Paul explains: "Atoms bind to each other to form molecules, the basic units of normal matter, be it DNA or water, and they usually do so following very precise rules. For instance, an oxygen atom normally binds to two other atoms, let us say hydrogen atoms, to form the water molecule, H2O. Each of these links is a chemical bond, which is related to a very particular relationship between a pair of electrons, the so-called Lewis pair.
"Sometimes, the oxygen atom, instead of binding to two other atoms, binds to only one but then doubly, i.e. the two pairs are used to bind the same two atoms. In the first case we have single bonds, in the second, double bonds. The number of these pairs of electrons making a bond has been traditionally called the bond order.
"The bond order is a very important concept in chemistry, but its rigorous definition has been difficult, so the concept has been languishing progressively over the years. One basic problem in finding a rigorous bond order is isolating atoms inside a molecule, because the rules of quantum mechanics, the theory that describes molecules, says that once several atoms join together their individuality is essentially lost.
Counting electron pairs
"However, in the last decades of the last century an elegant way to identify atoms-in-molecules was discovered by the late Richard Bader and his group. This approach is nowadays called Quantum Chemical Topology (QCT). In QCT there is a clear way to count the number of electron pairs that are shared between two atoms.
"We have shown that this descriptor, which is commonly called the delocalisation index, serves as a rigorous bond order. This way, chemists can now compute (and even measure in the not too distant future) bond orders, and use them to understand, rationalise, and design molecules with specific properties."
This is the first time that the QCT has been extensively examined as a measure for bond order, and is the largest study of its kind. This new research makes chemical bonding – a previously somewhat vague concept or simplified model – into a rigorous scientific framework for describing the behaviours of atoms and molecules.
This article is free to read in our open access, flagship journal Chemical Science: DOI: 10.1039/C8SC0089F