High-Accuracy Thermochemistry of Atmospherically Important Fluorinated and Chlorinated Methane Derivatives

Theory-Based Mechanism for Fluoromethane Combustion I: Thermochemistry and Abstraction Reactions

A new detailed chemical kinetic mechanism is presented for small fluorinated hydrocarbons. Ab initio electronic structure theory is used to provide heats of formation with subchemical accuracy. The ANL0 method is extended to include fluorine. The resulting heats of formation at 0 K are in excellent agreement with 36 benchmark species in the Active Thermochemical Tables, with a mean error of μ = -0.02 kJ/mol and a standard deviation of σ = 0.91 kJ/mol. The thermophysical properties for 92 small-molecule H/C/O/F species are computed. The rate coefficients for 40+ H-abstraction reactions involving H, O, F, OH, OF, HO₂, and various methyl radicals with CH₄, CH₃F, CH₂F₂, CHF₃, CH₂O, and CHFO are discussed. The computed rate constants are in excellent agreement with the available literature. Additionally, 30+ rate constants are provided for F abstraction, which are several orders of magnitude smaller than H abstraction. The thermophysical properties and rate constants are provided in a mechanism. This mechanism is the first in a series of theory-based investigations into the thermal destruction of per- and polyfluorinated species.

Route to Chemical Accuracy for Computational Uranium Thermochemistry

Benchmark spinor-based relativistic coupled-cluster calculations for the ionization energies of the uranium atom, the uranium monoxide molecule (UO), and the uranium dioxide molecule (UO₂) and for the bond dissociation energies of UO and UO₂ are reported. The accuracy of these calculations in the treatments of relativistic, electron-correlation, and basis-set effects is analyzed. The intrinsic convergence of the computed results and the favorable comparison with the experimental values demonstrate the unique applicability of the spinor representation of quantum-chemical methods to open-shell uranium-containing atomic and molecular species with uranium oxidation states ranging from U(0) to U(V).

Machine Learning of Coupled Cluster (T)-Energy Corrections via Delta (Δ)-Learning

Accurate thermochemistry is essential in many chemical disciplines, such as astro-, atmospheric, or combustion chemistry. These areas often involve fleetingly existent intermediates whose thermochemistry is difficult to assess. Whenever direct calorimetric experiments are infeasible, accurate computational estimates of relative molecular energies are required. However, high-level computations, often using coupled cluster theory, are generally resource-intensive. To expedite the process using machine learning techniques, we generated a database of energies for small organic molecules at the CCSD(T)/cc-pVDZ, CCSD(T)/aug-cc-pVDZ, and CCSD(T)/cc-pVTZ levels of theory. Leveraging the power of deep learning by employing graph neural networks, we are able to predict the effect of perturbatively included triples (T), that is, the difference between CCSD and CCSD(T) energies, with a mean absolute error of 0.25, 0.25, and 0.28 kcal mol^-1 (R² of 0.998, 0.997, and 0.998) with the cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets, respectively. Our models were further validated by application to three validation sets taken from the S22 Database as well as to a selection of known theoretically challenging cases.

The long road to calibrated prediction uncertainty in computational chemistry

Uncertainty quantification (UQ) in computational chemistry (CC) is still in its infancy. Very few CC methods are designed to provide a confidence level on their predictions, and most users still rely improperly on the mean absolute error as an accuracy metric. The development of reliable UQ methods is essential, notably for CC to be used confidently in industrial processes. A review of the CC-UQ literature shows that there is no common standard procedure to report or validate prediction uncertainty. I consider here analysis tools using concepts (calibration and sharpness) developed in meteorology and machine learning for the validation of probabilistic forecasters. These tools are adapted to CC-UQ and applied to datasets of prediction uncertainties provided by composite methods, Bayesian ensembles methods, and machine learning and a posteriori statistical methods.

Thermodynamic Properties: Enthalpy, Entropy, Heat Capacity, and Bond Energies of Fluorinated Carboxylic Acids

