Thermochemistry of the Smallest QOOH Radical from the Roaming Fragmentation of Energy Selected Methyl Hydroperoxide Ions

Authors

Kyle Covert, University of the PacificFollow
Judit Zádor, Sandia National LaboratoriesFollow
Andras Bodi, Paul Scherrer InstituteFollow
Krisztián Torma, University of the PacificFollow
Krisztina Voronova, University of the PacificFollow
Bálint Sztáray, University of the PacificFollow

Poster Number

9a

Lead Author Affiliation

Chemistry

Lead Author Status

Doctoral Student

Second Author Status

Staff

Third Author Status

Staff

Fourth Author Affiliation

Chemistry

Fourth Author Status

Doctoral Student

Fifth Author Affiliation

Chemistry

Fifth Author Status

Faculty

Sixth Author Affiliation

Chemistry

Sixth Author Status

Faculty

Introduction/Abstract

An understanding of the chemical reactions that take place in combustion systems, such as in engines and the atmosphere, is vastly important for modelling the climate and the impact these processes have on global warming. Methyl hydroperoxide (CH₃OOH, MHP) is the simplest organic hydroperoxide and plays an important role in low temperature autooxidation of combustion molecules; specifically, it was hypothesized to be a good candidate for forming a QOOH⁺ (CH₂OOH⁺) ion through a simple hydrogen atom loss carried out in a unimolecular dissociation study. Furthermore, a thermochemical network can be constructed to find the heat of formation for the QOOH radical, which is an extremely elusive species of interest that had yet to be fully studied before this experiment.

Purpose

The QOOH radical is the key to filling the gap in the low temperature autooxidation radical chain branching that takes place in combustion reactions. The goal of this experiment was to generate QOOH⁺ ions and derive their appearance energy (E₀) to construct a highly accurate thermochemical framework leading to the heat of formation of the QOOH radical .

Method

The dissociative photoionization of MHP was studied by photoelectron photoion coincidence (or shortly PEPICO) spectroscopy at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source (Villigen, Switzerland). Gas phase sample molecules were ionized by finely tuned VUV radiation with high energy resolution. Photoelectrons and photoions produced upon ionization were extracted from the ionization region and detected at opposite ends of the experimental setup in coincidence with each other, thus providing an opportunity for highly accurate energy selection of the photoions. The experimental data (relative ion abundances as a function of photon energy) was modeled using the Rice‒Ramsperger‒Kassel‒Marcus (RRKM) theory of unimolecular dissociations in order to map out all possible dissociation pathways involved in the studied process. Minima and transition state structures were located by ab initio quantum chemical calculations at high levels of theory.

Results

Four fragment ions were observed at m/z = 47, 29, 19, 15 after ionization of the parent molecule at 9.84 eV. All of the ions could be identified as, CH₂OOH⁺, HCO⁺, H₃O⁺, and CH₃⁺, respectively. Theses assignments were simply based off the small number of atoms in the parent molecule and were confirmed by a rigorous ab initio study on all the possible dissociation pathways. The fractional abundance of these ions was plotted against the photon energy in the range of 11.4 – 13.9 eV generating a breakdown diagram. QOOH⁺ was formed through a simple hydrogen atom loss, as was expected. The appearance energy was modelled with a high degree of accuracy which yielded a value of 11.647 ± 0.003 eV and a heat of formation for the QOOH radical of 74.2 ± 2.6 kJ mol^–1. The formation of HCO⁺ and H₃O⁺ involved a very dynamic process based on kinetics and roaming transition states. Even though a direct H₃O⁺ loss is the lowest energy pathway, HCO⁺ dominates because it is a much faster pathway. Furthermore, it was found that H₃O⁺ actually forms through a higher energy and faster consecutive pathway.

Significance

This experiment has yielded the first experimental heat of formation for the QOOH radical which can be used for combustion chemistry models and understanding other processes involving organic hydroperoxides. This should help create more accurate combustion models, which in turn gives more accurate predictions of how these processes contribute to global warming.

Location

DeRosa University Center

Format

Poster Presentation

Poster Session

Afternoon 1pm-3pm