A. E. Croce

Experimental and modeling study of the reaction C2F4 (+ M) ⇔ CF2 + CF2 (+ M).

The thermal dissociation reaction C2F4(+ M) → 2CF2(+ M) was studied in shock waves monitoring CF2 radicals by their UV absorption. The absorption coefficients as functions of wavelength and temperature were redetermined and are represented in analytical form. Dissociation rate constants as functions of bath gas concentration [M] and temperature, from previous and the present work, are presented analytically employing falloff expressions from unimolecular rate theory. Equilibrium constants are determined between 1200 and 1500 K. The data are shown to be consistent, with a C-C bond energy of 67.5 (±0.5) kcal mol(-1). High-pressure limiting rate constants for dissociation and recombination are found to be unusually small. This phenomenon can be attributed to an unusually pronounced anisotropy of the potential energy surface, such as demonstrated by quantum-chemical calculations of the potential energy surface.

Shock wave study of the thermal decomposition of CF3 and CF2 radicals.

The thermal dissociation reactions CF(3) + M --> CF(2) + F + M (reaction 1 ) and CF(2) + M --> CF + F + M (reaction 3 ) were studied behind shock waves. CF(2) radicals were monitored through their UV absorption. By working at very low reactant concentrations, the rate coefficients of the unimolecular processes could be derived. Reaction 1 was investigated between 1600 and 2300 K in the intermediate range of the falloff curves, at approximately 10 times larger bath gas pressures than employed in earlier work (Srinivasan, N. K.; Su, M.-C.; Michael, J. V.; Jasper, A. W.; Klippenstein, S. J.; Harding, L. B. J. Phys. Chem. A 2008, 112, 31). The combination of the two sets of data, together with theoretical modeling, allows one to construct falloff curves and to provide complete representations of the temperature and pressure dependences of the rate coefficients. Reaction 3 was studied in the limiting low-pressure range and, over the range 2900-3800 K, a rate coefficient k(3) = [Ar] 1.6 x 10(15) exp(-48,040 K/T) cm(3) molecule(-1) s(-1) was obtained. Representations of the rate coefficients over the full falloff curves were again derived by theoretical modeling.

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.

Experimental and modelling study of the unimolecular thermal decomposition of CHF3.

Abstract The unimolecular thermal decomposition reaction CHF3 (+M)→CF2 + HF (+M) was studied in shock waves by monitoring the UV absorption of the forming CF2 radicals. The results of the present and previous experiments on the temperature and pressure dependence of the rate constants are analyzed in terms of unimolecular rate theory. Falloff curves (for 1500–1900 K) are represented in terms of fitted limiting low pressure rate constants k0 = [Ar] 1.1 × 1016exp(−53.0 kcal mol−1/RT) cm3mol−1s−1, limiting high pressure rate constants k∞ = 1.25 × 1015 exp(−75.8 kcal mol−1/RT) s−1 from quantum-chemical calculations, and center broadening factors Fcent = 0.170(± 0.04) including strong and weak collision contributions from unimolecular rate theory. With these results, approximate analytical expressions of the falloff curves well represent the measured and calculated rate constants over wide ranges of pressure and temperature.

Temperature and Pressure Dependence of the Reaction 2CF3 (+ M) ⇔ C2F6 (+ M)†

Limiting low- and high-pressure rate coefficients as well as full falloff curves have been modeled by unimolecular rate theory for the recombination reaction 2CF(3) (+ M) --> C(2)F(6) (+ M) and the reverse dissociation of C(2)F(6). The results are compared with experimental data from the literature. Although there are considerable discrepancies (up to a factor of 5) between various experimental data near 300 K and the database for high temperatures is still limited, we try to conclude on the temperature dependence of the high-pressure rate coefficient. We suggest that there is only a small and probably positive temperature coefficient of the latter quantity. The present theoretical modeling seems to be in agreement with this experimental result, but it is in disagreement with conclusions from earlier theoretical work. The difference is attributed to different empirical assumptions about the anisotropy of the potential. It is shown that nearly all previous experiments (except high-temperature shock wave and very low pressure pyrolysis/photolysis experiments) correspond to nearly limiting high-pressure conditions.