Fluorinated carboxylic acids and their radicals are becoming more prevalent in environmental waters and soils as they have been produced and used for numerous commercial applications. Understanding the thermochemical properties of fluorinated carboxylic acids will provide insights into the stability and reaction paths of these molecules in the environment, in body fluids, and in biological and biochemical processes. Structures and thermodynamic properties for over 50 species related to fluorinated carboxylic acids with two and three carbons are determined with density functional computational calculations B3LYP, M06-2X, and MN15 and higher ab initio levels CBS-QB3, CBS-APNO, and G4 of theory. The lowest energy structures, moments of inertia, vibrational frequencies, and internal rotor potentials of each target species are determined. Standard enthalpies of formation, Δ_fH₂₉₈^°, from CBS-APNO calculations show the smallest standard deviation among methods used in this work. Δ_fH₂₉₈^° values are determined via several series of isodesmic and/or isogyric reactions. Enthalpies of formation are determined for fluorinated acetic and propionic acids and their respective radicals corresponding to the loss of hydrogen and fluorine atoms. Heat capacities as a function of temperature, C_p(T), and entropy at 298 K, S₂₉₈^°, are determined. Thermochemical properties for the fluorinated carbon groups used in group additivity are also developed. Bond dissociation energies (BDEs) for the carbon-hydrogen, carbon-fluorine, and oxygen-hydrogen (C-H, C-F, and O-H BDEs) in the acids are reported. The C-H, C-F, and O-H bond energies of the fluorinated carboxylic acids are in the range of 89-104, 101-125, and 109-113 kcal mol^-1, respectively. General trends show that the O-H bond energies on the acid group increase with the increase in the fluorine substitution. The strong carbon fluorine bonds in a fluorinated acid support the higher stability of the perfluorinated acids in the environment.

From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF3SF5) by Double Velocity Map Imaging

Trifluoromethylsulfur pentafluoride (CF₃SF₅) was valence threshold photoionized in a double imaging photoelectron photoion coincidence spectrometer using vacuum ultraviolet synchrotron radiation. In the 12.5-16.4 eV photon energy range, CF₃⁺, SF₅⁺, and SF₃⁺ cations were observed in both room temperature (RT) and molecular beam (MB) experiments. Their fractional abundances exhibited differences beyond the sample temperature. Kinetic energy analysis of the fragment ions confirmed the difference in the dissociative photoionization mechanism. In the RT experiment, the CF₃⁺ kinetic energies were extrapolated to a 11.84 ± 0.15 eV threshold, which was used in an ion cycle to determine the enthalpy of formation of CF₃SF₅ as Δ_fH°_298K(CF₃SF₅) = -1593 ± 16 kJ mol^-1. We also updated the enthalpy of formation of the sulfur pentafluoride radical as Δ_fH°_298K(SF₅) = -854 ± 7 kJ mol^-1 and discuss the discrepancy between the CF₃ ionization energy based on the Active Thermochemical Tables and the value anchored to the CF ionization energy. A computed reaction enthalpy network optimization resulted in Δ_fH°_298K(CF₃SF₅) = -1608 ± 20 kJ mol^-1. Both values for Δ_fH°_298K(CF₃SF₅) agree with previous ab initio ones in contrast to the original, experimental determination. SF₃⁺ is formed by F-transfer processes both in the RT and MB experiments. Although the same peaks were observed in both experiments, the lower SF₃⁺ onset energy and the more slowly rising CF₃⁺ kinetic energy release in the MB experiment revealed clustering and intracluster F-transfer reactions upon ionization. The monomer and dimer cation potential energy surfaces were explored to rationalize the observations. In the dimer cation, the observer CF₃SF₅ catalyzes fluorine transfer and promotes CF₄ formation, which ultimately leads to the SF₃⁺ fragment ion.

Theoretical investigations on mechanisms and kinetics of CH2XO2 (X=F, Cl) with Cl reaction in the atmosphere

The reactions of the CH₂XO₂ (X=F, Cl) with chlorine radical have been firstly investigated utilizing the BMC-CCSD//B3LYP method. The comprehensive calculations indicate that the association-elimination and S_N2 displacement reaction mechanisms existed on the singlet potential energy surface (PES), and H-abstraction and S_N2 displacement reaction mechanism existed on the triplet PES for the CH₂XO₂ (X=F, Cl) + Cl reactions. On the triplet PES, the dominant reactions are production of P3_X (CHXO₂ (X=F, Cl) + HCl) by direct H-abstraction. On the singlet PES, three energy-rich adducts, IM1_X (CH₂XOOCl (X=F, Cl)), IM2_X (CH₂XOClO (X=F, Cl)), and IM3_X (CH₂(OX)OCl (X=F, Cl)), are produced. RRKM-computed reveals that IM1_X (CH₂XOOCl (X=F, Cl)) produced by collisional stabilization occupied the reaction T ≤ 500 and 400 K, respectively, while P1_X (CHXO (X=F, Cl) + HOCl) are forecasted to be the dominant products at high temperatures. The atmospheric lifetime of CH₂FO₂ and CH₂ClO₂ in Cl is around 1.18 and 2.50 weeks, respectively. Time-dependent density functional theory (TDDFT) computations imply that IM1_X (CH₂XOOCl (X=F, Cl)) will photolyze under the sunlight. The current results could guide us to well understand the mechanism of the CH₂XO₂ (X=F, Cl) + Cl reactions and may be helpful to understand Cl-combustion chemistry. Graphical Abstract Predicted rate constant of the dominant pathways and the total rate constants at 760 Torr, N2 in the temperature region of 200-3000 K for the CH₂XO₂ (X=F, Cl) + Cl reactions.

