Chemistry Reference
In-Depth Information
produced by the two point charge at a distance apart equal to the internuclear separa-
tion of the molecule.
that is,
q
μ
μ
(23)
The working formulae to evaluate the dipole charge are reviewed below:
Pauling's Formula
Pauling, in an attempt to evaluate the dipole charge, plotted the percentage of ionic
charges against their electronegativity differences. Pauling [2] proposed an ansatz to
calculate the ionic character of the bond (i.e., static charge) was as
q = 1 - exp(-(χ
B
- χ
A
)
2
/4)
(24)
where χ
B
and χ
A
are the atomic electronegativities of atoms B and A respectively.
Nethercot's Formulae
Nethercot [35, 36] concluded that q could not be a simple function of electronegativity
difference of two atoms and he proposed two formulae to calculate the dipole moment
charges. His proposed ansatz(s) were
q
= 1 - exp(-3(χ
B
- χ
A
)
2
/2 χ
AM
2
) (25)
q = 1 - exp(-(χ
B
- χ
A
)
3/2
/χ
GM
3/2
) (26)
where χ
AM
and χ
GM
are the arithmetic mean (AM) and the geometric mean (GM) of
the two atomic electronegativities.
Barbe's Formula
Barbe [37] proposed a simple equation to calculate the dipole moment charges of het-
ero nuclear diatomic molecules as:
q
= (χ
B
- χ
A
)/χ
B
(27)
or, q = ∆χ/χ
B
(28)
where χ
B
> χ
A
.
COMPUTATION OF BOND MOMENT
The electric dipole moment is a measure of the separation of positive and negative
electrical charges in a system of charges that is a measure of the charge system's over-
all polarity. In the simple case of two point charges, one with charge +q and one with
charge -q, the electric dipole moment is
μ = q × d (29)
where d is the displacement vector and μ is the electric dipole moment vector
generated by bond charge.
If we take a series of di-atomics whose bond distances, d can be known to a satis-
factory accuracy. So, the dipoles could be calculated if the q's are known. Let us recast
the equation (29) to calculate the dipole in Debye unit.
μ = q e R
e
(30)
Search WWH ::
Custom Search