Diatomic Molecules with Different Atomic Nuclei

2-3 DIATOMIC MOLECULES WITH DIFFERENT ATOMIC NUCLEI 

We now turn to the diatomic molecules in which the atomic nuclei are different. We still use an LCAO description of the molecular orbitals, 

To estimate an energy diagram for the molecular orbitals in such a molecule we first have to examine the symmetry properties of the orbitals. 

The symmetry operations E, C, and av (reflection in a plane that contains the axis A-B) are present. All molecules that possess these symmetry properties have the point-group symmetry Coov The orbitals are characterized by symbols similar to those used for a homonuclear diatomic molecule, such as a, n, etc. The character table for CMV is given in Table 2-2. 

Since the atomic orbitals of the atomic nucleus with larger Z (B) have lower energy than the corresponding orbitals of the other atomic 

The last electron goes into an antibonding tt orbital. The molecule is paramagnetic since it possesses an unpaired electron. 

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Figure 2-19 The molecular orbitals for a diatomic molecule with different atomic nuclei. The diagram indicates that 4o repels 5a, making 5ct+ less stable than the lir orbitals.

In the previous example of H2 we saw that the covalent bond lowers the electronic energy by allowing the electrons to interact with more than one nucleus. In diatomic molecules with Dooh symmetry (i.e. composed of a single element), the sharing of electron density must be even between the two nuclei. When atoms of different elements form a diatomic molecule, the relevant symmetry will be Coov because the symmetry elements which interchange the nuclei are no longer valid. In addition, the nuclear charges will be unequal, and so we may expect the electron density to be biased toward the more attractive of the two atomic cores. 

An organic molecule is usually composed of many atoms, and the atoms are connected by covalent bonds. For an atom of a pure element that is not involved in bonding, the electrons reside in atomic orbitals closely associated only with that atom at discrete distances from the nucleus. These atomic orbitals are the S-, p-, and d-orbitals discussed in Section 3.1.2. In a molecule, the atoms of each element share electron density with another atom, and the electrons reside in orbitals that are different from the familiar atomic s-, p-, or d-orbitals. In other words, molecular orbitals are different in energy when compared to the corresponding atomic orbitals. Note that elements that are categorized as diatomic, including H2 or F2, and such entities are molecules, so the H-H and F-F bonds are covalent bonds. The shared electron density associated with one atom in the bond is directed toward the other atom in the bond. 

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