Reactions of C+ + Cl-, Br-, and I--A comparison of theory and experiment

Rate constants for the reactions of C⁺ + Cl^-, Br^-, and I^- were measured at 300 K using the variable electron and neutral density electron attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. Upper bounds of <10^-8 cm³ s^-1 were found for the reaction of C⁺ with Br^- and I^-, and a rate constant of 4.2 ± 1.1 × 10^-9 cm³ s^-1 was measured for the reaction with Cl^-. The C⁺ + Cl^- mutual neutralization reaction was studied theoretically from first principles, and a rate constant of 3.9 × 10^-10 cm³ s^-1, an order of magnitude smaller than experiment, was obtained with spin-orbit interactions included using a semiempirical model. The discrepancy between the measured and calculated rate constants could be explained by the fact that in the experiment, the total loss of C⁺ ions was measured, while the theoretical treatment did not include the associative ionization channel. The charge transfer was found to take place at small internuclear distances, and the spin-orbit interaction was found to have a minor effect on the rate constant.

On the Competition Between Electron Autodetachment and Dissociation of Molecular Anions

We treat the competition between autodetachment of electrons and unimolecular dissociation of excited molecular anions as a rigid-/loose-activated complex multichannel reaction system. To start, the temperature and pressure dependences under thermal excitation conditions are represented in terms of falloff curves of separated single-channel processes within the framework of unimolecular reaction kinetics. Channel couplings, caused by collisional energy transfer and "rotational channel switching" due to angular momentum effects, are introduced afterward. The importance of angular momentum considerations is stressed in addition to the usual energy treatment. Non-thermal excitation conditions, such as typical for chemical activation and complex-forming bimolecular reactions, are considered as well. The dynamics of excited SF₆^- anions serves as the principal example. Other anions such as CF₃^- and POCl₃^- are also discussed.

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5CHFCl: Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions

The gas-phase unimolecular reactions of C₂D₅CHFCl molecules with 94 kcal mol^-1 of vibrational energy have been studied by the chemical-activation experimental technique and by electronic-structure computations. Products from the reaction of C₂D₅CHFCl molecules, formed by the recombination of C₂D₅ and CHFCl radicals in a room temperature bath gas, were measured by gas chromatography-mass spectrometry. The 2,1-DCl (81%) and 1,1-HCl (17%) elimination reactions are the principal processes, but 2,1-DF and 1,1-HF elimination reactions also are observed. Comparison of experimental rate constants to calculated statistical rate constants provides threshold energies. The potential surfaces associated with C₂D₅(F)C: + HCl and C₂D₅(Cl)C: + HF reactions are of special interest because hydrogen-bonded adducts with HCl and HF with dissociation energies of 6.4 and 9.3 kcal mol^-1, respectively, are predicted by calculations. The relationship between the geometries and threshold energies of transition states for 1,1-HCl elimination and carbene:HCl adducts is complex, and previous studies of related molecules, such as CD₃CHFCl, CD₂ClCHFCl, C₂D₅CHCl₂, and halogenated methanes are included in the computational analysis. Extensive calculations for CH₃CHFCl as a model for 1,1-HCl reactions illustrate properties of the exit-channel potential energy surface. Since the 1,1-HCl transition state is submerged relative to dissociation of the adduct, inner and outer transition states should be considered for analysis of rate constants describing 1,1-HCl elimination and addition reactions of carbenes to HCl.

Thermochemical Properties: Enthalpy, Entropy, and Heat Capacity of C2-C3 Fluorinated Aldehydes. Radicals and Fluorocarbon Group Additivity

