The Meijer group’s interest in astrochemistry spans a wide range of areas within the field. Covering hydrogen formation on graphite surfaces to the ionisation behaviour of polycyclic aromatic hydrocarbons in ice and the formation of small, biological molecules from radical Interstellar Medium species as well as gas-phase reactions involving sulphur. We employ a variety of computational methods to tackle these problems including both electronic structure and quantum scattering methods. Our work is informed by and runs in tandem with developments in the experimental astrochemistry community to contribute to the understanding of the chemical complexity of space and the origins of life.
Through collaboration with the Weinstein group, an ultrafast spectroscopy research group at Sheffield, we strive to understand and explain the excited state dynamics of a variety of systems. Current work is focusing on donor-bridge-acceptor triads, such as platinum trans-acetylide complexes. These types of systems can be used to mimic natural photosynthetic systems and synthesise fuel (either directly or as electricity). During artificial photosynthesis, when a photon absorbs light the energy is transported to the reaction site via electron transfer (ET). This is a fundamental process in nature and is vital when it comes to the efficiency of artificial photosynthetic systems. Through the use of external perturbation, such as IR light, ET can start to be understood and ‘controlled’. Using theoretical calculations, such as time dependent density functional theory (TD-DFT) and MCTDH, we can begin to understand the excited state dynamics of these systems and follow ET.
We work closely with the engineering department at Sheffield to investigate and model jet fuel. Increases in performance and operating temperatures of jet engines has lead to the fuel being used to remove this excess heat. As a consequence, the fuel undergoes autoxidation through radical reaction pathways, causing the formation of insoluble gums. The oxidation process is a set of radical reaction mechanisms with initiation occurring through the basis autoxidation scheme. These reactions are linked to the secondary deposit forming reactions, known as Soluble Macromolecular Oxidatively Reactive Species (SMORS) mechanism, through the self reactions of peroxides and peroxyl radicals. They present a challenge to model with computational chemistry, due to them being intrinsically multi reference and open shell systems, and as such require high level multi reference methods to properly describe them. So far this work has demonstrated that these reaction involve open shell bi-radicals and present a lower energy route towards the formation of oxidation products. The self reaction of peroxyl radicals also appears to lead to the formation highly reactivity singlet oxygen as a product, which will accelerate the autoxidation of fuel by reacting directly with the hydrocarbons, acting as the initiation step.
Reactions and Structure of conformationally flexible molecules
As molecules become larger, they generally become more flexible. As a consequence the potential energy surface becomes more complicated with many local minima, which may or may not be accessible at thermal energies. Each of these minima will be a distinct structure with e.g. a distinct IR spectrum. We are currently working on methods to allow us to generate many minima, which can then be screened for further investigation. This work ties into a number of collaborations we have, such as with Prof. dr. Mathias Schäfer of the University of Cologne, who studies conformationally flexible molecules in the gas-phase using IRMPD spectroscopy as well as internal collaborations on the structure, reactivity, and properties of organic and organometallic compounds. This work is currently investigating hydrogen tunnelling at elevated temperatures in the gas phase.
Algorithm development for Quantum Dynamics Calculations
Quantum Dynamics calculations are significantly harder than standard electronic structure calculations due e.g. the exponential scaling with respect to the basis set size. We are working on methods that will allow us to solve the time-dependent Schrödinger equation more quickly. In particular, we develop efficient parallel methods to make calculations tractable. With Dr. Hill (UoS) we are also working on using AI techniques for generating potential energy surfaces for use in quantum dynamics calculations.