Theoretical Investigation of the gas-phase reactions of (CF3)2CHOCH3 with OH Radical

Connecting Gas-Phase Computational Chemistry to Condensed Phase Kinetic Modeling: The State-of-the-Art

In recent decades, quantum chemical calculations (QCC) have increased in accuracy, not only providing the ranking of chemical reactivities and energy barriers (e.g., for optimal selectivities) but also delivering more reliable equilibrium and (intrinsic/chemical) rate coefficients. This increased reliability of kinetic parameters is relevant to support the predictive character of kinetic modeling studies that are addressing actual concentration changes during chemical processes, taking into account competitive reactions and mixing heterogeneities. In the present contribution, guidelines are formulated on how to bridge the fields of computational chemistry and chemical kinetics. It is explained how condensed phase systems can be described based on conventional gas phase computational chemistry calculations. Case studies are included on polymerization kinetics, considering free and controlled radical polymerization, ionic polymerization, and polymer degradation. It is also illustrated how QCC can be directly linked to material properties.

Hydrogen bond docking site competition in methyl esters

The OH⋯O hydrogen bonds in the 2,2,2-trifluoroethanol (TFE)-methyl ester complexes in the gas phase have been investigated by FTIR spectroscopy and DFT calculations. Methyl formate (MF), methyl acetate (MA), and methyl trifluoroacetate (MTFA) were chosen as the hydrogen bond acceptors. A dominant inter-molecular hydrogen bond was formed between the OH group of TFE and different docking sites in the methyl esters (carbonyl oxygen or ester oxygen). The competition of the two docking sites decides the structure and spectral properties of the complexes. On the basis of the observed red shifts of the OH-stretching transition with respect to the TFE monomer, the order of the hydrogen bond strength can be sorted as TFE-MA (119cm^-1)>TFE-MF (93cm^-1)>TFE-MTFA (44cm^-1). Combining the experimental infrared spectra with the DFT calculations, the Gibbs free energies of formation were determined to be 1.5, 4.5 and 8.6kJmol^-1 for TFE-MA, TFE-MF and TFE-MTFA, respectively. The hydrogen bonding in the MTFA complex is much weaker than those of the TFE-MA and TFE-MF complexes due to the effect of the CF₃ substitution on MTFA, while the replacement of an H atom with a CH₃ group in methyl ester only slightly increases the hydrogen bond strength. Topological analysis and localized molecular orbital energy decomposition analysis was also applied to compare the interactions in the complexes.

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.

Theoretical investigation on mechanism and kinetics of the Cl-initiated hydrogen abstraction reactions of ethyl trifluoroacetate at 298 K

Theoretical investigations were carried out on the gas-phase reactions of CF3C(O)OCH2CH3, ethyl trifluoroacetate (ETFA) with Cl atoms by means of modern density functional theory methods. The optimized geometries, frequencies and minimum energy path were obtained with the hybrid density functional model MPWB1K using the 6-31+G(d,p) basis set. The single point energy calculations were refined further using the G2(MP2) method. Two conformers relatively close in energy were identified for ETFA; both are likely to be important in the temperature range of our study. The existence of transition states on the corresponding potential energy surface was ascertained by performing intrinsic reaction coordinate calculations. The rate constant at 298 K calculated theoretically using canonical transition state theory was found to be in good agreement with experimentally measured values. Our calculations suggest that H abstraction from the -CH2 group is kinetically and thermodynamically more favorable than abstraction from the -CH3 group. The atmospheric lifetime of ETFA with Cl atoms was determined to be 1.98 years. To the best of our knowledge, this work represents the first determination of the rate coefficients for the gas-phase reaction of chlorine atoms in ETFA.