Thermochemical properties of fluorinated aldehydes are important for understanding their stability and reactions in the environment and in thermal processes. Structures and thermochemical properties of C1 to C3 fluorinated aldehydes are determined by use of computational chemistry. Standard enthalpies of formation for 30 C₂- and C₃-fluorinated aldehydes and 31 radicals were calculated with 11 different ab initio and density functional theory methods: CBS-APNO, CBS-4M, CBS-QB3, M06-2X, ωB97X, B3LYP, G-2, G-3, G-4, and W1U via several series of isodesmic reactions. Entropy, S°₂₉₈, and heat capacities, C _p( T)'s (300 ≤ T/K ≤ 1500) from vibration, translation, and external rotation contributions are calculated on the basis of the vibration frequencies and structures obtained from the B3LYP/6-31++G(d,p) density functional method. Potential barriers for the internal rotations are also from this method and used to calculate hindered rotor contributions to S°298 and Cp(T)'s using direct integration over energy levels of the internal rotational potential curves. Literature data on standard enthalpies of formation of fluorinated aldehydes are compared. Thermochemical properties for the fluorinated carbon groups CO/C/F, C/CO/F3, C/CO/F/H2, C/C/CO/F/H, C/C/CO/F2, and C/C/CO/F/H are developed. Non-next nearest neighbor terms for the strong interactions resulting from fluorine atoms on adjacent and on second nearest carbon atoms are unfortunately, needed. The required non-next-neighbor interactions significantly reduce the practical application of group additivity for thermochemical properties of highly fluorinated halocarbons.

Critical evaluation of the enthalpies of formation for fluorinated compounds using experimental data and high-level ab initio calculations

The ab initio method for prediction of the enthalpies of formation for CHON-containing organic compounds proposed earlier (J. Chem. Theory Comput. 2018, 14, 5920-5932) has been extended to their fluorinated derivatives. A single experimental Δ_f H ^o _m is typically available for compounds in this scope. Thus, a priori evaluation of the data quality was found to be inefficient despite all available experimental data for C₁─C₃ hydrofluorocarbons and 34 data points for medium-size organofluorine compounds being considered. The training set was derived by analyzing consistency of the experimental and predicted values and removal of outliers. Significant issues in the experimental data, including inconsistency across different laboratories, were identified and potential causes for these problems were discussed. A conservative estimate of uncertainty for the experimental Δ_f H ^o _m of organofluorine compounds was proposed.

Analysis of the Five Unimolecular Reaction Pathways of CD2ClCHFCl with Emphasis on CD2Cl(F)C: and CD2Cl(Cl)C: Formed by 1,1-HCl and 1,1-HF Elimination

The five unimolecular HX and DX (X = F, Cl) elimination pathways of CD₂ClCHFCl* were examined using a chemical activation technique; the molecules were generated with 92 kcal mol^-1 of vibrational energy in a room-temperature bath gas by a combination of CD₂Cl and CHFCl radicals. The total unimolecular rate constant was 9.7 × 10⁷ s^-1, and branching fractions for each channel were 0.52 (2,1-DCl), 0.29 (1,1-HCl), 0.10 (2,1-DF), 0.07 (1,1-HF), and 0.02 (1,2-HCl). Comparison of the individual experimental rate constants to calculated statistical rate constants gave threshold energies for each process as 63, 72, 66, 73, and 70 kcal mol^-1, listed in the same order as the branching fractions. The 1,1-HCl and 1,1-HF reactions gave carbenes, CD₂Cl(F)C: and CD₂Cl(Cl)C:, respectively, as products, which have hydrogen-bonded complexes with HCl or HF in the exit channel of the potential energy surface. These carbenes have energy in excess of the threshold energy for D atom migration to give CDCl═CDF and CDCl═CDCl, and the subsequent cis-trans isomerization rates of the dihaloethenes can provide information about energy disposal by the 1,1-HX elimination reactions. Electronic structure calculations provide information for transition states of CD₂ClCHFCl and hydrogen-bonded complexes of carbenes with HF and HCl. In addition, D atom migration in both free carbenes and in complexes formed by the carbene hydrogen bonding to HCl or HF is explored.

High Accuracy Quantum Chemical and Thermochemical Network Data for the Heats of Formation of Fluorinated and Chlorinated Methanes and Ethanes

Reliable heats of formation are reported for numerous fluorinated and chlorinated methane and ethane derivatives by means of an accurate thermochemical protocol, which involves explicitly correlated coupled-cluster calculations augmented with anharmonic, scalar relativistic, and diagonal Born-Oppenheimer corrections. The theoretical results, along with additional experimental data, are further enhanced with the help of the thermochemical network approach. For 28 species, out of 50, this study presents the best estimates, and discrepancies with previous reports are also highlighted. Furthermore, the effects of the less accurate theoretical data on the results yielded by thermochemical networks are discussed.

