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Physical chemistry 284

multiplicity of 1 (a singlet), a single unpaired electron has Σ=½ and multiplicity 2 (a
doublet) two unpaired electrons have Σ=1 and multiplicity 3 (a triplet).
The orbital angular momentum is described by the quantum number Λ. For an
electron in a σ orbital, Λ=0, for a π orbital, Λ=±1, etc. Pairs of degenerate π orbitals
contribute no angular momentum, as their contributions cancel out.
The molecular term symbol is created as for an atomic term symbol. The value of Λ is
denoted by Σ for Λ=0 (note that this label is not related to the symbol for the spin angular
momentum), Π for Λ=1, ∆ for Λ=2, etc. (c.f. S,P,D…for atomic term symbols), and the
1
3
multiplicity of the spin angular momentum is added to this as a superscipt (e.g. Π, ∆).
The parity of the overall wavefunction may be added as a subscript. The ground state
1
3
term symbol for nitrogen is Σ g, and that of oxygen is Σ g, for example.
Heteronuclear diatomic molecules
Diatomic molecules composed of two different elements, such as CO or NO are termed
heteronuclear. Molecular orbitals in heteronuclear diatomic molecules are constructed in
the same manner as those in homonuclear molecules. The bonding differs from that of a
homonuclear diatomic molecule in the form of the LCAO:
ψ(MO)=c 1ψ 1+c 2ψ 2

For a molecular orbital in a homonuclear diatomic molecule, the mixing coefficients, c 1
and c 2 (see Topic H3) are equal whereas the mixing coefficients for corresponding
atomic orbitals are no longer equal in a heteronuclear species.
As a result of the inequality of the mixing coefficients, all the molecular orbitals have,
to varying degrees, unequal distributions over the two nuclei. Electrons therefore spend
more time around one atom than the other, on average, and this gives rise to a dipole over
the length of the bond. The greater the energy difference between the corresponding
atomic orbitals, the greater the difference between the mixing coefficients, and the greater
the localization of electrons around one of the atoms. This is illustrated schematically for
the carbon monoxide molecule in Fig. 6. In the highest occupied π bonding orbital, the
mixing coefficient for the oxygen 2p orbitals is greater than that of the carbon 2p orbitals,
so giving the π orbital a higher degree of oxygen 2p character than