Gas-Phase Radical Reactions Characterised by Mass Spectrometry: Kinetics, Product Detection, and Modelling

Orientated electric fields (OEFs) can influence radical reactivity and exploiting these interactions is a relatively new and rapidly growing field in molecular science. Electrostatic catalysis, however, is difficult to harness in practice as electrostatic interactions are highly directional but molecules are typically unaligned in bulk conditions. Distonic radical ions – species with localised and spatially-separated charge and radical sites – sidestep this challenge as they are comprised of a fixed internal charge site that can be relocated using synthetic strategies to control the orientation and electrostatic field strength induced at the radical site. In this thesis, gas–phase distonic radical ion–molecule reactions are investigated with careful consideration of internal OEFs. Experimental techniques including ion mobility, ion–trap mass spectrometry, and photodissociation action spectroscopy are combined with quantum chemical methods and statistical reaction-rate modelling to study these reactions.

In the first study, the reactivity of eighteen distonic radical ions with the simple hydrocarbons ethylene and acetylene are measured at 300 K and 2.5 mTorr using a modified commercial mass spectrometer. The distance between the radical and charged functional group modulates the reactivity of these reactions by three orders of magnitude. Rate coefficients, modelled using 4-point potential energy schemes combined with a RRKM theory master equation model, are in good agreement with the experimental values. This model provides a framework to rationalise and predict the electrostatic effects on the rate of distonic radical ion-molecule reactions.

Next, a combination of ion-mobility filtering and laser-equipped quadrupole ion–trap mass spectrometry was used to probe the effect of modifying internal OEFs around the same substrate molecule. The gas-phase reaction kinetics of two seperated quinazoline distonic radical protonation isomers with ethylene are measured with this technique. The protonation site variation drives a 100% increase in the radical reactivity of this system, primarily due to through–space (electrostatic) rather than through-bond effects. Quantum chemical methods specifically designed to calculate long-range interactions, such as double-hybrid density functional theory, was required to rationalize the experimentally measured difference in reactivity.

Another study examines the gas-phase ion–molecule reaction of the 2-dehydrobenzonitrileH+ distonic radical cation + propyne. This radical undergoes a rapid primary and secondary propyne reaction to yield two nitrogen-containing tricyclic aromatic hydrocarbon products. Quantum chemical calculations identify four candidates for these major products but this analysis is inconclusive. Thus, a combination of reaction UV photodissociation action spectroscopy and hole burning techniques was deployed to verify these assignments. It is shown that such techniques provide structural information which can assign these ion-molecule reaction products and thus verify complex calculated reaction mechanisms.

This thesis extends the study of charge effects by applying similar modelling frameworks to the oxidation of the neutral phenyl radical (c-C6H5). The rapid association of the phenyl radical with oxygen has previously been modelled as a barrierless process with little temperature dependence. However, with double-hybrid density functional methods (DSD-PBEP86-D3(BJ)/aug-cc-pVTZ), a submerged transition state stationary point was located along the entrance pathway of this reaction. Using this potential energy landscape, experimental rate coefficients for the addition of molecular oxygen to the phenyl radical were reproduced with a 4-point RRKM-ME kinetic model. This work highlights that purportedly barrierless radical oxidation reactions may proceed through stationary points.

These results join a growing body of evidence that suggest that the reaction rates and product branching ratios of radical reactions can be systematically controlled using the internal OEFs induced by distonic radical ions.