Curve crossing covalent-ionic

Figure 8.4. The first four E states of LiF near the ionic-covalent curve crossing region. The electric dipole moment is also given showing its change in relation to the crossing of the first two states. The energy curves refer to the left vertical axis and the dipole moment to the right.

Fig. 3-5.—Energy curves for the sodium chloride molecule. At very large internuclear distances the curve for the ionic structure lies above that for the covalent structure. The curves cross at 10.5 A, and at smaller inter-nuclear distances the ionic structure is the more stable one ...

Fig. 7 shows the changes in the occupation numbers of the covalent CH bonds, ionic CH bonds, covalent HH bond, ionic HH bond, and the other (doubly ionic) structures along IRC. The origin of the horizontal axis corresponds to TS and the left end of each curve to the equilibrium structure. The occupation numbers of CH and HH covalent bond structures change rapidly near TS and the curves cross immediately after TS (0.1 bohr(amu)1/2), while the occupation numbers of CH and HH ionic bond structures change slowly. 

The reactions of Sn with Cl2 and Br2 show forward scattering of the SnX product, with about 30—45% of the reaction energy appearing as translation of the products in the case of Sn + Cl2 [424]. The exact contribution to the reaction of the various spin-orbit states of tin, Sn(3P012), is unknown. Similarities between the results for Sn + Cl2 and those for Li + Cl2 [297] suggest an electron-jump mechanism, although the ionic—covalent curve crossing radius is quite small for Sn + Cl2 (— 2.9 A). 

The optical spectra of alkali halides provide another example of the effect of ionic-covalent curve crossing on spectra. 

The differential elastic cross section for scattering of potassium on several halogen compounds has been measured between 1 and 100 eV103104 in order to get information on the curve crossing between the ionic and covalent configuration in these systems. This experimental approach could also be used in cases where electronic excitation is important. 

One of the most intuitive examples to understand curve crossing and potential energy curve interaction, which complements the discussion of predissociation, is that of the interaction of ionic and covalent potentials (e.g. + B and A - - B) exhibiting the... 

Figure 15.5 Potential energy curve crossing, exemplified for ionic-covalent crossing of curves with the same symmetry (here E ). Solid lines diabatic curves, l/dia dashed lines adiabatic curves, l id internuclear separation of curve crossing...

Fig 7 a, b. Curve crossings of the HL (covalent) and ionic structures and the formation of ionic type reaction intermediates in ... 

A monotonic decrease of the cross section was measured in both channels for collision energies from 0.1 to 1 eV (attained by high-speed rotor acceleration of the atoms). This behaviour is typical of an ionic/covalent curve crossing mechanism and suggests that the reactions are initiated by an electron jump Xe + X2 Xe+... 

TTiese results were explained in Ref. 68 considering the crossings of the incoming Xe + IBr covalent curve with the ionic curves (1), Xe+ + IBr (X 2 +-), and (11), Xe+ + IBr(2ni/2)- The former (outer) crossing will lead to XeBr excimer formation, reaction (32b), because the IBr (X 2 ) ground state dissociates... 

The potentials during the approach motion of the reactants can be much modified when the reactants are not isolated but are coupled to an environment. In the gas phase, if the ionization potential is not low, the energy AEq needed to form an ion-pair is high and the curve crossing of the ionic and covalent potentials does not occur as the reactants approach (or recede). In water or another polar solvent. 

A related type of covalent-ionic curve cro.ssing occurs in the S l and El mechanisms. In these cases, though, the positive ion R is an encumbered carbenium ion, much larger than the H, and does not allow a compact ion-paired structure with significant electrostatic stabilization. Here, the solvent has to be involved and to stabilize the ion-paired structure below the crossing point of the two covalent forms. A similar description applies to the EIcb mechanism with the exception that here the ionic form is... 

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