The Hückel Molecular Orbital Method
In general the
calculation of all the matrix elements is not straightforward.
The Hückel
method makes some severe approximations as to the values of the Hamiltonian and
the overlap matrix elements:
Hückel Approximations:
(i) Hrr = a for all conjugated carbon
atoms r
(ii) Hrs = b for any conjugated carbon link r-s,
i.e
if atoms r and s are adjacent
Hrs = 0 otherwise
(iii) Srs = 0 for r ¹ s, i.e. zero overlap for r ¹ s.
·
a is often referred to as a 'Coulomb
integral'.
·
Use of equation (3) shows that a is the atomic
2p energy.
·
b is often referred to as the
'resonance integral' or 'bond integral' since the value of b determines
the strength of the bonding interactions.
·
Both a and b are
NEGATIVE.