MOs of a Heteronuclear Diatomic Molecule

Rgure 2.3 MOs of a heteronuclear diatomic molecule. (pA and [Pg.31]

Acciuate MO expressions require the solution of the Roothaan equations for their determination. For qualitative discussion (but not quantitative work), it is useful to have simple approximations for heteronuclear diatomic MOs. In the crudest approximation, we can take each valence MO of a heteronuclear diatomic molecule as a linear combination of two AOs and < 6, one on each atom. (As the discussions of CO and BF show, this approximation is often quite inaccurate.) From the two AOs, we can form two MOs ... 

As another example of a heteronuclear diatomic molecule, consider the MO diagram for HF ... 

Diatomic molecules such as CO and NO, formed from atoms of two different elements, are called heteronuclear. We construct MOs for such molecules by following the procedure described earlier, with two changes. First, we use a different set of labels because heteronuclear diatomic molecules lack the inversion symmetry of homonuclear diatomic molecules. We therefore drop the g and u subscripts on the MO labels. Second, we recognize that the AOs on the participating atoms now correspond to different energies. For example, we combine the 2s AO of carbon and the 2s AO of oxygen to produce a bonding MO (without a node). 

We have restricted all of this to homonuclear diatomic molecules. These are obviously a very small subset of the possible diatomic molecules. It is time to move on to heteronuclear molecules. We already know what needs to be considered. Let s write some configurations first, then look at the MOs in detail. 

The treatment of heteronuclear diatomic molecules by LCAO-MO theory is not fundamentally different from the treatment of homonuclear diatomics, except that the MO s are not symmetric with respect to a plane perpendicular to and bisecting the intemuclear axis. The MO s are still constructed by forming linear combinations of atomic orbitals on the two atoms, but since the atoms are now different we must write them < a+ 0b> where A is not in general equal to 1. Thus these MO s will not in general represent non-polar bonding. As examples let us consider HC1, CO, and NO. 

The principles we have used in developing an MO description of homonuclear diatomic molecules can be extended to heteronuclear diatomic molecules—those in which the two atoms in the molecule are not the same—and we conclude this section with a fascinating heteronuclear diatomic molecule—nitric oxide, NO. 

In this section, we return to MO theory and apply it to heteronuclear diatomic molecules. In each of the orbital interaction diagrams constructed in Section 2.3 for hotnonuclear diatomics, the resultant MOs contained equal contributions from each atomic orbital involved. This is represented in equation 2.5 for the bonding MO in H2 by the fact that each of the wavefunctions and V 2 contributes equally to V mo> and the representations of the MOs in H2 (Figure 2.4) depict symmetrical orbitals. Now we look at representative examples of diatomics in which the MOs may contain different atomic orbital contributions, a scenario that is typical for heteronuclear diatomics. 

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