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Mulder, E.

Department of Theoretical Philosophy

The Molecular Structure Hypothesis: Foundations, Representations, Pluralism and Realism

In this thesis, I explore different philosophical positions with respect to the molecular structure hypothesis, which conceives molecules as collections of atoms linked by a network of bonds, as well as with respect to notions, concepts and entities underlying the molecular structure hypothesis. Despite its central position in chemical explanation, the truth of the molecular structure hypothesis has been challenged. The classical conception of a molecule as an object in three-dimensional space, with a determinate structure and geometry, is incompatible with the symmetry requirements of the quantum mechanical description of an isolated molecule. This is known as the molecular structure problem.

I discuss quantum decoherence and the Quantum Theory of Atoms in Molecules as two possible methods of resolving the molecular structure problem. The former explains the emergence of semi-classical properties such as molecular shape and structure upon interaction of a molecule with its environment and the latter defines these notions in terms of the topological properties of the charge density. I argue that neither of these provides a satisfactory explanation for the phenomenon of isomerism, nor do these theories explain how or if collapse of the wavefunction occurs. Therefore, neither of these theories resolves the molecular structure problem.

I argue that, in the absence of a thorough explanation of the emergence of the classical world from the quantum realm, a plurality of perspectives is required in order to describe the various properties of molecules. I propose to extend van Fraassen's conception of observability to include observations made by means of methods or instruments that have been developed by the process of epistemic iteration, in which knowledge is created and justified in a stepwise process. On this view, molecular shape, molecular structure, atoms, electrons and bonded interactions between atoms are observable. I argue that we should therefore adopt a position of moderate realism with respect to molecular shape and structure, as well as to entities underlying these notions, such as atoms, electrons and bonded interactions between atoms. On this view, representations of molecules and their shape and structure are similar to maps, such as railway maps or topographical maps, in the sense that they describe certain features of a system with some amount of realism.

Last modified:03 December 2013 11.53 a.m.