Atomic orbitals homonuclear diatomic molecules

The energies of The energies of the molecular orbitals of atomic orbitals homonuclear diatomic molecules of free atoms... 

Figure 7.14 Molecular orbital energy level diagram for first-row homonuclear diatomic molecules. The 2p, 2py, 2p atomic orbitals are degenerate in an atom and have been separated for convenience. (In O2 and F2 the order of

In the molecular orbital description of homonuclear diatomic molecules, we first build all possible molecular orbitals from the available valence-shell atomic orbitals. Then we accommodate the valence electrons in molecular orbitals by using the same procedure we used in the building-up principle for atoms (Section 1.13). That is,... 

The molecular orbital energy-level diagrams of heteronuclear diatomic molecules are much harder to predict qualitatitvely and we have to calculate each one explicitly because the atomic orbitals contribute differently to each one. Figure 3.35 shows the calculated scheme typically found for CO and NO. We can use this diagram to state the electron configuration by using the same procedure as for homonuclear diatomic molecules. 

There exists no uniformity as regards the relation between localized orbitals and canonical orbitals. For example, if one considers an atom with two electrons in a (Is) atomic orbital and two electrons in a (2s) atomic orbital, then one finds that the localized atomic orbitals are rather close to the canonical atomic orbitals, which indicates that the canonical orbitals themselves are already highly, though not maximally, localized.18) (In this case, localization essentially diminishes the (Is) character of the (2s) orbital.) The opposite situation is found, on the other hand, if one considers the two inner shells in a homonuclear diatomic molecule. Here, the canonical orbitals are the molecular orbitals (lo ) and (1 ou), i.e. the bonding and the antibonding combinations of the (Is) orbitals from the two atoms, which are completely delocalized. In contrast, the localization procedure yields two localized orbitals which are essentially the inner shell orbital on the first atom and that on the second atom.19 It is thus apparent that the canonical orbitals may be identical with the localized orbitals, that they may be close to the localized orbitals, that they may be identical with the completely delocalized orbitals, or that they may be intermediate in character. 

The molecular orbital diagram for the nitrogen monoxide molecule is shown in Figure 4.6. The orbitals are produced from the same pairs of atomic orbitals as in the cases of the homonuclear diatomic molecules of Section 4.2. 

Figure 2-12 Energy diagrams for a homonuclear diatomic molecule. Note that the differences in the energy levels of the atoms are larger than the energy differences between the molecular orbitals. Diagram (a) is appropriate for no interaction between 2s and 2p levels, and diagram (b) is appropriate for substantial interaction between 2s and 2p levels. Refer to pp. 36-38.

A comparison of EH and CNDO with experimental data has been made by Baetzold (30) for other metal homonuclear diatomic molecules. This work has employed the orbital exponents of Clementi et al. (10,11) and experimental atomic data for ionization potentials. Table III lists representative data for transition metal molecules calculated by CNDO and EH. No one procedure is universally superior to another. 

Up to now we have only considered combining two atoms of the same element to form homonuclear diatomic molecules. Now we shall consider what happens when the two atoms are different. First of all, how do the atomic orbitals of different elements differ They have the same sorts of orbitals Is, 2s, 2p, etc. and these orbitals will be the same shapes but the orbitals will have different energies. For... 

In this section we consider homonuclear diatomic molecules (those composed of two identical atoms) formed by elements in Period 2 of the periodic table. The lithium atom has a 1 s22s electron configuration, and from our discussion in the previous section, it would seem logical to use the Li Is and 2s orbitals to form the MOs of the Li2 molecule. However, the Is orbitals on the lithium atoms are much smaller than the 2s orbitals and therefore do not overlap in space to any appreciable extent (see Fig. 14.33). Thus the two electrons... 

A study of the contributions of the individual molecular orbital densities to the total force exerted on a nucleus in homonuclear diatomic molecules was made by Bader et al. (1967a). This study shows that the atomic polarizations binding the nuclei in these molecules do not arise primarily from the IcTg and l(Tu molecular orbitals whose densities correspond to slightly polarized Is-like atomic distributions. Indeed, in C2 these orbital densities exert small antibinding forces on the nuclei. It is the density of the 2atomic forces which bind the nuclei, while the densities of the 2cr and 3cTg orbitals are responsible for the oppositely directed atomic first moments. 

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