Theoretical Studies on the Mechanisms and Dynamics of OH Radicals with CH2FCF2OCHF2 and CH2FOCH2F

Theoretical insight into OH- and Cl-initiated oxidation of CF3OCH(CF3)2 and CF3OCF2CF2H &fate of CF3OC(X•)(CF3)2 and CF3OCF2CF2X• radicals (X=O, O2)

In this study, the mechanistic and kinetic analysis for reactions of CF₃OCH(CF₃)₂ and CF₃OCF₂CF₂H with OH radicals and Cl atoms have been performed at the CCSD(T)//B3LYP/6-311++G(d,p) level. Kinetic isotope effects for reactions CF₃OCH(CF₃)₂/CF₃OCD(CF₃)₂ and CF₃OCF₂CF₂H/CF₃OCF₂CF₂D with OH and Cl were estimated so as to provide the theoretical estimation for future laboratory investigation. All rate constants, computed by canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT), are in reasonable agreement with the limited experimental data. Standard enthalpies of formation for the species were also calculated. Atmospheric lifetime and global warming potentials (GWPs) of the reaction species were estimated, the large lifetimes and GWPs show that the environmental impact of them cannot be ignored. The organic nitrates can be produced by the further oxidation of CF₃OC(•)(CF₃)₂ and CF₃OCF₂CF₂• in the presence of O₂ and NO. The subsequent decomposition pathways of CF₃OC(O•)(CF₃)₂ and CF₃OCF₂CF₂O• radicals were studied in detail. The derived Arrhenius expressions for the rate coefficients over 230–350 K are: k_T(1)= 5.00 × 10⁻²⁴T^3.57 exp(−849.73/T), k_T(2)= 1.79 × 10⁻²⁴T^4.84 exp(−4262.65/T), k_T(3)= 1.94 × 10⁻²⁴T^4.18 exp(−884.26/T), and k_T(4) = 9.44 × 10⁻²⁸T^5.25 exp(−913.45/T) cm³ molecule⁻¹ s⁻¹.

Atmospheric chemistry of ethers, esters, and alcohols on the lifetimes, temperature dependence, and kinetic isotope effect: an example of CF3CX2CX2CX2OX with OX reactions (X = H, D)

Mechanisms and kinetics of the reaction of CF₃CX₂CX₂CX₂OX with OX (X= H, D) radical are investigated on a sound theoretical basis.

Kinetics and mechanism of gas-phase reaction of CF3OCH2CH3 (HFE-263) with the OH radical — a theoretical study

In the present study, the density functional method with recently developed M06 functionals has been used to study the reaction of CF3OCH2CH3 with the OH radical. All possible hydrogen abstraction and displacement reaction channels have been modeled. The minimum energy path on the respective potential energy surface and energetics were calculated at the M06-2X/6-311++G(d,p) level of theory. Two different reaction mechanisms were considered: (i) reactant and product complexes called the complex mechanism and (ii) the direct mechanism (reactant → transition state → product). Tunneling corrections were made using the Eckart unsymmetrical potential. The overall rate constant calculated by the complex mechanism (keff = 1.8 × 10−13 cm3 molecule−1 s−1) has been found to be in good agreement with the experimentally determined value (1.5 ± 0.25 × 10−13 cm3 molecule−1 s−1), while the rate constant calculated by the direct mechanism (kD = 7.6 × 10−14 cm3 molecule−1 s−1) is about two times lower than the experimental value. The theoretical studies show that hydrogen atom abstraction from the –CH2– site is the most favorable reaction pathway and the reaction involves prereactive and product complexes before leading to stable product formation.

An experimental and theoretical study of the kinetics of the reaction between 3-hydroxy-3-methyl-2-butanone and OH radicals

Absolute experimental and theoretical rate constants are determined for the first time for the reaction of 3-hydroxy-3-methyl-2-butanone with OH as a function of temperature. The atmospheric implications are discussed.

