrams in chemical reactivity and bonding. Adv Quantum Chem 26 99-163... 

The general question is whether the avoided crossing diagram is an oversimplified model or does it faithfully account for the barrier problem Specifically (i) Can the adiabatic curve be reproduced rigorously and shown to follow the energetic behavior that has been postulated by the qualitative theory (ii) Do the / G, and B parameters follow the predicted trends or estimations (iii) Can a mixture of only two valence bond curves reproduce an accurate adiabatic potential surface ... 

Quantitative Valence Bond Computation of a Curve Crossing Diagram for a Model Sn2 Reaction, H + CH3H HCH3 + H . 

Valence Bond Computations of Curve Crossing Diagrams for a Gas Phase Reaction,... 

This article discusses key elements of the valence bond (VB) curve crossing diagram which has been formulated to meet the requirements of an interface model, and has been applied in the last decade and a half to a variety of reactivity problems in organic as well as organometallic chemistry. "... 

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