Effect of External Charges in MO Treatments

The approach discussed above can provide a qualitative description of the effect of external fields on bond-breaking processes. For example, consider the H2 molecule (HA — HB) in the presence of an Li+ ion 3 A away from HB on the A-B axis. To study this problem, we assume that there is no charge migration to the Li location (so that Pc = 0) and that fiAC = pBC = 0 since the Li+ ion is sufficiently far from HA and HB. In this case, we can write the H matrix as 

The main feature of the new wave function is its polarization by the field of the ion. That is, the presence of the ion changes the effective ionization energies of HA and HB and tends to pull electrons toward HB. The effect of the external ion on the potential surface for bond breaking is shown in Fig. 1.4. 

Solution 1.6. Use Program l.B, which is given at the end of this chapter. 

The Huckel approach does not really consider the interaction between the bonding electrons. Including this electrostatic interaction in the calcula- 

FIGURE 1.4. A simplified MO potential surface for an H2 molecule in the presence of a Li+ ion. The Li+ is held 3 A from IIfi and the bond length RAB is changed by moving H . 

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