Theoretical investigation of the mechanisms and dynamics of the reaction CHF2OCF 2CHFCl+Cl

The reaction of CHF2OCF2CHFCl with atomic chlorine was studied using B3LYP/6-311G(d,p), BHandHLYP/6-311G(d,p), and M06-2X/6-311G(d,p) methods and further using CCSD(T) and QCISD(T) methods. Two hydrogen abstraction channels were found for the title reaction. Dynamics calculations were followed by means of canonical variational transition state with the small-curvature tunneling correction between 220 and 2,000 K. Our rate constant k = 2.90 × 10(-15) cm(3) molecule(-1) s(-1) is in reasonable agreement with the available data (3.20 ± 0.32) × 10(-15) cm(3) molecule(-1) s(-1) at 296 K. The three-parameter Arrhenius expression (in the unit of cm(3) molecule(-1) s(-1)) for the title reaction is given as k (T) = 1.38 × 10 (-19)  T (2.57) exp (-2622.95/T).

Methyl-perfluoroheptene-ethers (CH3OC7F13): measured OH radical reaction rate coefficients for several isomers and enantiomers and their atmospheric lifetimes and global warming potentials

Mixtures of methyl-perfluoroheptene-ethers (CH3OC7F13, MPHEs) are currently in use as replacements for perfluorinated alkanes (PFCs) and poly-ether heat transfer fluids, which are persistent greenhouse gases with lifetimes >1000 years. At present, the atmospheric processing and environmental impact from the use of MPHEs is unknown. In this work, rate coefficients at 296 K for the gas-phase reaction of the OH radical with six key isomers (including stereoisomers and enantiomers) of MPHEs used commercially were measured using a relative rate method. Rate coefficients for the six MPHE isomers ranged from ∼ 0.1 to 2.9 × 10(-12) cm(3) molecule(-1) s(-1) with a strong stereoisomer and -OCH3 group position dependence; the (E)-stereoisomers with the -OCH3 group in an α- position relative to the double bond had the greatest reactivity. Rate coefficients measured for the d3-MPHE isomer analogues showed decreased reactivity consistent with a minor contribution of H atom abstraction from the -OCH3 group to the overall reactivity. Estimated atmospheric lifetimes for the MPHE isomers range from days to months. Atmospheric lifetimes, radiative efficiencies, and global warming potentials for these short-lived MPHE isomers were estimated based on the measured OH rate coefficients along with measured and theoretically calculated MPHE infrared absorption spectra. Our results highlight the importance of quantifying the atmospheric impact of individual components in an isomeric mixture.

A theoretical investigation on the conformation and the interaction of CHF₂OCF₂CHF₂ (desflurane II) with one water molecule

The conformation and the interaction of CHF₂OCF₂CHF₂ (desflurane II) with one water molecule is investigated theoretically using the ab initio MP2/aug-cc-pvdz and DFT-based M062X/6-311++G(d,p) methods. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes along with a natural bond orbital (NBO) analysis including the NBO charges, the hybridization of the C atom and the intra- and intermolecular hyperconjugation energies. In the two most stable conformers, the CH bond of the F2HCO- group occupies the gauche position. The hyperconjugation energies are about the same for both conformers and the conformational preference depends on the interaction between the non-bonded F and H atoms. The deprotonation enthalpies of the CH bonds are about the same for both conformers, the proton affinity of the less stable conformer being 3 kcal mol−1 higher. Both conformers of desflurane II interact with water forming cyclic complexes characterized by CH…O and OH…F hydrogen bonds. The binding energies are moderate, ranging from −2.4 to −3.2 kcal mol−1 at the MP2 level. The origin of the blue shifts of the ν(CH) vibrations is analyzed. In three of the complexes, the water molecule acts as an electron donor. Interestingly, in these cases a charge transfer is also directed to the non bonded OH group of the water molecule. This effect seems to be a property of polyfluorinated ethers.