The premise on which this text is based is that the vast majority of chemical phenomena
may be qualitatively understood by the judicious use of simple orbital interaction dia￾grams. The material borrows heavily from the pioneering work of Fukui [1, 2], Wood￾ward and Ho¨mann [3], Klopman [4], Salem [5], Ho¨mann [6], and many others whose
work will be acknowledged throughout including Fleming: Frontier Orbitals and Organic
Chemical Reactions [7], from which a number of illustrative examples are extracted. If
there is uniqueness to the present approach, it lies in the introduction of the a and b of
simple HuÈckel molecular orbital theory as reference energy and energy scale on which to
draw the interaction diagrams, mixing s and s* orbitals and nonbonded orbitals with
the usual p orbitals of SHMO theory on the same energy scale. This approach is di½cult
to justify theoretically, but it provides a platform on which the reader can construct his
or her interaction diagrams and is very useful in practice. Numerous illustrations from
the recent literature are provided.