Molecular Orbitals for Homonuclear Diatomic Molecules

The shapes and energy ordering of the molecular orbitals for homonuclear diatomic molecules. The scheme on the left is applicable to 02 and F2, while that on the right is applicable to other diatomics of the same period. 

Molecular orbitals for homonuclear diatomic molecules can be formed by adding together atomic orbitals on each atom. 

Molecular Orbitals for Homonuclear Diatomic Molecules of the Second-Period Elements... 

Wahl plotted the contours of the near Hartree-Fock molecular orbitals of homonuclear diatomic molecules from H2 through F2. Figure 13.20 shows these plots for Li2. 

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. 

Valence bond theory does agree fairly well with molecular orbital (MO) theory for homonuclear diatomic molecules that can obey the octet rule H2 (single bond, bond order = 1), Li2 (single bond, bond order = 1), N2 (triple bond, bond order = 3), 02 (double bond, bond order = 2), F2 (single bond, bond order = 1). However, for those molecules that don t, it is more difficult to know if they exist or not and what bond orders they have. MO theory allows us to predict that He2, Be2 and Ne2 do not exist since they have bond orders = 0, and that B2 has bond order = 1 and C2 has bond order = 2. 

Figure 4.2 The molecular orbital diagrams for homonuclear diatomic molecules of the second short period, Li. to Ne2. Diagram (a) is appropriate for 02, F2 and Ne2, diagram (b) for the molecules Li2 to N2...

Heteronuclear diatomic molecules are naturally somewhat more complicated than the homonuclear comprehensive comparisons with homonuclear molecules were given by Mulliken [15]. The atomic orbital coefficients in the molecular orbitals ofheteronu-clear diatomic molecules are no longer determined by symmetry alone, and the electrons in the molecular orbitals may be shared equally between atoms, or may be almost localised on one atom. The molecular orbitals can still be classified as a or n, but in the absence of a centre-of-symmetry the g/u classification naturally disappears. Some heteronuclear molecules contain atoms which are sufficiently similar that the molecular orbitals resemble those shown in figure 6.7. In many other cases, however, the atoms are very different. This is particularly the case for hydride systems, like the HC1 molecule,... 

Apply the Aufbau Principle to find molecular orbital descriptions for homonuclear diatomic molecules and ions... 

Figure 9-5 shows molecular orbital energy level diagrams for homonuclear diatomic molecules of elements in the first and second periods. Each diagram is an extension of the... 

Partial orbital energy-level diagram of NO. The molecular orbitals are labelled in the two ways that we introduced for homonuclear diatomic molecules. Note that we reach higher numbers when labelling the a and it orbitals than we did for N2 or 02. This is because we can no longer distinguish g and u, and so have to label all a and all it orbitals consecutively. 

TABLE 13.1 Molecular-Orbital Nomenclature for Homonuclear Diatomic Molecules ... 

For homonuclear diatomic molecules, a g or m right subscript is added to the term symbol to show the parity of the electronic states belonging to the term. Terms arising from an electron configuration that has an odd number of electrons in molecular orbitals of odd parity are odd (u) all other terms are even (g). This is the same rule as for atoms. 

When the atoms in a heteronuclear diatomic molecule are close to one anoth in a row of the periodic table, the molecular orbitals have the same relative order of energies as those for homonuclear diatomic molecules. In this case you can obtain the electron configurations in the same way, as the next example illustrates. 

Write the molecular orbital configuration of the diatomic molecule BN. What is the bond order of BN Is the substance diamagnetic or paramagnetic Use the order of energies that was given for homonuclear diatomic molecules. 

For homonuclear diatomic molecules with more than four electrons, we require additional molecular orbitals beyond the erg 1 and a 1 orbitals. For U2 and Be2 we can use the <7g2j and the cr 2 orbitals in Eq. (20.2-9) and Eq. (20.2-10). The electron configuration of Li2 in the ground state is... 

The sequence of energies of the molecular orbitals in homonuclear diatomics is normally determined by experiment, and for most diatomic molecules of first row dements is as shown in Fig. 1.7. 

Consider the correlation diagram for homonuclear diatomic molecules (Fig. 1.10), constructed in the same way as that for Hj. The significance of the non-crossing rule becomes apparent when we consider the details of this diagram. In terms of symmetry the Og 2s molecular orbital could correlate with either the 2s- or the 3s-orbital of the united atom. Similarly the Og 2p molecular orbital could correlate with either the 2s or the 3s united atom orbital. As a consequence of the non-crossing rule, the correlation of the... 

Here is how we assign electrons to the molecular orbitals of the diatomic molecules of the second-period elements We start with the and a g orbitals filled. Then we add electrons, in order of increasing energy, to the available molecular orbitals of the second principal shell. Figure 11-26 shows the electron assignments for the homonuclear diatomic molecules of the second-period elements. Some molecular properties are also listed in the figure. 

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

Hurley, A. C., Proc. Roy. Soc. [London) A216, 424, The molecular orbital theory of chemical valency. XIII. Orbital wave functions for excited states of a homonuclear diatomic molecule."... 

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,... 

FIGURE 3.31 Atypical molecular orbital energy-level diagram for the homonuclear diatomic molecules Li2 through N2. Each box represents one molecular orbital and can accommodate up to two electrons. 

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