Oxygen anion (O- ) and hydroxide anion (HO- ) reactivity with a series of old and new refrigerants

The reactivity of a series of commonly used halogenated compounds (trihalomethanes, chlorofluorocarbon, hydrochlorofluorocarbon, fluorocarbons, and hydrofluoroolefin) with hydroxide and oxygen anion is studied in a compact Fourier transform ion cyclotron resonance. O^- is formed by dissociative electron attachment to N₂ O and HO^- by a further ion-molecule reaction with ammonia. Kinetic experiments are performed by increasing duration of introduction of the studied molecule at a constant pressure. Hydroxide anion reactions mainly proceed by proton transfer for all the acidic compounds. However, nucleophilic substitution is observed for chlorinated and brominated compounds. For fluorinated compounds, a specific elimination of a neutral fluorinated alkene is observed in our results in parallel with the proton transfer reaction. Oxygen anion reacts rapidly and extensively with all compounds. Main reaction channels result from nucleophilic substitution, proton transfer, and formal H₂⁺ transfer. We highlight the importance of transfer processes (atom or ion) in the intermediate ion-neutral complex, explaining part of the observed reactivity and formed ions. In this paper, we present the first reactivity study of anions with HFO 1234yf. Finally, the potential of O^- and HO^- as chemical ionization reagents for trace analysis is discussed.

The Unimolecular Reactions of CF3CHF2 Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to the 1,2-H Atom Transfer, 1,1-HF and 1,2-HF Elimination Reactions, and the Dependence of Threshold Energies on the Number of F-Atom Substituents in the Fluoroethane Molecules

The recombination of CF₃ and CHF₂ radicals in a room-temperature bath gas was used to prepare vibrationally excited CF₃CHF₂* molecules with 101 kcal mol^-1 of vibrational energy. The subsequent 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions were observed as a function of bath gas pressure by following the CHF₃, CF₃(F)C: and C₂F₄ product concentrations by gas chromatography using a mass spectrometer as the detector. The singlet CF₃(F)C: concentration was measured by trapping the carbene with trans-2-butene. The experimental rate constants are 3.6 × 10⁴, 4.7 × 10⁴, and 1.1 × 10⁴ s^-1 for the 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions, respectively. These experimental rate constants were matched to statistical RRKM calculated rate constants to assign threshold energies (E₀) of 88 ± 2, 88 ± 2, and 87 ± 2 kcal mol^-1 to the three reactions. Pentafluoroethane is the only fluoroethane that has a competitive H atom transfer decomposition reaction, and it is the only example with 1,1-HF elimination being more important than 1,2-HF elimination. The trend of increasing threshold energies for both 1,1-HF and 1,2-HF processes with the number of F atoms in the fluoroethane molecule is summarized and investigated with electronic-structure calculations. Examination of the intrinsic reaction coordinate associated with the 1,1-HF elimination reaction found an adduct between CF₃(F)C: and HF in the exit channel with a dissociation energy of ∼5 kcal mol^-1. Hydrogen-bonded complexes between HF and the H atom migration transition state of CH₃(F)C: and the F atom migration transition state of CF₃(F)C: also were found by the calculations. The role that these carbene-HF complexes could play in 1,1-HF elimination reactions is discussed.

Shock Wave and Theoretical Modeling Study of the Dissociation of CH2F2. I. Primary Processes

The unimolecular dissociation of CH₂F₂ leading to CF₂ + H₂, CHF + HF, or CHF₂ + H is investigated by quantum-chemical calculations and unimolecular rate theory. Modeling of the rate constants is accompanied by shock wave experiments over the range of 1400-1800 K, monitoring the formation of CF₂. It is shown that the energetically most favorable dissociation channel leading to CF₂ + H₂ has a higher threshold energy than the energetically less favorable one leading to CHF + HF. Falloff curves of the dissociations are modeled. Under the conditions of the described experiments, the primary dissociation CH₂F₂ → CHF + HF is followed by the reaction CHF + HF → CF₂ + H₂. The experimental value of the rate constant for the latter reaction indicates that it does not proceed by an addition-elimination process involving CH₂F₂* intermediates, as assumed before.

Accurate Theoretical Thermochemistry for Fluoroethyl Radicals

An accurate coupled-cluster (CC) based model chemistry was applied to calculate reliable thermochemical quantities for hydrofluorocarbon derivatives including radicals 1-fluoroethyl (CH₃-CHF), 1,1-difluoroethyl (CH₃-CF₂), 2-fluoroethyl (CH₂F-CH₂), 1,2-difluoroethyl (CH₂F-CHF), 2,2-difluoroethyl (CHF₂-CH₂), 2,2,2-trifluoroethyl (CF₃-CH₂), 1,2,2,2-tetrafluoroethyl (CF₃-CHF), and pentafluoroethyl (CF₃-CF₂). The model chemistry used contains iterative triple and perturbative quadruple excitations in CC theory, as well as scalar relativistic and diagonal Born-Oppenheimer corrections. To obtain heat of formation values with better than chemical accuracy perturbative quadruple excitations and scalar relativistic corrections were inevitable. Their contributions to the heats of formation steadily increase with the number of fluorine atoms in the radical reaching 10 kJ/mol for CF₃-CF₂. When discrepancies were found between the experimental and our values it was always possible to resolve the issue by recalculating the experimental result with currently recommended auxiliary data. For each radical studied here this study delivers the best heat of formation as well as entropy data.

