In-Bond Orbitals: A Comprehensive Graphical Expression of Orbital Interaction

Abstract

Abstract The minimal linear-combination-of-atomic-orbitals (LCAO) has a clear chemical meaning, namely, that a molecular orbital consists of atomic orbitals of the member atoms. In organic chemistry, this concept has been applied to graphical expression of orbital interactions. However, the minimal LCAO method is inadequate to illustrate such interatomic interactions. Based on a general idea in quantum chemistry, we have proposed pictorial ``in-bond orbitals'' to compensate for the shortcomings of the minimal LCAO scheme without destroying its simple concept. In-bond orbitals well express the qualitative electronic structures of bonding and antibonding internuclear regions. Furthermore, 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 the introduction of a minimal number of in-bond orbitals is a simple method to solve the continuity problem of the correlation diagram and to improve the electronic structure, (4) a selection scheme for in-bond orbitals is proposed, 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 carbon atoms of norbornene.