Valence Bond Diagrams for Chemical Reactivity

Chapter 6 describes two VB-diagrams (VBD) that model reactivity: the VB state-correlation-diagram (VBSCD) and the VB configuration-mixing diagram (VBCMD). VBSCDs involve two curves that stretch from the ground states of reactants ( R ) and products ( P ) to their respective promoted states, R → P * and P → R *, with promotion energies G R and G P . The two curves avoid their crossing and generate a transition state ( TS ), which is stabilized, relative to the crossing point, by a resonance energy B . These diagrams enable us to conceptualize barrier formation and estimate the energy barriers, based on G R , G P , B, and the reaction energy , ΔE RP . Moving along a coordinate, which reduces B to zero, creates a funnel for the respective photochemical reaction. VBCMDs involve an intermediate curve, which stretches below the crossing point of the VBSCD, and defines a reaction intermediate (e.g., SiH 5 − vs. CH 5 − ). Using fragment orbitals (FOs) instead of AOs generate FO–VB diagrams that enable to derive selection-rules.