Characterising Heterocyclic Rings Through Quantum Chemical Topology

In this chapter, ring structures are characterised through their calculated properties within the theory of quantum chemical topology (QCT). QCT properties of the atoms within a ring can predict the properties at a special point, a so-called ring critical point (RCP). Both the RCP properties and the atomic properties according to QCT successfully distinguish between different ring structures. There are four features of a ring that are responsible for its ring atom properties: (i) the number of heteroatoms within the ring, (ii) the heteroatom’s element, (iii) the ring substituent, and (iv) the substituent site. Interestingly, the four features affect the ring’s properties independently. Therefore, a change in a heteroatom’s element will always affect the ring’s properties the same amount, irrespective of other ring features. This is called ring characteristic orthogonality. For substituent types, it is the atom of the substituent connecting the substituent to the ring that dominates the effect on the ring, rather than the entire substituent itself. Using these relationships between ring features and QCT properties opens up the possibility of improving ring structures in areas such as drug design.