E. Tschuikow‐Roux

Reactions of ground state chlorine atoms with fluorinated methanes and ethanes

The reactions of photochemically generated Cl(2PJ) atoms with a number of fluorohydrocarbons have been investigated in the temperature range 8–95 °C by the competitive photochlorination technique using CH4 as a primary standard. Relative and absolute rate parameters are reported for CH3F, CH2F2, CH3CH2F, CH2FCH2F, CH3CHF2, CH3CF3, CH2FCHF2, CHF2CHF2, and several auxiliary reactions including CH3Cl, C2H6, and C3H8. The internal competition for hydrogen abstraction in asymmetric fluorethanes is examined in detail. The reactivity trends are discussed and it is found that the activation energies in the fluoromethane series correlate with the known C–H bond dissociation energies. The hydrogen reactivity in the fluoroethane series for which a sufficient data base of DH°(C–H) values is not available is best rationalized in terms of inductive effects and resonance interactions.

Thermal decomposition of nitrogen trifluoride in shock waves

The thermal decomposition of nitrogen trifluoride has been reinvestigated behind incident shock waves in dilute (1%) NF3/Ar mixtures over the temperature range 1150–1530 K and at 0.80–1.81 atm total pressures. The reaction kinetics were followed by monitoring NF2 radical concentrations in absorption at 260 nm. In several experiments, the infrared emission at 5.17 μ was also monitored. Under the experimental conditions used, the reversible dissociation NF3+M?NF2+F+M is known to be first order in both NF3 and Ar concentrations, and the low pressure limit second‐order rate constant was found to be k1(cm3 mol−1 s−1) =1016.61±0.13 ×exp[−(48.0±0.8 kcal mol−1)/RT] using an approximate iterative procedure. A numerical solution of the differential rate equation yielded comparable results. The temperature dependence of the absorption coefficient for NF2 and the equilibrium constant were also determined.

Structures, barriers for rotation and inversion, vibrational frequencies, and thermodynamic functions of ethyl, α‐fluoroethyl, and α,α‐difluoroethyl radicals: An abinito study

The potential energy surfaces of CH3CHF and CH3CF2 radicals were studied by ab initio molecular orbital methods using the gaussian 86 system of programs at the UHF/6‐31G* level of theory. CH3CHF has C1 symmetry and exists as a single pair of enantiomeric conformations. The difluoro species has only a single stable structure, of Cs symmetry. The barriers to rotation about the C–C bond, and the barriers hindering pyramidal inversion at the nonplanar radical center were located for each compound by analytical methods. The monofluoro and difluoro species had similar rotation barriers at the UMP2/6‐31G*//6‐31G* level, 1.68 and 2.26 kcal/mol, respectively, after inclusion of zero‐point vibrational energy differences. The inversion barriers were substantially different, 0.54 and 10.45 kcal/mol, respectively. Vibrational frequencies, as well as moments of inertia for overall and internal rotations, are reported for each species and for the ethyl radical. Calculated heat capacities, entropies, ent...

Nonequilibrium Unimolecular Decomposition of Ethylene

Some of the experimental data in the literature on the nonequilibrium decomposition of ethylene have been examined in the light of calculations using a simple Rice–Ramsperger–Kassel–Marcus treatment. An activation energy of approximately 80 kcal mole−1 is indicated for the molecular elimination of hydrogen. Of the energy available to the products of the initial processes by which excited ethylenes are formed, the fraction retained by ethylene does not appear to be constant, as was previously concluded using the classical RRK theory of unimolecular reactions. In the case of ethylene produced in the photolysis of 3‐methyldiazirine, an alternative explanation for the limiting yield of acetylene at low pressure is offered, based upon an arbitrary energy distribution selected to match the experimental data.

Structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of CH2FCH2, CHF2CH2, and CF3CH2 radicals: An ab initio study

The equilibrium geometries, rotational barriers, and harmonic vibrational frequencies for β‐fluoroethyl (CH2FCH2), β,β‐difluoroethyl (CHF2CH2), and β,β,β‐trifluoroethyl (CF3CH2) radicals have been determined by ab initio molecular‐orbital techniques using the gaussian 86 system of programs at the unrestricted Hartree–Fock, UHF/6‐31G* level of theory. Three conformational minima in the potential‐energy surface were found for the CH2FCH2 and CHF2CH2 radicals, while only one was found for CF3CH2. The radical centers for all three species are nonplanar, but the effect on the geometries of these radicals in replacing β‐position hydrogen by fluorine atoms is much weaker than is the case for α‐position fluorine substituted ethyl radicals. Transition structures for internal rotation were also located. Estimates for the correction of electron correlation effects were obtained by single‐point calculations using second‐order Moller–Plesset perturbation theory (UMP2). The rotation about the C–C bond is almost free fo...

