62nd Annual Report on Research 2017

In combustion, two key intermediates in the oxidation of hydrocarbons are the hydro-peroxy-alkyl (QOOH) and hydro-peroxy-alkyl-peroxy (OOQOOH) radicals. These elusive species, formed from the isomerization of peroxy radicals (ROO), are highly reactive and have remained largely undetected in experiments. We seek to directly observe them and follow their kinetics using two novel and sensitive techniques, cavity ringdown spectrometer (CRDS) and laser-induced fluorescence (LIF) action spectroscopy. In the first year, we have performed calculations to identify plausible candidates and their precursors, undertaken preliminary IR-CRDS and NIR-CRDS experiments, and reassembled the LIF action spectrometer.

Peroxy radicals are ubiquitous in combustion. They often undergo unimolecular hydrogen shifts to form QOOH. Further reaction with O₂ forms OOQOOH, which subsequently decomposes to multiple OH radicals and other species. This isomerization and the subsequent chemistry is believed to be the most important radical chain-branching mechanism in low temperature (< 900 K) combustion [1]. Previous work has shown that hydrogen tunneling is key to forming QOOH [2], and so better understanding this reaction is important to improve our fundamental understanding of potential energy surfaces and reaction dynamics, as well as to benchmark theoretical calculations, which are used extensively in combustion modeling.

Savee et al. first observed a QOOH species by multiplexed photoionization mass spectrometry (MPIMS) [2], but no further detections have been reported. A critical issue in initial experiments is identifying species where thermodynamics and kinetics favor stabilizing QOOH. We have modeled the chemistry and found that the most viable routes to form sufficient QOOH concentrations are from isomerization of 2,4-cycloheptadienyl and 3,5-cyclooctadienyl peroxy radicals; these decrease the energy of QOOH relative to its ROO isomer through resonance stabilization of the carbon-centered electron radical in QOOH [2].

We find that initiating the system by the reactions of 1,3-cycloheptadiene or 1,3-cyclooctadiene with atomic chlorine will form observable concentrations of QOOH and OOQOOH, but side chemistry may interfere with the detection of these species. We are considering alternative approaches, such as the synthesis of iodo-substituted precursors. Upon photolysis, they will dissociate to selectively form alkyl radicals and iodine atoms and therefore bypass the side chemistry produced from the chlorine-initiated oxidation. For instance, 5-iodo-1,3-cycloheptadiene, which has never before been synthesized, can be formed selectively by treating cycloheptatriene with gaseous HCl in glacial acetic acid to form 5-chloro-1,3-cycloheptadiene [3], followed by a Finkelstein reaction to exchange the chlorine atom for iodine.

We calculated the potential energy surfaces (PES) of the QOOH compounds and their vibrational transitions. The PES calculations indicate there are a large number of conformers for these species, but the –OH stretches should absorb at the same frequency. The predicted frequencies at roughly 3600 cm^-1 agrees closely with previously published literature on stable alkyl hydroperoxides [4] and our group’s previous work on the –OH stretches of HOR and HOROO radicals [5]. We have also collaborated with Caltech Professor Tom Miller to apply new quantum rate theories based on Feynman path integrals to model the hydrogen tunneling mechanism more accurately.

We have begun experiments using our mid-IR and near-IR pulsed laser photolysis cavity ringdown instruments. The mid-infrared instrument works primarily in the 3200-3750 cm^-1 range, while the near-IR is in the 7000-8000 cm^-1 region. We have used these instrument to study related hydroxyl-methyl-peroxy [6], the intermediates from the isomerization of n-butoxy and 2-pentoxy [5], and the products of the OH + NO₂ reaction [7]. Experiments on QOOH await synthesis of the relevant precursors. By using our recently developed temperature control cell, we will be able to go to higher temperatures where QOOH and OOQOOH form in increased amounts. We will also be able to go to higher pressures than the previous MPIMS work, which was limited to 8 torr.

We are re-assembling an IR-action spectrometer previously used for to observe the first -OH overtone of HOONO [8] using vibrationally-mediated photodissociation spectroscopy. We have completed much of the initial work constructing this instrument and recently acquired a high repetition rate laser ideal for kinetics detected by LIF.

The work to date has introduced our group to the fascinating chemistry of free radicals in combustion. The pathways open at high temperature have forced us to rethink how hydrocarbon radicals and their oxygenated counterparts behave. The team of students, led by Joey Messinger, a rising 3rd year student, has also been exposed to quantum chemical calculations of highly fluxional radicals and the complexities in computing their spectra. Our search is even leading us to consider the synthesis of hydrocarbon precursors.

1. Zador, J., C.A. Taatjes, and R.X. Fernandes, Prog. Energy Combustion Science, 2011 37 371.
2. Savee, J.D., et al. Science, 2015 347 643.
3. Mayr, H. et al., Tetrahedron, 1986, 42 6663
4. Moller, K.H. et al. Journal of Physical Chemistry A, 2017 121, 2951.
5. Sprague, M.K., et al. Journal of Physical Chemistry A, 2012 116 6327.
6. Sprague, M.K., et al., Journal of Physical Chemistry A 2013 117 10006.
7. Mollner, A.K., et al., Science, 2010, 330 646.
8. Fry, J.L., et al. Journal of Chemical Physics, 2004, 121 1432.