The Nature of Bonding in Diatoms

Abstract

Having established the nature and strength of bonds in largely classical chemical language in the previous chapter, we must now consider the way in which the wave mechanical theory of electronic structure can lead to an understanding at a first-principles level of the nature of the bonding in (a) homonuclear and (b) heteronuclear diatoms. This will be done first with particular reference to the single bond in the H₂ molecule. Here, of course, one of the major successes of the early work using quantum mechanics was the theory of Heitler and London (1927) on the nature of the covalent bond in H₂. In many respects, the ideas underlying their pioneering work remain those which, in principle, are most deeply embedded in the chemist’s concepts. Their viewpoint was that molecules are built, first and foremost, out of atoms, which retain some of their own distinctive features, even when bound into a molecule. This picture, however, in spite of the efforts of researchers like Moffitt and others [see, e.g., the review by Balint-Kurti and Karplus (1974)] has proved the most difficult one to develop quantitatively from the Schrödinger equation. Therefore, a lot of emphasis has gone into developing the major alternative theory in which the electrons are thought to occupy molecular orbitals which belong to the molecule as a whole. Though, conceptually, this method is fundamentally different from the Heitler-London or valence bond method, in fact the atomic orbitals in most treatments are used as the basic building blocks from which the molecular orbitals are constructed.