Computational study of H-abstraction reactions from CH3OCH2CH2Cl/CH3CH2OCH2CH2Cl by Cl atom and OH radical and fate of alkoxy radicals

Multichannel gas-phase reactions of CH₃OCH₂CH₂Cl/CH₃CH₂OCH₂CH₂Cl with chlorine atom and hydroxyl radical have been investigated using ab initio method and canonical variational transition-state dynamic computations with the small-curvature tunneling correction. Further energetic information is refined by the coupled-cluster calculations with single and double excitations (CCSD)(T) method. Both hydrogen abstraction and displacement processes are carried out at the same level. Our results reveal that H-abstraction from the -OCH₂- group is the dominant channel for CH₃OCH₂CH₂Cl by OH radical or Cl atom, and from α-CH₂ of the group CH₃CH₂- is predominate for the reaction CH₃CH₂OCH₂CH₂Cl with Cl/OH. The contribution of displacement processes may be unimportant due to the high barriers. The values of the calculated rate constants reproduce remarkably well the available experiment data. Standard enthalpies of formation for reactants and product radicals are calculated by isodesmic reactions. The Arrhenius expressions are given within 220-1200 K. The atmospheric lifetime, ozone depleting potential (ODP), ozone formation potential (OFP), and global warming potential (GWP) of CH₃OCH₂CH₂Cl/CH₃CH₂OCH₂CH₂Cl are investigated. Meanwhile, the atmospheric fate of the alkoxy radicals are also researched using the same level of theory. To shed light on the atmospheric degradation, a mechanistic study is obtained, which indicates that reaction with O₂ is the dominant path for the decomposition of CH₃OCH(O•)CH₂Cl, the C-C bond scission reaction is the primary reaction path in the consumption of CH₃CH(O•)OCH₂CH₂Cl in the atmosphere.

HIGHLIGHTS

Ab initio method and canonical variational transition-state theory are employed to study the kinetic nature of hydrogen abstraction reactions of CH₃OCH₂CH₂Cl/CH₃CH₂OCH₂CH₂Cl with Cl atom and OH radical and fate of alkoxy radicals (CH₃OCH(O•)CH₂Cl/CH₃CH(O•)OCH₂CH₂Cl).

Chemical Activation Study of the Unimolecular Reactions of CD3CD2CHCl2 and CHCl2CHCl2 with Analysis of the 1,1-HCl Elimination Pathway

Chemically activated C₂D₅CHCl₂ molecules were generated with 88 kcal mol^-1 of vibrational energy by the recombination of C₂D₅ and CHCl₂ radicals in a room temperature bath gas. The competing 2,1-DCl and 1,1-HCl unimolecular reactions were identified by the observation of the CD₃CD═CHCl and CD₃CD═CDCl products. The initial CD₃CD₂C-Cl carbene product from 1,1-HCl elimination rearranges to CD₃CD═CDCl under the conditions of the experiments. The experimental rate constants were 2.7 × 10⁷ and 0.47 × 10⁷ s^-1 for 2,1-DCl and 1,1-HCl elimination reactions, respectively, which corresponds to branching fractions of 0.84 and 0.16. The experimental rate constants were compared to calculated statistical rate constants to assign threshold energies of 54 and ≈66 kcal mol^-1 for the 1,2-DCl and 1,1-HCl reactions, respectively. The statistical rate constants were obtained from models developed from electronic-structure calculations for the molecule and its transition states. The rate constant (5.3 × 10⁷ s^-1) for the unimolecular decomposition of CHCl₂CHCl₂ molecules formed with 82 kcal mol^-1 of vibrational energy by the recombination of CHCl₂ radicals also is reported. On the basis of the magnitude of the calculated rate constant, 1,1-HCl elimination must contribute less than 15% to the reaction; 1,2-HCl elimination is the major reaction and the threshold energy is 59 kcal mol^-1. Calculations also were done to analyze previously published rate constants for chemically activated CD₂ClCHCl₂ molecules with 86 kcal mol^-1 of energy to obtain a better overall description of the nature of the 1,1-HCl pathway for 1,1-dichloroalkanes. The interplay of the threshold energies for the 2,1-HCl and 1,1-HCl reactions and the available energy determines the product branching fractions for individual molecules. The unusual nature of the transition state for 1,1-HCl elimination is discussed.

