Fast Dissociations of Halogenated Methanes: A Thermochemical Network

The work presented in this chapter has been published as a journal article entitled ‘A Halomethane Thermochemical Network from iPEPICO Experiments and Quantum Chemical Calculations’ in 2012 by J. Harvey, R. P. Tuckett and A. Bodi, in the Journal of Physic

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Fast Dissociations of Halogenated Methanes: A Thermochemical Network

4.1 Preamble The work presented in this chapter has been published as a journal article entitled ‘A Halomethane Thermochemical Network from iPEPICO Experiments and Quantum Chemical Calculations’ in 2012 by J. Harvey, R. P. Tuckett and A. Bodi, in the Journal of Physical Chemistry A, volume 116, issue 39, pages 9696–9705. The majority of the data collection and analysis was performed by the author, however, the assistance lent by Ms Nicola Rogers, Drs Mathew Simpson Andras Bodi, Melanie Johnson and Professor Richard Tuckett during beamtime with the collection of the data is gratefully acknowledged. The modelling program was developed by Sztáray et al. [1]. The threshold photoelectron spectra can be found in Appendix C.

4.2 Introduction This chapter focuses on the study of fast dissociations of halogenated methanes using threshold photoion photoelectron coincidence techniques. It will be demonstrated how the primary piece of information yielded from such experiments, the 0 K onset energy E0 for the production of the first photodissociation daughter ion, can be used to construct a network comprised of enthalpies of formation of neutral and ion species from which more updated and new thermochemical values can be derived. Eleven enthalpies of formation are updated, including that for CBrClF2. Importantly, enthalpies of formation reported in the literature for several neutral and ion species which were derived using purely computational methods are also confirmed. Calculations were performed using the GAUSSIAN 09 computational suit [2]. Rate constants calculated with Rice–Ramsperger–Kassel–Marcus (RRKM) theory at arbitrarily chosen transition state geometries along the dissociation coordinate J. Harvey, Modelling the Dissociation Dynamics and Threshold Photoelectron Spectra of Small Halogenated Molecules, Springer Theses, DOI: 10.1007/978-3-319-02976-4_4,  Springer International Publishing Switzerland 2014

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4 Fast Dissociations of Halogenated Methanes: A Thermochemical Network

show that the dissociations were confirmed to be fast (rates in excess of 107 s-1). Fast dissociative photoionization processes in threshold PEPICO experiments are modelled simply by taking into consideration the thermal energy distribution of the neutral molecule, which yields the energy distribution of the ion as a function of photon energy [1]. G3B3 [3] and W1 [4] composite methods (see Chap. 3) were used to determine the neutral and ion energetics, which were utilised along with previously reported energies [5] in the construction of the thermochemical network shown in Sect. 4.3.4. The experimental onset energies in the thermochemical network provide rigid links between the neutral and the ion enthalpies of formation. In the shallow well instances, the initial abundance of the first daughter ion is non-zero. Even in such cases, the fit was required to reproduce the disappearance energy range of the parent signal, thus giving the E0 value. The photon energy, at whi