MOLECULAR ORBITALS: HETERONUCLEAR DIATOMICS

The molecular orbitals that explain the motion of a single electron in a molecule consisting of two unequal nuclear charges will not show the g and u symmetry properties of the homonuclear diatomic case. The molecular orbitals within the heteronuclear case will generally be concentrated more around one nucleus than the other. Since the molecular axis is still an axis of symmetry.

By the difference and sum of single atomic orbitals on each centre, in simple numerical calculations the molecular orbitals are sometimes approximated their limiting form. The molecular orbital is termed as approximated mathematically by a linear combination of atomic orbitals and the method is termed as the LCAO-MO method. It has to be understood that the LCAO-MO technique using a limited number of atomic orbitals gives only an approximation to the true molecular orbital. The concept of a molecular orbital is completely free of the additional concept of approximating it in terms of atomic orbitals, apart from the case of the separated atoms. Though, by using a large number of atomic orbitals centred on each nucleus in the construction of a single molecular orbital enough mathematical flexibility can be accomplished to approximate the exact form of the molecular orbital extremely close.

When the LCAO approximation using a limited number of atomic orbitals is generally a poor one for quantitative purposes, it does provide a helpful guide for the prediction of the qualitative characteristics of the molecular orbital. There are two simple situations that must be met if atomic orbitals on dissimilar centres are to interact considerably and form a molecular orbital which is delocalized over the whole molecule. Both the atomic orbitals must have approximately similar orbital energy and they must possess similar symmetry characteristics with respect to the internuclear axis. We shall refer the molecular orbitals in LiH, CH and HF to demonstrate how molecular orbital theory describes the bonding in heteronuclear molecules, and to observe how well the forms of the orbitals in these molecules can be rationalized in terms of the symmetry and energy criteria set out above.