PHOtolysis Reaction Mechanisms by Emerging and New Technologies – PhoRMENT

Photochemistry controls a vast array of the natural and man-made processes known to science: photosynthesis, atmospheric and combustion chemistry, plasma technology, solar energy. Despite the importance of the subject, experimental limitations have inhibited the study of even the simplest processes by which small gas-phase molecules interact with chemically active ultra violet (UV) light. In this project you will exploit new and emerging technologies such as UV LEDs, chemosensors (developed in York) for sensitive and selective free-radical detection, online mass spectrometry, and modern theoretical computational methods to study atmospherically important photolysis reactions.

Among the largest classes of chemicals emitted or produced in the atmosphere are “carbonyls” – organic molecules containing one or more carbonyl functionality. Carbonyls are used in in a vast array of industrial applications, including directly as solvents, pesticides and biofuels, and as reagents for production of pharmaceuticals, polymers and aromachemicals. Carbonyls are ubiquitous in both indoor and outdoor air. Direct emissions are supplemented by in-situ production as virtually all organic compounds break-down through atmospheric oxidation processes via multiple generations of carbonyl intermediates, where they can significantly impact on air quality and health.

Figure 1. Norrish type I (radical) and type II (molecular) are important photolysis reaction pathways for long chain aldehyde species under atmospheric conditions [1] (see also Figure 2)
The photochemistry of carbonyls is therefore both interesting and important (see Figure 1). Unusually amongst atmospheric organics, they are broken down by abundant UV-A radiation (Figure 2). Therefore this project is particularly timely, as the photochemical environment is rapidly changing indoors. LED lighting is replacing fluorescent and incandescent technology, whilst UV and plasma based air filtration systems are increasingly used in an effort to enhance indoor air quality and to suppress virus spread.

Figure 2. Experimental data for the photolysis of a series of n-aldehyde carbonyl species (C1 to C7); cross-section (left) and quantum yield data (right). QY data shows that as the carbon chain increases, the product yields from the radical channel decreases as the molecular and cyclisation channels increase (see Figure 1)

Objectives:

To determine absorption cross-sections and photolysis quantum yields for a variety of important gas-phase carbonyl species; to assess photolysis rates and product branching ratios, over a range of indoor and outdoor conditions, and hence air quality impacts; to identify chemical structures and additional functionalities in carbonyls impacting on photolysis rates [2].

Experimental Approach

  • Characterisation and development of a newly commissioned fast-flow reactor, coupled to chemosensor radical traps and on-line mass spectrometry for quantum yield determinations
  • UV-vis spectroscopy techniques for absorption cross-section measurements
  • GAUSSIAN quantum chemical toolkit for theoretical thermodynamic calculations and chemical structure determination
  • Development of models incorporating the master chemical mechanism (MCM, york.ac.uk) for experimental design and the assessment of environmental impact assessment.

Training

The student will work under the supervision of Dr Terry Dillon (laboratory experiments, GUASSIAN theoretical calculations), Dr Andrew Rickard (chemical mechanism development, laboratory experiments) and Dr Victor Chechik (chemosensor design and applications).  The student will be based in the Wolfson Atmospheric Chemistry Laboratories (WACL), part of the Department of Chemistry at the University of York (www.york.ac.uk/chemistry/research/wacl/). WACL is a world leading facility bringing together experts in atmospheric measurements, lab-studies and Earth system modelling to form the UK’s largest integrated atmospheric science research team.

Dr Dillon has a wealth of experience of studying a wide range of atmospherically important reactions in the laboratory, as well as using GAUSSIAN quantum chemical modelling tools for theoretical calculations. Dr Rickard has interests that span mechanistic chemistry of complex gas- and condensed- phase systems, kinetic modelling of chemical processes and the chemistry of reactive radical intermediates. He currently curates the internationally renowned Master Chemical Mechanism (mcm.york.ac.uk). Drs Dillon, Rickard and the wider WACL team will provide comprehensive training in all kinetic techniques and instrumentation required. Dr Chechik is an expert on organic free-radical chemistry and recently developed bespoke radical chemosensor traps suitable for the identification and quantification of gas-phase free-radicals. You will develop transferrable skills in design and preforming fast flow gas-phase kinetic experiments, spectrometric characterisation techniques (e.g., UV-vis spectroscopy, PTR-MS (Proton Transfer Reaction mass spectrometry)), quantum chemical calculations, chemical mechanism development and evaluation, numerical / data skills and kinetic model analysis. Training will be provided in all areas, and we expect to establish international collaborations with a number of colleagues in the area of chemical kinetics and mechanism development.

The studentship is offered as part of the NERC PANORAMA Doctoral Training Programme,  which will provide training in addition to that offered by the department in York.  A wide range of training opportunities are available, including courses aimed at specific science objectives, at improving your transferable skills and putting your work into a wider scientific context.  Dr Rickard also works for the National Centre for Atmospheric Science (NCAS, https://ncas.ac.uk/en/air-quality), and thus the student will have access to the wider resources that NCAS provides including the Arran Atmospheric Measurement Summer School, the Earth System Science Summer School (ES4), and future further developments in computations and data analysis.

The student should have a strong background in the physical sciences (i.e. a good degree in chemistry, physics, engineering or similar science), a keen interest in environmental issues, and an aptitude and enthusiasm for experimental work.

We appreciate that this PhD project encompasses several different science and technology areas, and we don’t expect applicants to have experience in many of these fields. The project is well supported with experienced scientists and training in these new techniques and disciplines is all part of the PhD.

References

[1]  Newland, M. J., Rea, G. J., Thuner, L. P.; Henderson, A. P., Golding, B. T., Rickard, A. R., Barnes, I., and Wenger, J. Photochemistry of 2-butenedial and 4-oxo-2-pentenal under atmospheric boundary layer conditions. Phys. Chem. Chem. Phys., 21, 1160−1171, DOI: 10.1039/c8cp06437g, 2019

[2]  Vereecken, L., Aumont, B., Barnes, I., Bozzelli, J. W., Goldman, M. J., Green, M. H., Madronich, S., Mcgillen, M. R., Mellouki, A., Orlando, J. J., Picquet-Varrault, B., Rickard, A. R., Stockwell, W. R., Wallington, T. J., and Carter, W. P. L.: Perspective on mechanism development and structure-activity relationships for gas-phase atmospheric chemistry, Int. J. Chem. Kinet., 50, 435–469, https://doi.org/10.1002/kin.21172, 2018