The theory was developed under the following assumptions:
-electrons
in the field of 
-skeleton - AO (the
carbon p-atomic orbitals perpendicular to the molecular plane) are taken into
account-system
(Coulomb integrals) and 
(Resonance
integrals) if i and j belong to the bonded atoms. 
It is convenient to choose the carbon Coulomb integral as a reference (zero)
energy level and 
as an energy unit. In this system 
cc= 0
and 
cc
= -1.
The Hückel method can be extended so that heteroatoms can be included
into the -system [1]. The
parameters are collected in the following table
| -Atom |
Number of-electrons |
|
|
Bond | Ref | Example |
| B
|
0
|
1
|
-0.7 | B - C | 1, 2 | |
| -0.8 | B - N | 1, 2 | ||||
 C |
1 | 0 | -1 | C - C | 1, 2 | |
| N |
1 | -0.5 | -0.8 | N - C | 1, 2 |  |
| :N | 2 | -1.5 | -1 | N - C | 1, 2 |  |
| N+ | 2 | 2 | -0.7 | N - O | 1, 2 | |
| O |
1 | -1 | -0.8 | N - C | 1, 2 | |
| :O | 2 | -2 | -1 | N = O | 1, 2 | |
| O+ | -2.5 | -1 | N - C | 1, 2 | ||
| F |
2 | -3 | -0.7 | F - C | 1, 2 | |
| Cl |
2 | -2 | -0.4 | Cl - C | 1, 2 | |
| Br |
2 | -1.5 | -0.3 | Br - C | 1, 2 | |
| CH3 |
2 | -2 | -0.7 | C - CH3 | a) | |
| Ca |
1 | 0.1 | -0.8 | Ca - C  |
b) | |
| C |
1 | 0.1 | -3 | C CH3 |
b) | |
| H3 |
1 | 0.5 | -3 | C CH3 |
b) |
a) CH3 is treated as one "atom"
b) in the three-"atom" fragment: > Ca
- C H3 . Here Ca
is the alpha carbon atom.
1. H. H. Greenwood, Computing Methods in Quantum Organic Chemistry, Wiley-Interscience, NY, 1972.
2. A. Streitwieser, Molecular Orbital Theory for Organic Chemists, John Wiley, 1961, p 135.
see also
http://web.uccs.edu/chemistry/orglab/hmo_demo/hetero.html
http://www.chem.swin.edu.au/courses/ntu/sch330/exp9.html
http://neon.chem.swin.edu.au/modules/mod3/h_hetero.shtml
http://web.uccs.edu/danderso/chem532/532hmopr.html
ŠNikolai V. Shokhirev, F. Aann Walker 2003