Theoretical Study of the Absorption Spectrum and the Thermochemistry of the CF3OSO3 Radical

The UV-visible absorption spectrum of the recently reported CF3OSO3 radical has been studied by using the time-dependent generalization of the density functional theory (TDDFT). For this a set of eleven hybrid functionals combined with the 6-311+G(3df) basis set were employed. The main features of the three experimental absorption bands of CF3OSO3 recorded over the 220 - 530 nm range are well reproduced by the calculations. A dissociation enthalpy for the CF3O-SO3 bond of 19.1 kcal mol−1 is predicted at the BAC-G3MP2//B3LYP/6-311+G(3df) level of theory

KINETICS OF THE RECOMBINATION REACTIONS OF CCLF WITH CF2 AND WITH CCLF

Abstract The laser flash photolysis–absorption technique has been used to study the CClF+CF 2 +M→C 2 ClF 3 +M and CClF+CClF+M→C 2 Cl 2 F 2 +M recombination reactions at 297 K. Both reactions were found to be essentially at the second-order regime from 10 Torr of the CClF and CF 2 radicals precursor C 2 ClF 3 , up to 750 Torr of mixtures of C 2 ClF 3 /He. The derived limiting high-pressure rate coefficients are (9±2)×10 −13 and (1.2±0.2)×10 −12 cm 3 molecule −1 s −1 , respectively. An analysis in terms of the ab initio density functional theory and the statistical adiabatic channel model (SACM) indicates that the formation of chlorine atoms by unimolecular decomposition of the initially formed C 2 ClF 3 or C 2 Cl 2 F 2 energized adducts can be discarded at room temperature. Employing isodesmic reaction schemes at the B3LYP/6-311++G(d,p) level of theory, the enthalpy of formation of C 2 Cl 2 F 2 , C 2 ClF 2 and C 2 F 3 are calculated to be −78.3±4, −18.1±4 and −59.9±4 kcal mol −1 , respectively.

Theoretical Kinetics Study of the Reactions Forming the ClCO Radical Cycle in the Middle Atmosphere of Venus

Abstract A quantum-chemistry and kinetics study of key chemical reactions involved in the ClCO radical cycle of the Venus atmosphere: Cl + CO + M → ClCO + M (1); ClCO + O2 + M → ClC(O)OO + M (2) and ClC(O)OO + Cl → CO2 + ClO + Cl (3) (M = CO2), has been performed at 150–300 K. Unimolecular reaction rate theories on potential energy features derived at the G4//B3LYP/6-311+G(3df) ab initio composite level were employed. Limiting low pressure rate coefficients calculated for Reaction (1) are in good agreement with recommended experimental values. The present results validate rate coefficient values measured for Reaction (2) over relevant strato-mesosphere Venusian conditions. Rate coefficients calculated by the SACM/CT for Reaction (3) are given by k3 = 2.5 × 10−11 (T/300)0.5 cm3 molecule–1 s–1. In the absence of experimental data, these values provide the first reliable prediction for k3.

Theoretical Study of the Electronic Spectrum of Disulfur Monoxide

The near ultraviolet-visible absorption spectrum of disulfur monoxide ( S2O) has been theoretically studied by using the time-dependent density functional theory (TD-DFT) and the equation of motion coupled-cluster singles and doubles approach (EOM-CCSD) combined with the AUG-cc-PVQZ basis set. From this, analytical expressions for the absorption coefficient over the 250 - 340 nm range are reported for the first time. The computed molecular structure and the vibrational frequencies for the ground and third electronically excited state S2O (C 1Aʹ), responsible of the observed spectrum, are compared with available data.

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.