Abstract
Does quantum mechanics accommodate chemical intuitions about bonds, or sweep them away in favour of something new? In this paper I look at foundational issues arising for two revisionary efforts to embed the chemical bond into quantum mechanics. Energy and structure: Since the 1930s, a standard way to explain the stability of a molecule within quantum chemistry has been to use correlation diagrams based on Molecular Orbital (MO) theory. Such diagrams allow estimates of the total electronic energy of a molecule and comparison with the separated atoms: H2 exists because its energy is lower than that of two isolated hydrogen atoms, while He2 does not exist because its energy would be higher than that of two isolated helium atoms. Hendry (2008) has distinguished the structural and the energetic conceptions of the chemical bond. The structural conception tries to identify the explanatory role of the covalent bond in chemistry, then work out what realises that role. The energetic view draws on physical rather than chemical explanation: how the stability of molecules is explained within quantum mechanics. Taking MO theory as a guide, this suggests that facts about bonds are determined by facts about bonding at the level of whole molecules. This would be a radical revision of a longstanding chemical concept, but perhaps quantum mechanics forces that on us: Weisberg (2008) has challenged the quantum-mechanical adequacy of the more retentionist structural view. The Quantum Theory of Atoms in Molecules (QTAIM): The late Richard Bader (1990) analysed the distribution of electron density within molecules, arguing that it is possible to construct well-motivated quantum-mechanical correlates of some classical chemical concepts, including bonds between atoms. However, Bader thought that QTAIM provides not bonds but ‘bond paths’ and that chemical intuitions which cannot be justified by quantum mechanics should be discarded. Esser (2019) has argued further that QTAIM provides a third ‘interactive’ conception of bonding. In this paper I will respond to Weisberg’s challenge to the structural conception, but also issue my own challenge to the energetic view: the identification of a bond with a change in energy breaks down for anything more complex than diatomic species. I also argue that Bader’s dismissal of classical ideas about bonds as mere ‘chemical intuition’ is too quick: this is a well-developed body of theory that has made important contributions to the development of chemistry. It should be given a robust defence, not a dismissal.
