María P. Badenes

Experimental and theoretical study of the recombination reaction F + FC(O)O + M → FC(O)OF + M

The kinetics of the F+FC(O)O+M→FC(O)OF+M recombination reaction (1) has been experimental and theoretically investigated. Both F atoms and FC(O)O radicals were generated by 193-nm laser flash photolysis of FC(O)OO(O)CF in mixtures with He or CF4 at 295 K. From the extrapolation of the fall-off curves measured from 55 to 790 Torr total pressure, the high pressure rate coefficient (2.1±0.3)×10−11 cm3 molecule−1 s−1 and the low pressure rate coefficients (1.6±0.3)×10−29[He], (3.3±0.7)×10−29[CF4] and (7.1±1.5)×10−29[FC(O)OO(O)CF] cm3 molecule−1 s−1 were determined. The rate coefficient for the reaction between F atoms and FC(O)OO(O)CF has been found to be (1.5±0.5)×10−15 cm3 molecule−1 s−1. The limiting rate coefficients were analyzed using the unimolecular reaction theory on an ab initio potential energy surface derived at the G3S level of theory. Structural properties, harmonic vibrational frequencies and heats of formation for the FC(O)O radical and for the cis and trans conformers of FC(O)OF calculated with the B3LYP/6-311+G(3df) functional are presented. The values −87.9, −103.4 and −104.6 kcal mol−1, respectively, for the heats of formation of the three molecules were obtained at 298 K from isodesmic energies computed at the G3//B3LYP/6-311++G(3df,3pd) level.

Theoretical Study of the Heats of Formation of SF, FSO, FSO2 and HSO Radicals

Standard heats of formation of SF, FSO, FSO2 and HSO radicals have been calculated with the hybrid B3LYP density functional using extended basis sets and with CBS-4M, CBS-q, CBS-Q and CBS-QB3 model chemistries. The values computed from bond dissociation enthalpies and atomization energies are typically 2-3 kcal mol-1 higher than those obtained from a large number of isodesmic reactions. Application of isodesmic reactions calculations with energies computed at the CBS-q level of theory yielded our best values of 0.2±1.0, -69.9±2.0, -96.2±3.0, and -4.4±1.5 kcal mol-1 for SF, FSO, FSO2 and HSO respectively. The values for the FSO and FSO2 reduce markedly the literature uncertainties while the values for the SF and HSO are in very good agreement with previous ab initio estimates.

Quantum Chemical and Kinetics Study of the Thermal Gas Phase Decomposition of 2-Chloropropene

A detailed theoretical study of the kinetics of the thermal decomposition of 2-chloropropene over the 600-1400 K temperature range has been done. The reaction takes place through the elimination of HCl with the concomitant formation of propyne or allene products. Relevant molecular properties of the reactant and transition states were calculated for each reaction channel at 14 levels of theory. From information provided by the BMK, MPWB1K, BB1K, M05-2X, and M06-2X functionals, specific for chemical kinetics studies, high-pressure limit rate coefficients of (5.8 ± 1.0) × 10(14) exp[-(67.8 ± 0.4 kcal mol(-1))/RT] s(-1) and (1.1 ± 0.2) × 10(14) exp[-(66.8 ± 0.5 kcal mol(-1))/RT] s(-1) were obtained for the propyne and allene channels, respectively. The pressure effect over the reaction was analyzed through the calculation of the low-pressure limit rate coefficients and falloff curves. An analysis of the branching ratio between the two channels as a function of pressure and temperature, based on these results and on computed specific rate coefficients, show that the propyne forming channel is predominant.

Thermal Decomposition of 3-Bromopropene. A Theoretical Kinetic Investigation

A detailed kinetic study of the gas-phase thermal decomposition of 3-bromopropene over wide temperature and pressure ranges was performed. Quantum chemical calculations employing the density functional theory methods B3LYP, BMK, and M06-2X and the CBS-QB3 and G4 ab initio composite models provide the relevant part of the potential energy surfaces and the molecular properties of the species involved in the CH2═CH-CH2Br → CH2═C═CH2 + HBr (1) and CH2═CH-CH2Br → CH2═CH-CH2 + Br (2) reaction channels. Transition-state theory and unimolecular reaction rate theory calculations show that the simple bond fission reaction ( 2 ) is the predominant decomposition channel and that all reported experimental studies are very close to the high-pressure limit of this process. Over the 500-1400 K range a rate constant for the primary dissociation of k2,∞ = 4.8 × 10(14) exp(-55.0 kcal mol(-1)/RT) s(-1) is predicted at the G4 level. The calculated k1,∞ values lie between 50 to 260 times smaller. A value of 10.6 ± 1.5 kcal mol(-1) for the standard enthalpy of formation of 3-bromopropene at 298 K was estimated from G4 thermochemical calculations.

