Molecular orbital of homonuclear diatomic molecules

TABLE 6.3 Molecular Orbitals of Homonuclear Diatomic Molecules... 

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 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. 

We consider symmetry in Chapter 3, but it is useful at this point to consider the labels that are commonly used to describe the parity of a molecular orbital. A homonuclear diatomic molecule (e.g. H2, CI2) possesses a centre of inversion (centre of symmetry), and the parity of an MO describes the way in which the orbital behaves with respect to this centre of inversion. 

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 of heteronuclear 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-synunetry 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 HCl molecule. 

The molecular orbital description of period 2 diatomic molecules leads to bond orders in accord with the Lewis structures of these molecules. Further, the model predicts correctly that O2 should exhibit param pietism, which leads to attraction of a molecule into a magnetic field due to the influence of unpaired electrons. Molecules in which all the electrons are paired exhibit diamagnetism, which leads to weak repulsion from a magnetic field. The molecular orbitals of heteronuclear diatomic molecules are often closely rdated to those of homonuclear diatomic molecules. 

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

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. 

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 limitation of the above analysis to the case of homonuclear diatomic molecules was made by imposing the relation Haa = Hbb> as in this case the two nuclei are identical. More generally, Haa and for heteronuclear diatomic molecules Eq. (134) cannot be simplified (see problem 25). However, the polarity of the bond can be estimated in this case. The reader is referred to specialized texts on molecular orbital theory for a development of this application. 

This section consists of the formal molecular orbital treatment of homonuclear diatomic molecules of the second short period. The molec-... 

Molecular orbitals in heteronuclear diatomic molecules. 167-175 in homonuclear diatomic molecules, 160-166 of metallocenes. 670-673 in octahedral complexes. 414-418... 

The electronic states of homonuclear diatomic molecules may now be built up by feeding the electrons into the various orbitals, provided that the relative order of molecular orbital energies is known. This has been determined by Mullikan from molecular spectra data and is generally found to be ... 

A buildup principle analogous to that for atoms exists for molecules. The order of filling molecular orbitals from the valence shells in the case of homonuclear diatomic molecules, where x is the bond axis, is as follows ... 

In thissection we cons de [ homonuclear diatomic molecules (those composed of two identical atoms) of elements in Period 2 of the periodic table. Since the lithium atom has a 15 25 electron configuration, it would seem that we should use the Li I5 and 2s orbitals to form the molecular orbitals of the Ll2 molecule. However, the I5 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. 9.31). Thus the two electrons in each I5 orbital can be assumed to be localized and not to participate in the bonding. To participate in molecular orbitals, atomic orbitals must overlap in space. This means that only the valence orbitals of the atoms contribute significantly to the molecular orbitals of a particular molecule. 

The validity of molecular orbital theory is supported by its ability, unlike valence bond theory, to correctly predict certain properties of homonuclear diatomic molecules of elements in the first and second periods. What prediction would valence bond theory make about the paramagnetism of these molecules For which molecules does molecular orbital theory make a different prediction ... 

Figure 10.34 shows the relative energies of the molecular orbitals obtained from 2s and 2p atomic orbitals. This order of molecular orbitals reproduces the known electron configurations of homonuclear diatomic molecules composed of elements in the second row of the periodic table. The order of filling is... 

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