Photolysis of C3O2 in the Presence of CH3F at 1470 Å

Carbon suboxide (C3O2) has been photolyzed at the 1470‐A Xe resonance line at 25°C in the presence of CH3F, with and without added inert gases. The principal reaction products include acetylene, vinyl fluoride, CO, and CO2. The results are interpreted in terms of the participation of an electronically excited C2O (singlet) radical produced in the primary photochemical process. The energy of excitation of C2O is considerably higher than the reported 1Δ, 1Σ, and 3π excited states of C2O. A tentative estimate places the value at ∼ 3.1 eV above the ground‐state C2O (3Σ).

Vacuum Ultraviolet Photolysis of Fluoroethylenes. I. Vinyl Fluoride at 1470 Å

The photolysis of vinyl fluoride at 1470 A and pressures in the range 1–150 torr, yields in addition to C2H2 and C2HF, 1,1‐C2H2F2. The yield of 1,1‐C2H2F2 with respect to that of C2H2, increases rapidly with decreasing pressure. It is proposed that F atoms are liberated as a result of the decomposition of certain excited C2H3F molecules. The F atoms in turn react with C2H3F to give a chemically activated difluoroethyl radical with a small excess energy with respect to a decomposition which yields almost exclusively 1,1‐C2H2F2. The details of this preliminary proposal are discussed in relation to the measured relative yields of C2H2, C2HF, and 1,1‐C2H2F2.

Vacuum–ultraviolet (147 nm) photodecomposition of 1,1,2‐trichloro‐2,2‐difluoroethane

The 147 nm photolysis of CF2ClCHCl2 has been investigated at 25 °C as a function of reactant pressure, conversion, and nitric oxide as additive. In the absence of NO the observed reaction products are CF2CHCl, CF2CCl2, and the diastereomers of (CF2ClCHCl)2. At constant reactant pressure the quantum yields of the olefin decrease with increasing conversion and there is a corresponding increase in the quantum yield of the C4 product. For fixed values of conversion the olefin quantum yields decrease with increasing reactant pressure and approach limiting values at ∼100 Torr. The addition of NO completely suppresses the formation of the chlorofluorobutanes, while it enhances the olefin quantum yields at higher conversion. These observations are interpreted in terms of reactions of chlorine atoms which result either directly (by near simultaneous expulsion of two Cl atoms), or via the dissociation of an excited Cl2* molecule produced by molecular elimination in the primary process. Chlorine atoms abstract hydro...

RRKM Theory Calculation of the Unimolecular Decomposition of Hexafluoroethane: Thermal Activation

The thermal decomposition of hexafluoroethane is discussed in terms of the quantum‐statistical RRKM theory of unimolecular reactions. The falloff behavior is calculated at several temperatures using the approximate rotation–vibration energy‐level sums of Whitten and Rabinovitch. A very loose, Gorin‐type activated complex model with five active internal rotations is found to be consistent with earlier shock‐tube results. Centrifugal effects, associated with a significant extension of the critical bond length in the activated complex, are found to be important and must be taken into account. The approximate method is also applied to C2H6 and satisfactory agreement is found with previous exact computations.

Structures, barriers for internal rotation and inversion, vibrational frequencies, and thermodynamic functions of CH2FCF2, CHF2CF2, and CF3CF2 radicals: An abinitio study

The equilibrium geometries, rotational and inversion barriers, and harmonic vibrational frequencies for α, β difluoroethyl (CH2FCHF), α, β, β trifluorethyl (CHF2CHF), and α, β, β, β tetrafluoroethyl (CF3CHF) radicals have been determined by a b i n i t i o molecular orbital techniques using the gaussian 86 system of programs at the unrestricted Hartree–Fock (UHF)/6–31G* level of theory. Three conformational minima in the potential energy surface were found for the CH2FCHF, and CHF2CHF radicals, while only one was found for the CF3CHF radical. The radical centers for all three species are nonplanar. The barriers of rotation about the C–C bond and the inversion barriers of the radical center were located for each compound by analytical methods. Vibrational frequencies, as well as moments of inertia for overall and internal rotation are reported for each species. Calculated heat capacities,entropies, and enthalpy and free energy functions are tabulated as a function of temperature. Several isodesmic/homodesmic reactions have been studied for the purpose of obtaining theoretical heats of formation of the three radicals for which experimental values are not available. The theoretical heats of formation (ΔH 0 f,298 ) thus evaluated are −56.3 kcal/mol, −108.0 kcal/mol, and −164.5 kcal/mol for CH2FCHF, CHF2CHF, and CF3CHF, respectively. These data are used to evaluate ΔH 0 f,T , ΔG 0 f,T , and K f,T for all three radicals as a function of temperature.