Theoretical study of the equilibrium structure, vibrational spectrum, and thermochemistry of the peroxynitrate CF2BrCFBrOONO2.

The results of a theoretical study of the molecular structure and conformational mobilities of the peroxynitrate CF(2)BrCFBrOONO(2) and its radical decomposition product CF(2)BrCFBrOO are reported in this paper. The most stable structures were calculated from ab initio G3(MP2)B3 and G4(MP2) methods and from density functional theory at the B3LYP/6-311+G(d) and B3LYP/6-311+G(3df) levels of theory. The equilibrium conformation of CF(2)BrCFBrOONO(2) indicates that the bromine atoms lie in position anti to each other and possess a COON dihedral angle of 114°. A quantum statistical analysis shows that about 40% of the internal rotors can freely rotate at room temperature. Our best values for the standard enthalpies of formation of CF(2)BrCFBrOONO(2) and CF(2)BrCFBrOO at 298 K obtained from isodesmic reactions at the G3(MP2)//B3LYP/6-311+G(3df) level of theory are -144.7 and -127.0 kcal mol(-1). From these values and the enthalpy of formation of the NO(2) radical, a CF(2)BrCFBrOO-NO(2) bond dissociation enthalpy of 26.0 ± 2 kcal mol(-1) was estimated.

Kinetics of formation of the novel peroxide FC(O)OO(O2)SF

The high-pressure rate coefficient for the formation of the new peroxide FC(O)OO(O2)SF from recombination of FC(O)O and FS(O2)O radicals has been determined by laser flash photolysis at 296 K; density functional theory calculations indicate peroxide stabilization and allow estimation of an O–O bond dissociation energy of 20.6 ± 3 kcal mol−1.

Role of the Recombination Channel in the Reaction between the HO and HO2 Radicals

The kinetics of the gas phase recombination reaction HO + HO2 + He → HOOOH + He has been studied between 200 and 600 K by using the SACM/CT model and the unimolecular rate theory. The molecular properties of HOOOH were derived at the CCSD(T)/aug-cc-pVTZ ab initio level of theory, while relevant potential energy features of the reaction were calculated at the CCSD(T)/aug-cc-pVTZ//CCSD(T)/aug-cc-pVDZ level. The resulting high and low pressure limit rate coefficients are k∞ = 3.55 × 10-12 (T/300)0.20 cm3 molecule-1 s-1 and k0 = [He] 1.55 × 10-31 (T/300)-3.2 cm3 molecule-1 s-1. The rate coefficients calculated over the 6 × 10-4 - 400 bar range are smaller at least in a factor of about 60 than the consensus value determined for the main reaction channel HO + HO2 → H2O + O2, indicating that the recombination pathway is irrelevant.

Experimental and Quantum Chemical Study of the Oxidation Mechanism of CF2CFBr by O2

Abstract The oxidation mechanism of bromotrifluoroethylene, CF2CFBr, by molecular oxygen has been investigated in the gas phase at 416 and 460 K. Based on the CF2CFBr consumption, the yields of the principal product CF2BrC(O)F were 77% and 70% at 416 and 460 K, respectively. Smaller amounts of CF2O, CFBrO and only traces of CF3C(O)F and CO2 were also formed. In the absence of O2, no consumption of CF2CFBr was observed. The oxidation follows a chain mechanism propagated by Br atoms. The energetics of the relevant reaction pathways was studied using different density functional theory methods including more accurate high-level G3 ab initio calculations. The studies revealed that the oxidation is initiated by attack on the carbon atom of the CF2 group of CF2CFBr by the O2. The predicted activation energy for this process is 26.6 kcal mol–1. All other electronic energy barriers and reaction intermediates are below the reagents energy. The theoretical calculations support the proposed reaction mechanism based on previous experimental studies.