Thermochemical Properties and Bond Dissociation Energies for Fluorinated Methanol, CH3-xFxOH, and Fluorinated Methyl Hydroperoxides, CH3-xFxOOH: Group Additivity

Oxygenated fluorocarbons are routinely found in sampling of environmental soils and waters as a result of the widespread use of fluoro and chlorofluoro carbons as heat transfer fluids, inert materials, polymers, fire retardants and solvents; the influence of these chemicals on the environment is a growing concern. The thermochemical properties of these species are needed for understanding their stability and reactions in the environment and in thermal process. Structures and thermochemical properties on the mono- to trifluoromethanol, CH3-xFxOH, and fluoromethyl hydroperoxide, CH3-xFxOOH (1 ≤ x ≤ 3), are determined by CBS-QB3, CBS-APNO, and G4 calculations. Entropy, S°298, and heat capacities, Cp(T)'s (300 ≤ T/K ≤ 1500) from vibration, translation, and external rotation contributions are calculated on the basis of the vibration frequencies and structures obtained from the B3LYP/6-31+G(d,p) density functional method. Potential barriers for the internal rotations are also calculated from this method and used to calculate hindered rotor contributions to S°298 and Cp(T)'s using direct integration over energy levels of the internal rotational potentials. Standard enthalpies of formation, ΔfH°298 (units in kcal mol(-1)) are CH2FOOH (-83.7), CHF2OOH (-138.1), CF3OOH (-193.6), CH2FOO(•) (-44.9), CHF2OO(•) (-99.6), CF3OO(•) (-153.8), CH2FOH (-101.9), CHF2OH (-161.6), CF3OH (-218.1), CH2FO(•) (-49.1), CHF2O(•) (-97.8), CF3O(•) (-150.5), CH2F(•) (-7.6), CHF2(•) (-58.8), and CF3(•) (-112.6). Bond dissociation energies for the R-OOH, RO-OH, ROO-H, R-OO(•), RO-O(•), R-OH, RO-H, R-O(•), and R-H bonds are determined and compared with methyl hydroperoxide to observe the trends from added fluoro substitutions. Enthalpy of formation for the fluoro-hydrocarbon oxygen groups C/F/H2/O, C/F2/H/O, C/F3/O, are derived from the above fluorinated methanol and fluorinated hydroperoxide species for use in Benson's Group Additivity. It was determined that fluorinated peroxides require interaction terms O/CH2F/O, O/CHF2/O, and O/CF3/O, as opposed to the common (O/C/O) group in hydrocarbons, resulting from interactions of the peroxide oxygen with the fluorines. Hydrogen bond dissociation increment (HBI) groups are also developed.

A theoretical insight into atmospheric chemistry of HFE-7100 and perfluoro-butyl formate: reactions with OH radicals and Cl atoms and the fate of alkoxy radicals

The calculated rate constants for C₄F₉OCH₃ + OH/Cl reactions are found to be 1.94 × 10⁻¹⁴ and 1.74 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, respectively, at 298 K. The atmospheric lifetime and global warming potential for HFE-7100 are computed to be 2.12 years and 155.3, respectively.

Theoretical studies on kinetics, mechanism and thermochemistry of gas-phase reactions of HFE-449mec-f with OH radicals and Cl atom

A theoretical study on the mechanism and kinetics of the gas phase reactions of CF3CHFCF2OCH2CF3 (HFE-449mec-f) with the OH radicals and Cl atom have been performed using meta-hybrid modern density functional M06-2X using 6-31+G(d,p) basis set. Two conformers have been identified for CF3CHFCF2OCH2CF3 and the most stable one is considered for detailed study. Reaction profiles for OH-initiated hydrogen abstraction are modeled including the formation of pre-reactive and post-reactive complexes at entrance and exit channels. Our calculations reveal that hydrogen abstraction from the CH2 group is thermodynamically and kinetically more facile than that from the CHF group. Using group-balanced isodesmic reactions, the standard enthalpies of formation for HFE-449mecf and radicals generated by hydrogen abstraction, are also reported. The calculated bond dissociation energies for CH bonds are in good agreement with experimental results. The rate constants of the two reactions are determined for the first time in a wide temperature range of 250-450K. The calculated rate constant values are found to be 9.10×10(-15) and 4.77×10(-17)cm(3)molecule(-1)s(-1) for reactions with OH radicals and Cl atom, respectively. At 298K, the total calculated rate coefficient for reactions with OH radical is in good agreement with the experimental results. The atmospheric life time of HFE-449mec-f is estimated to be 0.287 years.

