In-bond orbitals: A comprehensive graphical expression of orbital interaction

The minimal linear-combination-of-atomic-orbitals (LCAO) has a clear chemic al meaning, namely, that a molecular orbital consists of atomic orbitals of the member atoms. In organic chemistry, this concept has been applied to g raphical expression of orbital interactions. However, the minimal LCAO meth od is inadequate to illustrate such interatomic interactions. Based on a ge neral idea in quantum chemistry, we have proposed pictorial "in-bond orbita ls" to compensate for the shortcomings of the minimal LCAO scheme without d estroying its simple concept. In-bond orbitals well express the qualitative electronic structures of bonding and antibonding internuclear regions. Fur thermore, they are continuously correlated with the AOs of the united atom. In this paper, (1) we discuss the conceptual problems of the minimal LCAO expression in interaction between atoms, (2) we show the importance of the contribution of AOs with higher quantum numbers to the chemical interaction nominally given by AOs with the lowest quantum number, (3) we show that th e introduction of a minimal number of in-bond orbitals is a simple method t o solve the continuity problem of the correlation diagram and to improve th e electronic structure, (4) a selection scheme for in-bond orbitals is prop osed, and (5) as an example, the concept of the in-bond orbital is applied to the prediction of the direction of pyramidalization of the olefinic carb on atoms of norbornene.