Ionic Curve

Figure 17. Approximate one-dimensional ionic and covalent diabatic potentials adapted from Reference 48 for CFjBr. Solid and dashed ionic curves are Rittner-type potentials for parent and fragment ions respectively, and heads and tails are covalent curves. The ionic asymptote is denoted by the arrow. The crossings are avoided dotted curves for the "crossing" near 4.3 A are the adiabatic curves resulting from configuration interaction between the diabatic ionic and covalent curves. (Adiabatic curves for the other crossings are omitted for simplicity.)...

In hydrogen fluoride the situation is different. For this molecule the ionic curve and the covalent curve are nearly coincident in the neighborhood of the equilibrium internuclear distance. In conse-... 

The covalent curve of Figure 3-5 has been drawn in the same way as for the HX curves. The ionic curve represents a Coulomb attractive potential and a repulsive potential b/R9f in which the constant b is given values which lead (with use of the Madelung constant and corresponding constant in the repulsive potential) to the correct interatomic distance in the corresponding crystal (Chap. 13). Polarization is neglected. 

Based on these differences, it is possible to represent the VBCMDs for typical nucleophilic substitution reactions on Si versus C as shown in Figs. 6.16a and b. In Fig. 6.16a, the ionic curve X SiL3 + X is very stable in the pentacoordinated geometry due to the electrostatic energy of the triple ion structure, much like the case of the (FIIF) species discussed before. Consequently, the pentacoordinated (XSiL3X) species will generally be a... 

FIGURE 6.Ans.2 Qualitative dissociation energy curves for the R—X bond in a polar solvent. (1) The covalent and ionic curves, m (s) and (T>r(s), shown in regular lines, while the gas- phase ionic curve [4>i(g)] is shown in a dashed line, (2) Covalent and ionic curves in solvent (thin curves), and their avoided crossing leading to the ground-state and twin-excited states (bold curves). 

We have chosen to model the diabatic ionic curve (H - -(R)) as a truncated Rittner potential with a turning-off function for the polarization term ... 

The parameter Y is essentially fixed by the requirement that the ionic curve pass through the crossing point, change repulsive term is small and... 

Based on our definition for the "practical" diabatic curves, we require that the ionic curve agree with the inner wall of the adiabatic curve. The fitted parameters A and p are listed in Table V. 

Figure 5 illustrates the essentially experimental hybrid potentials and the fitted ionic curve of LiH. Note that the ionic curve starts to deviate from the X z potential in the avoided crossing region and runs parallel to the ionic portion of the A z potential slightly outside the crossing region and crosses... 

Figure 6 illustrates the RKR turning points and the ionic curves of NaH, KH, RbH and CsH. The ionic curves all seem to be satisfactory in view of the very simple model we have employed. 

Figure 5. LiFl potential energy curves. The /I 2 and A 2 curves are the hybrid potentials of Ref. 24. Key -------, ionic curve of equation 1 , ionic curve without R polarization term and vertical------------------------, Rc.

Mechanism of production of excited states excited ionic curves... 

Any two-state VBSCD can be transformed into a VBCMD where the Heitler-London (HL) and ionic VB structures are plotted explicitly as independent curves, instead of being combined into state curves [11]. As a rule, ionic structures, which are the secondary VB configurations of polar-covalent bonds, lie above the covalent Heitler-London (HL) stmctures at the reactant and product geometries, and generally they cross the two HL stmctures above their own crossing point. In many cases, the ionic curve is low enough in... 

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