Photofragmentation spectra of halogenated methanes in the VUV photon energy range

In this paper an investigation of the photofragmentation of dihalomethanes CH2X2 (X = F, Cl, Br, I) and chlorinated methanes (CH(n)Cl(4-n) with n = 0-3) with VUV helium, neon, and argon discharge lamps is reported and the role played by the different halogen atoms is discussed. Halogenated methanes are a class of molecules used in several fields of chemistry and the study of their physical and chemical proprieties is of fundamental interest. In particular their photodissociation and photoionization are of great importance since the decomposition of these compounds in the atmosphere strongly affects the environment. The results of the present work show that the halogen-loss is the predominant fragmentation channel for these molecules in the VUV photon energy range and confirm their role as reservoir of chlorine, bromine, and iodine atoms in the atmosphere. Moreover, the results highlight the peculiar feature of CH2F2 as a source of both fluorine and hydrogen atoms and the characteristic formation of I2(+) and CH2(+) ions from the photofragmentation of the CH2I2 molecule.

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].

IR Band profiling of dichlorodifluoromethane in the greenhouse window: high-resolution FTIR spectroscopy of ν2 and ν8

The IR spectrum of dichlorodifluoromethane (i.e., R12 or Freon-12) is central to its role as a major greenhouse contributor. In this study, high-resolution (0.000 96 cm(-1)) Fourier transform infrared spectra have been measured for R12 samples either cooled to around 150 K or at ambient temperature using facilities on the infrared beamline of the Australian Synchrotron. Over 14,000 lines of C(35)Cl2F2 and C(35)Cl(37)ClF2 were assigned to the b-type ν2 band centered around 668 cm(-1). For the c-type ν8 band at 1161 cm(-1), over 10,000 lines were assigned to the two isotopologues. Rovibrational fits resulted in upper state constants for all these band systems. Localized avoided crossings in the ν8 system of C(35)Cl2F2, resulting from both a direct b-axis Coriolis interaction with ν3 + ν4 + ν7 and an indirect interaction with ν3 + ν4 + ν9, were treated. An improved set of ground state constants for C(35)Cl(37)ClF2 was obtained by a combined fit of IR ground state combination differences and previously published millimeter wave lines. Together these new sets of constants allow for accurate prediction of these bands and direct comparison with satellite data to enable accurate quantification.

An integrated experimental and quantum-chemical investigation on the vibrational spectra of chlorofluoromethane

The vibrational analysis of the gas-phase infrared spectra of chlorofluoromethane (CH2ClF, HCFC-31) was carried out in the range 200-6200 cm(-1). The assignment of the absorption features in terms of fundamental, overtone, combination, and hot bands was performed on the medium-resolution (up to 0.2 cm(-1)) Fourier transform infrared spectra. From the absorption cross section spectra accurate values of the integrated band intensities were derived and the global warming potential of this compound was estimated, thus obtaining values of 323, 83, and 42 on a 20-, 100-, and 500-year horizon, respectively. The set of spectroscopic parameters here presented provides the basic data to model the atmospheric behavior of this greenhouse gas. In addition, the obtained vibrational properties were used to benchmark the predictions of state-of-the-art quantum-chemical computational strategies. Extrapolated complete basis set limit values for the equilibrium geometry and harmonic force field were obtained at the coupled-cluster singles and doubles level of theory augmented by a perturbative treatment of triple excitations, CCSD(T), in conjunction with a hierarchical series of correlation-consistent basis sets (cc-pVnZ, with n = T, Q, and 5), taking also into account the core-valence correlation effects and the corrections due to diffuse (aug) functions. To obtain the cubic and quartic semi-diagonal force constants, calculations employing second-order Møller-Plesset perturbation (MP2) theory, the double-hybrid density functional B2PLYP as well as CCSD(T) were performed. For all anharmonic force fields the performances of two different perturbative approaches in computing the vibrational energy levels (i.e., the generalized second order vibrational treatment, GVPT2, and the recently proposed hybrid degeneracy corrected model, HDCPT2) were evaluated and the obtained results allowed us to validate the spectroscopic predictions yielded by the HDCPT2 approach. The predictions of the deperturbed second-order perturbation approach, DVPT2, applied to the computation of infrared intensities beyond the double-harmonic approximation were compared to the accurate experimental values here determined. Anharmonic DFT and MP2 corrections to CCSD(T) intensities led to a very good agreement with the absorption cross section measurements over the whole spectral range here analysed.