Chemistry made simple
How our bodies function, how chemical plants for refining ores and making plastics work, and how functional materials like photovoltaics, organic conductors, organic light-emitting diodes, etc., operate requires a knowledge of how and why chemical reactions occur. Historically, different types of chemical processes have been described using different languages. Covalent bonding in organic molecules, aromaticity, polar bonding in water, ionic bonding in salts, hydrogen bonding in biology, and metallic bonding in materials all have their traditional methods of description.
While the theories of these chemical processes have all been very successful, Chemistry teaching is complex as students must learn many principles rather than a single guiding one. Also, it has not been possible to consider chemistry from an abstract mathematical perspective as there is no unifying description of all processes. This hampers doing things that are really different. For example, at the moment there is much interest in making qubits for quantum computers, and many options have been tried exploiting properties of materials and molecules and light. Quantum mechanics controls chemistry and so chemistry is the natural first choice for finding a quantum qubit. But which chemical process would be the most suited? The absence of a general theory for chemistry means that such questions cannot be answered. To make a good quantum qubit, should one manipulate covalent bonding, hydrogen bonding, or metallic bonding?
Research by Professor Jeffrey Reimers in the SHU International Centre for Quantum and Molecular Structure has found out how to construct a general chemical theory and then put it to work to solve century-old challenges to chemical understanding as well as to indicate the best ways to build chemical quantum qubits. The work completes the vision of Fritz London in 1928 in which he foresaw what is now known as diabatic models depicting general transformations from reactants to products across transition states. To date, this approach had only been applied with great success to electron transfer problems, through the 1950-60s adiabatic electron transfer theory of Noel Hush.
In four papers just published in the journal Physical Chemistry Chemical Physics, Reimers combined with Hush (Sydney University), Laura McKemmish (Imperial College London) and Ross McKenzie (University of Queensland) to show how similar success could be obtained for general chemical reactions.
Easy answers then arise to complex chemical questions: Why is benzene aromatic (high symmetry, all 6 C-C bond lengths equal) whilst ammonia is pyramidal (low symmetry non-planar sp3)? The answer is simple: their general theory expressed all chemical reactions in terms of 3 parameters, and the ration of two of these, the ratio of the resonance energy to the reorganization energy, determines the symmetry. The theory can also answer a related previously unanswered question: why is ammonia sp3 whereas phosphine and all other related compounds are octahedral? The answer is in the Rydberg orbital ordering, an effect that makes the chemistry of the first row of the periodic table different to that of the later rows.
The work offers significant advances in both the basic understanding of chemistry and in the development of new materials.
These four new papers plus one related older one are:
- “A unified diabatic description for electron transfer reactions, isomerization reactions, proton transfer reactions, and aromaticity” PCCP 17 (2015) P24598, http://dx.doi.org/10.1039/c5cp02236c
Since London proposed the use of diabatic models for general chemical reactions in 1928, great success has been found only for applications in electron-transfer theory. We have found the critical required element that allows general reactions to be treated as London envisaged. This allows any one-step reaction not passing through a conical intersection to be described using the three critical parameters well known in electron-transfer theory- the electronic coupling (“resonance” energy), the reorganization energy, and the exothermicity. This paper shows how it works by comparing standard results from adiabatic electron-transfer theory to isomerization reactions like ammonia inversion and proton transfer to aromaticity. A unified theory of chemical reactivity is thus presented.
- “Bond angle variations in XH3 [X = N, P, As, Sb, Bi]: the critical role of Rydberg orbitals exposed using a diabatic state model” PCCP 17 (2015) P24618,
This examines detailed experimental and calculated results for NH3, PH3, AsH3, SbH3, and BiH3 using the generalized diabatic approach. While simple approaches like VSEPR theory account for the differences in bond angle between NH3 and the other molecules by assuming that it is inherently tetrahedral while the others are inherently octahedral, the diabatic approach provides the first simple explanation for these critical experimental facts. The results are expected to be generally indicative of why the chemistry of the first row is so different to that of later rows. For the NH3 series, the bond angle in the absence of resonance is identified by the diabatic model as being 86.7° while that in the associated radical cations is 101.5°.
- “Non-adiabatic effects in thermochemistry, spectroscopy and kinetics: the general importance of all three Born–Oppenheimer breakdown corrections” PCCP 17 (2015) P24641, http://dx.doi.org/1039/C5CP02238J
This looks at different types of effects caused by Born-Oppenheimer breakdown, considering the whole parameter space available to the diabatic model. This parameter space then spans all possible types of chemical processes that do not involve direct passage over conical intersections. A wide range of analytical solutions and/or full numerical solutions are obtained. These then form a basis for assessment of numerical methods commonly employed to treat Born-Oppenheimer breakdown. Born-Oppenhaimer breakdown is then shown to be tightly connected with the mathematics of the Cusp Catastrophe. It is also shown to be tightly connected with the delocalization of wavefunctions as the transition state is crossed, an essential aspect of the trditional description of general electron-transfer problems using adiabatic electron-transfer theory.
- “Electron–vibration entanglement in the Born–Oppenheimer description of chemical reactions and spectroscopy” PCCP 17 (2015) P24666
Born-Oppenheimer breakdown intrinsically involves the entanglement of nuclear and electronic degrees of freedom. Here this entanglement is used as a measure of the fundamental nature of Born-Oppenheimer breakdown, considered over the full parameter space available to chemical reactions not involving passage through conical intersections. It is shown to provide quite a different representation of the effect than is obtained by considering properties such as state energies and transition moments. Possibly measureable entanglement in chemical dynamics at high energy is also considered.
- “Quantum entanglement between electronic and vibrational degrees of freedom in molecules” Chem. Phys. 135, (2011) 244110
Chemical qubits based on electron-vibration entanglement have often been proposed as possible qubits in quantum information processors. Spanning the whole parameter space available to chemical reactions not involving passage over conical intersections, we consider the viability of these proposals through examination of the extent of environmental isolation required to maintain entanglement.