Advances in theoretical chemistry are now enabling reliable predictions to be made on the observable properties of molecules. A new Theoretical Chemistry special collection of Biographical Memoirs of Fellows of the Royal Society describes this progress through the contributions of many of the leading pioneers in this field in the 20th century. Sir David Clary FRS, Emeritus Professor of Chemistry at the University of Oxford, traces this history.
The fundamentals of modern theoretical chemistry link back to the founders of quantum mechanics, including Erwin Schrödinger, Paul Dirac, Werner Heisenberg and Wolfgang Pauli. Ever since Schrödinger’s publication of the theory of wave mechanics in 1926 it has been realised that essentially all problems of molecular science can, in principle, be described by solutions of Schrödinger’s equation. However, it is only recently with the development of fast electronic computers that useful numerical results have become available. Accordingly, simple pictorial descriptions of orbitals, the solutions of Schrödinger’s equation for one electron, were the dominant tool used in research for many years.
The first publication which showed that Schrödinger’s wave mechanics could be applied to molecules was by Walter Heitler and Fritz London in 1927, just one year after Schrödinger’s great paper. They proposed a valence bond formulation in which the wave function for the simplest hydrogen molecule was expressed in terms of products of atomic orbitals on each atom. Linus Pauling at Caltech then exploited this valence bond approach to explain the structures of more complicated molecules. This work won him the Nobel Prize for Chemistry in 1954. However, it is the mathematical simplicity of molecular orbitals (MOs), which are linear combination of atomic orbitals, that enabled the MO approach to become the preferred theory.
Fig. 1. The MO diagram of Lennard-Jones illustrating the triple bond in N2. Taken from J. E. Lennard-Jones, The Electronic Structure of Some Diatomic Molecules, Trans. Faraday. Soc., 25, 668 (1929).
The first Professor of Theoretical Chemistry was John Lennard-Jones, who was appointed at Cambridge University in 1932. His early 1929 paper on the MO theory for diatomic molecules explained the paramagnetism in O2 and the strong triple bond in N2. (Fig. 1). He also made well-known contributions to describing intermolecular potentials. The subsequent development of theoretical chemistry in the UK is closely linked with Lennard-Jones, and his students and colleagues (Fig. 2).
Fig. 2. Cambridge Theoretical Chemistry group, 1950. In the first row, Sir John Lennard-Jones is in the centre and Frank Boys is third from the left. The inset is John Pople. Copyright unknown.
Charles Coulson, Lennard-Jones’ first PhD student, also made many contributions to MO theory including the first applications to polyatomic molecules. In a colourful academic career at Dundee, King’s London and Oxford, Coulson supervised many outstanding research students including Peter Higgs of the boson fame. The work of Coulson did much to make organic chemists aware of the power of MOs in explaining the strengths and lengths of chemical bonds between carbon atoms.
Coulson’s brilliant student Christopher Longuet-Higgins was appointed in 1954 at the very young age of 31 to the Chair in Cambridge previously held by Lennard-Jones. Even as an undergraduate at Oxford, Longuet-Higgins had used MO theory to explain the bridging role of hydrogen atoms in enabling boron hydrides to form. At Cambridge, he established a highly active research school which concentrated on simple qualitative applications of MO theory leading to a variety of novel predictions relevant to molecular spectroscopy. He also made several fundamental contributions to molecular quantum mechanics including the first realisation of a sign change in a wave function about a conical intersection, which is now known as the Berry Phase. The work of Coulson and Longuet-Higgins was largely on organic molecules whereas their colleague Leslie Orgel developed influential orbital theories for inorganic chemistry such as his energy level splitting diagrams for d orbitals in transition metal compounds.
The development of a general computer package for molecular orbital calculations was pioneered by John Pople, another student of Lennard-Jones. Pople exploited the mathematical advantages of the Gaussian orbitals of his former Cambridge colleague Frank Boys and did much to accelerate the move of theoretical chemistry from simple pictorial MO theories to computational approaches. However, electron correlation provided a major stumbling block that for many years prevented theoretical chemists from calculating results with the chemical accuracy required by experimentalists. The Density Functional Theory (DFT) of Walter Kohn was a major advance. DFT has enabled results of good accuracy to be obtained with reasonable computational expense for more complex molecular problems. Nicholas Handy, a student of Boys, also contributed significantly to these developments. The award of the 1998 Nobel Prize in Chemistry to Pople and Kohn was very warmly received by the theoretical chemistry community.
A potential energy surface (PES) is the electronic energy expressed as a function of the nuclear coordinates in a molecule. It arises following the approximate separation of electronic and nuclear motion first demonstrated by Max Born and Robert Oppenheimer. Practical mathematical expansions of the PES were developed by John Murrell , a student of Longuet-Higgins. James K. G. Watson derived a rigorous Hamiltonian for calculating vibrational states of triatomic molecules from the PES. The theory for the longer-range terms in the PES was developed by David Buckingham, a student of Pople who followed Longuet-Higgins in the Cambridge Chair in 1969. The PES is needed for calculations on the dynamics of molecules including chemical reaction rates, molecular vibrations and the simulation of liquids and other condensed phases. These have become very active topics for research in recent years, although many of the contributors are still alive and so do not yet have Biographical Memoirs! In due course, these Memoirs will provide an interesting compilation describing advances in theoretical chemistry in the 21st century.
Read the memoirs discussed here and more in the new Theoretical Chemistry special collection. All memoirs are published online as they are ready before being compiled into two volumes per year. Sign up for alerts or keep an eye on our website for new memoirs as they appear.