Theoretical investigation on the atmospheric fate of CF3C(O)OCH 2O radical: alpha-ester rearrangement vs oxidation at 298 K

A theoretical study on the mechanism of the thermal decomposition of CF(3)C(O)OCH(2)O radical is presented for the first time. Geometry optimization and frequency calculations were performed at the MPWB1K/6-31 + G(d, p) level of theory and energetic information further refined by calculating the energy of the species using G2(MP2) theory. Three plausible decomposition pathways including α-ester rearrangement, reaction with O(2) and thermal decomposition (C-O bond scission) were considered in detail. Our results reveal that reaction with O(2) is the dominant path for the decomposition of CF(3)C(O)OCH(2)O radical in the atmosphere, involving the lowest energy barrier, which is in accord with experimental findings. Our theoretical results also suggest that α-ester rearrangement leading to the formation of trifluoroacetic acid TFA makes a negligible contribution to decomposition of the title alkoxy radical. The thermal rate constants for the above decomposition pathways were evaluated using canonical transition state theory (CTST) at 298 K.

Theoretical investigation on gas-phase reaction of CF3CH2OCH3 with OH radicals and fate of alkoxy radicals (CF3CH(O•)OCH3/CF3CH2OCH2O•)

Detailed theoretical investigation has been performed on the mechanism, kinetics and thermochemistry of the gas phase reactions of CF3CH2OCH3 (HFE-263fb2) with OH radicals using ab-initio and DFT methods. Reaction profiles are modeled including the formation of pre-reactive and post-reactive complexes at entrance and exit channels, respectively. Our calculations reveal that hydrogen abstraction from the CH2 group is thermodynamically and kinetically more facile than that from the CH3 group. Using group-balanced isodesmic reactions, the standard enthalpies of formation for CF3CH2OCH3 and radicals (CF3CHOCH3 and CF3CH2OCH2) are also reported for the first time. The calculated bond dissociation energies for the CH bonds are in good agreement with experimental results. At 298K, the calculated total rate coefficient for CF3CH2OCH3+OH reactions is found to be in good agreement with the experimental results. The atmospheric fate of the alkoxy radicals, CF3CH(O)OCH3 and CF3CH2OCH2O are also investigated for the first time using the same level of theory. Out of three plausible decomposition channels, our results clearly point out that reaction with O2 is not the dominant path leading to the formation of CF3C(O)OCH3 for the decomposition of CF3CH(O)OCH3 radical in the atmosphere. This is in accord with the recent report of Osterstrom et al. [CPL 524 (2012) 32] but found to be in contradiction with experimental finding of Oyaro et al. [JPCA 109 (2005) 337].

Theoretical investigation of atmospheric chemistry of volatile anaesthetic sevoflurane: reactions with the OH radicals and atmospheric fate of the alkoxy radical (CF3)2CHOCHFO: thermal decomposition vs. oxidation

Reaction profile (kcal mol⁻¹) for (CF₃)₂CHOCHFO radical at the M06-2X/6-311++G(d,p) level.

A review of molecular-level mechanism of membrane degradation in the polymer electrolyte fuel cell

Chemical degradation of perfluorosulfonic acid (PFSA) membrane is one of the most serious problems for stable and long-term operations of the polymer electrolyte fuel cell (PEFC). The chemical degradation is caused by the chemical reaction between the PFSA membrane and chemical species such as free radicals. Although chemical degradation of the PFSA membrane has been studied by various experimental techniques, the mechanism of chemical degradation relies much on speculations from ex-situ observations. Recent activities applying theoretical methods such as density functional theory, in situ experimental observation, and mechanistic study by using simplified model compound systems have led to gradual clarification of the atomistic details of the chemical degradation mechanism. In this review paper, we summarize recent reports on the chemical degradation mechanism of the PFSA membrane from an atomistic point of view.

Computational study on the thermal decomposition and isomerization of the CH3OCF2O• radical

The geometries of the reactant, products, and transition states involved in the decomposition pathways of the CH3OCF2O• radical formed during the photooxidation of CH3OCHF2 (HFE-152a) have been optimized and characterized at the DFT-B3LYP level of theory using the 6–311G(d,p) basis set. Single-point energy calculations have been made at the G2M (CC,MP2) level of theory. Out of the four prominent decomposition channels considered, the β-C–O bond scission is found to be the dominant path involving a barrier height of 9.78 kcal mol–1 (1 cal = 4.184 J). The thermal rate constant for the above decomposition pathway is evaluated using canonical transition state theory (CTST) and was found to be 5.27 × 104 s–1 at 298 K and 1 atm (1 atm = 101.325 kPa).