J-Coupling: a rigorous definition of the emergent chemical bond
The chemical bond is an emergent property used to understand the structure and properties of materials. Despite its widespread use it is not a well defined object. This project aims to put the concept of bonding on a physical footing through the study of an observable quantity known as J-coupling.
So called ‘first-principles’ theories aim to predict the properties of materials by solving the fundamental equations which govern the behaviour of electrons and nuclei - the laws of quantum mechanics. Indeed, using modern computing power approaches such as density functional theory can now predict many of the observable properties of materials with often remarkable accuracy. While first-principles technique tell us what properties a material may posses, they do not directly address why the material has the property - a prerequisite to understanding how the material might be adapted to enhance that property. Chemists use many concepts in order to understand the structure and properties of materials. Descriptions of atomic level structure use the concept of bonding; pairs of atoms might be said to be bonded and that bonding further classified as strong or weak, covalent or ionic and further classes such as hydrogen bonds or halogen bonds used. However, these classifications do not correspond to some directly measured observable.
Solid-state NMR is a uniquely sensitive probe of atomic scale structure, which uses the interaction between the magnetic moment of an atomic nucleus and radio- frequency radiation to provide information on the local environment of an atom. An NMR experiment can be modelled using an effective spin Hamiltonian, in which all of the degrees of freedom except the nuclear and external fields have been integrated out. The remaining parameters directly correspond to observables in an NMR experiment. Some of these interactions can be assigned to a single atomic site (e.g. the chemical shift interaction), However, there are two NMR interactions that couple pairs of atoms: The direct dipolar interaction, D, is determined by the distance between the nuclei – it is strictly a measure of the relative geometric positions of the atoms in a material. In contrast an indirect interaction, known as Jcoupling, couples pairs of nuclei through an interaction mediated by the electrons. NMR J-coupling is therefore commonly regarded as providing a direct map of the atomic connectivities in a material, and implicitly, the presence or absence of a bond and its strength. However, our calculations, matched with experiment have shown that this simple interpretation is not always true. We have found Jcouplings close to zero for pairs of atoms with a strong covalent bond, significant J-couplings across both “weak” hydrogen bonds and even coupling between pairs of atoms without any conventional bond. We propose to explore this connection between J-coupling and atomic structure and the way it influences how scientists view bonding. The project will greatly benefit from the interaction with the Durham Philosophy Department, who have a world-leading reputation in the philosophy of chemistry. In particular we identify three research questions:
1- To what extent is there a link between J-coupling as an experimental observable and the chemist’s emergent concept of ‘bonding’. In particular, is J-coupling an appropriate measure of so-called “weak bonds” such as hydrogen bonds, halogen bonds and other close contact feature such as pi-pi interactions?
2- What is the relationship between “bonding” as characterized by J-coupling – and a crystallographer’s geometric description of crystal structure as determined by diffraction based experiments?
3- Can J-coupling in solid materials be regarded as a fundamental property of a crystal’s unit cell, or does the symmetry breaking associated with the coupling mechanism impose a nonperiodic approach?