Reaction kinetics using mass spectrometric detection

To date, we have measured rate coefficients for R+O₂, R+NO, R+NO₂, R+Cl₂, R+Br₂, R+I₂, R+HCl, R+DCl (tunneling effect), and R+HBr reactions (see more in ). In some cases, the reverse reaction back to reactants is sufficiently fast, and we can determine both the forward and reverse rate coefficients (and therefore, equilibrium constants) from the experimental traces. Interestingly, our study on CH₂I + O₂ and CH₂Br + O₂ reactions from the year 2006 [1] was a crucial initiator for the discovery that the smallest Criegee intermediate, CH₂OO, can be produced by the CH₂I + O₂ reaction [2]. This was an important discovery and it played a huge role in the first direct observation of a Criegee intermediate [2,3]. This scientific breakthrough led to many high-impact publications that studied the reactivity Criegee intermediates. Consequently, our understanding about the importance of Criegee intermediates in the atmosphere has greatly changed.

The LP-PIMPS apparatus is designed to study the kinetics of gas-phase radical reactions as a function of temperature and pressure. The experiments are conducted in a temperature-controlled laminar flow reactor that is coupled to a photoionization quadrupole mass spectrometer. A pulsed exciplex laser is used to to photolyse suitable precursors to produce the radical under study homogeneously in a tubular flow reactor. Radical concentrations are kept low to ensure that radical-radical and radical-precursor reactions are negligible. The flow in the reactor typically consists mainly (> 95%) of an inert bath gas (He or N₂). However, for slow reactions the reactant can make up to 50% of the flow. All measurements are performed under pseudo-first-order conditions, meaning that the reactant concentration (O₂, NO, NO₂, etc…) is in huge excess over the radical concentration. An advantage of doing the measurements under pseudo-first-order conditions is that bimolecular rate coefficients can be determined even if the exact radical concentration is unknown. The photoionization quadruple mass spectrometer is used with a multichannel scaler (MCS) to monitor the radical decay signal in real-time. It is also possible to monitor the formation signals of reaction products. It is worth noting that we measure the kinetics of reactions directly and do not use relative rate methods. A detailed description of the apparatus is given in ref [4].

With different reactor material and coating combinations we are able to achieve very low wall rates (below 15 s^-1) for many radicals. This makes the experimental apparatus especially good for measuring rate coefficients and equilibrium constants for R + O₂ ↔ RO₂ equilibrium reactions. For resonance-stabilized hydrocarbon radicals (RSHRs), this equilibrium is already observed at very low temperatures, (300–400 K), and at 500 K, the equilibrium overwhelmingly favours the reactants. Unless there are low-barrier reaction channels that permit the RO₂ adduct to react further, these reactions are dead-ends in combustion environments. This explains the soot-forming propensity of RSHRs, of which propargyl (C₃H₃) and allyl (C₃H₅) are the most important. To understand soot formation in combustion environments, the competition between the self-reactions and oxygen reactions of RSHRs needs to be understood.

Fig. 1 A schematic figure of the laser photolysis photoionisation mass spectrometer (LP-PIMS) apparatus.

References

1. Arkke J. Eskola, Dorota Wojcik-Pastuszka, Emil Ratajczak, Raimo S. Timonen: Kinetics of the reactions of CH₂Br and CH₂I radicals with molecular oxygen at atmospheric temperatures. Phys. Chem. Chem. Phys., 2006, 8, 1416-1424.
2. Oliver Welz, John D. Savee, David L. Osborn, Subith S. Vasu, Carl J. Percival, Dudley E. Shallcross, Craig A. Taatjes: Direct kinetic measurements of Criegee Intermediate (CH₂OO) formed by reaction of CH₂I with O₂. Science, 2012, 335 (6065), 204-207.
3. Johanna L. Miller: Criegee chemistry is captured. Physics Today, 2012, 65 (3), 17-19.
4. A. J. Eskola, R. S. Timonen: Kinetics of the reactions of vinyl radicals with molecular oxygen and chlorine at temperatures 200 – 362 K. Phys. Chem. Chem. Phys., 2003, 5, 2557-2561.