A theoretical study of ROX (R=H, CH3; X=F, Cl, Br) enthalpies of formation, ionization potentials and fluoride affinities

doi:10.1016/S0009-2614(98)01451-1

Chemical Physics Letters

Volume 301, Issues 1–2, 19 February 1999, Pages 10-18

https://doi.org/10.1016/S0009-2614(98)01451-1 Get rights and content

Abstract

We report the results of a systematic Gaussian2 ab initio study of the ROX (R=H, CH₃; X=F, Cl, Br) series. The calculated standard enthalpies of formation (ΔH_f^298K) provide the following estimates for the previously undetermined R=CH₃ series; ΔH_f=−94.9, −74.0, and −57.0 kJ mol⁻¹ for X=F, Cl, and Br, respectively. The calculated ionization potentials (IP) provide an estimate of 10.24 eV for the previously undetermined IP of CH₃OBr. The first determination of fluoride affinities for ROX species are presented and are shown to depend strongly on the orientation of the F⁻+ROX complex and on the identity of the halogen substituent.

Introduction

The species ROX (R=H, CH₃; X=Cl, Br) are of interest in atmospheric chemistry because of their key role in ozone depletion. HOCl and HOBr are produced by the heterogeneous reaction of the halogen reservoir species ClONO₂ and BrONO₂, respectively, with H₂O on polar stratospheric clouds. These reactions are crucial steps in the conversion of reservoir chlorine and bromine species to active ozone-depleting species in the Antarctic stratosphere [1]. CH₃OCl and CH₃OBr are produced by the gas phase reaction of the methane oxidation intermediate CH₃O₂ with ClO and BrO, respectively. These species have been proposed as part of a catalytic ozone destruction mechanism which indicates that CH₃O₂ chemistry may speed ozone destruction by halogens [2]. The knowledge of accurate enthalpies of formation (ΔH_f) for these species is of vital importance in determining the thermodynamically feasible pathways for the production and loss of these molecules in the atmosphere. Additionally, because of the widespread use of mass spectrometric analytical methods in atmospheric chemistry, it is useful to investigate the thermodynamics of ion formation for these species; in particular, the ionization potential (IP) is relevant to positive ion electron impact and photoionization mass spectrometry methods and the fluoride affinity (FA) is relevant to negative ion fluoride transfer chemical ionization mass spectrometry methods. The species ROF (R=H, CH₃) are of interest in synthetic chemistry because of their unique properties as synthons of novel cationic reagents [3]. While spectroscopic 4, 5, 6, 7, 8and theoretical 3, 9, 10, 11, 12, 13studies have been performed on a number of ROX species (in particular, the R=H series and X=F series), primarily to determine structural information, enthalpies of formation and ionization potentials for the complete R=CH₃ series have not been systematically determined. To our knowledge, no previous work has addressed the fluoride affinities of the R=H or R=CH₃ series. In order to address these remaining issues, we report the results of a systematic Gaussian2 (G2) ab initio study of ROX species with a focus on the determination of neutral and ionic thermodynamic properties of importance in atmospheric chemistry research.

Section snippets

Computational methods

All electronic structure calculations were performed using the GAUSSIAN94 package [14]. Recently, the accuracy of the G2 compound method [15]for the determination of ion thermodynamic properties was assessed, and G2 theory was found to be accurate to an average absolute deviation of 0.06 eV for both ionization potentials and electron affinities for the 146 molecules included in the G2 set [16]. Due to this impressive performance for ion thermodynamic properties, G2 theory was used to calculate

Structures and natural charges

The MP2(full)/6-31G(d) geometries of HOX and CH₃OX (X=F, Cl, Br), as well as the cationic and fluoride adduct species, are given in Table 1. Schematic representations for these species are included in Fig. 1, and natural charge information for each species is included in Table 2. Note that because the CH₃OX species maintain C_s symmetry, two of the hydrogen atoms are equivalent and labeled H₂ in Fig. 1. Geometries and energies were also determined for the CH₂OCl and CH₂OBr species, and the

Conclusions

The enthalpies of formation for all ROX species (R=H, CH₃; X=F, Cl, Br) have been calculated using isodesmic reactions and the G2 ab initio method, resulting in the first determination of ΔH_f for the complete R=CH₃ series. These calculations are in agreement with previous experimental work for the HOX species, with a standard deviation of 9.5 kJ mol⁻¹ from the experimental values. Each anionic analog of ROX was found to dissociate into RO and X⁻ fragments. Additionally, the ionization

Acknowledgements

The authors acknowledge support from the Camille and Henry Dreyfus Foundation, the American Chemical Society – Petroleum Research Fund, Research Corporation, the Michigan Space Grant Consortium, and the National Science Foundation.

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A theoretical study of ROX (R=H, CH3; X=F, Cl, Br) enthalpies of formation, ionization potentials and fluoride affinities

Abstract

Introduction

Section snippets

Computational methods

Structures and natural charges

Conclusions

Acknowledgements

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A theoretical study of ROX (R=H, CH₃; X=F, Cl, Br) enthalpies of formation, ionization potentials and fluoride affinities