Infrared multiphoton dissociation of CF3Br

The CF₂, Br, and F products of 9.2 μ multiphoton dissociation of CF₃Br have been spectroscopically monitored. Primary dissociation leads to CF₃ + Br, while secondary dissociation of CF₃ leads to CF₂ + F. Both the Br and F atoms are found to have average recoil energies close to those of thermal (300 K) atoms. Comparison of these results to calculations based on RRKM theory indicates that CF₃Br dissociates from levels 2–5 photons above the CF₃ + Br threshold.     

The effect of vibrational structure on multiphoton dissociation spectra of CF3Br

Results on IR multiphoton dissociation of rotationally cold CF₃Br molecules are presented and interpreted. Model calculations have been performed extending current theories and using known linear absorption data. Strong dependence of the dissociation spectra on excitation to the first discrete levels has been recognized and discussed.     

Multiphoton laser-induced chemistry of trifluorobromomethane

The multiphoton induced chemistry of trifluorobromomethane (CF₃Br), using a pulsed TEA CO₂ laser operating in the R branch of the 00°1–02°0 transition, and the effects of pulse number, pressure, and exciting wavelength has been investigated. The photolysis products are C₂F₆, CF₄, CF₂Br₂, and Br₂. A reaction scheme is proposed to account for the observed products and their dependence on experimental conditions. The product yield are found to increase with CF₃Br pressure but to decrease with increasing added bath gas pressure (N₂ or NO). Varying the exciting wavelength caused a variation in product yields resulting in a spectrum similar to that for single photon absorption but red shifted by ≈10 cm^?1. It was also observed that at the wavelength employed ¹²CF₃Br is preferentially dissociated at pressures below 2 Torr. Finally a comparison is made between the results of this work and those done on other freons and SF₆.     

Enrichment of carbon-13 in the products of the multiphoton dissociation of CF3X Compounds

The separation factors for the enrichment of carbon-13 in the hexafluorcethane produced from the multiphoton dissociation of CF₃I, CF₃Br and CF₃C1 are reported as a function of CO₂-laser line. The bromide and chloride are significantly more selective than the iodide both at room temperature and at ?50°C. This is a consequence of the spectroscopy of the individual molecules.     

Schwingungsspektren und Kraftkonstanten symmetrischer Kreisel. XXI [1]. Das Schwingungs-Rotationsspektrum von CF379Br und CF381Br im Bereich von v2

Vibrational Spectra and Force Constants of Symmetric Tops. XXI. Rovibrational Spectrum of CF₃⁷⁹Br and CF₃⁸¹Br in the v₂ Region The rovibrational spectrum of CF₃⁷⁹Br and of natural CF₃Br has been recorded in the region of the fundamental v₂ near 750 cm^?1 in the i.r. with a resolution of 0.04 cm^?1. From the analysis of the spectra the values of v₀, the anharmonicity constants x₂₃, x₂₅ and x₂₆ as well as B″ and B′ from the J structure have been obtained both for CF₃⁷⁹Br and CF₃⁸¹Br. v₂ displays Fermi resonance with 2v₃ which is located near 700 cm^?1.     

Increase in isotopic selectivity with pressure in the multiphoton infrared decomposition of CF3Br

The C-13 isotopic selectivity in the multiphoton decomposition of CF₃Br has been shown to increase with increasing pressure of substrate for photolysis far to the red of the resonant, low intensity absorption band using relatively low fluence radiation. However, the quantum efficiency would appear to decrease as pressure is increased.     

Angle-resolved photoelectron spectra of CF3Br and CF2Br2 in the photon energy range 29–115 eV

Photoelectron asymmetry parameters β and partial photoionization cross sections have been measured for ionization from the molecular orbitals of CF₃Br and CF₂Br₂ using synchrotron radiation in the photon energy range from 19 to 115 eV. Cooper minima are observed in the β spectra for ionizations from orbitals with substantial bromine 4p character. The measured positions of the minima have been compared to the results of non-relativistic calculations performed for different elements of the periodic system. Several shape resonances have been observed in β spectra associated with orbitals of predominantly F(2p) character. Ionization from the Br(3d) subshell has also been observed and studied in some detail. Auger transitions involving 3d electrons have been observed at higher photon energies.     

Electrochemical and Chemical Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

The electroreduction of the halofluoromethanes CF₃Br, CF₂Br₂ and CF₂BrCl has been studied in high‐pressure stainless steel autoclaves at different cathodes [Pt, steel (V2A, V4A), glassy carbon (GC)] and in various solvent‐supporting electrolyte systems (SSE), e.g. DMF/[Bu₄N]Br, NMP/[Bu₄N]BF₄ etc. The reduction potentials for CF₃Br increase from Pt (–1.6 V) < V2A (–1.8 V) < GC (–2.1 V) and are lower for CF₂Br₂ and CF₂BrCl suggesting a reductive cleavage of C‐X bonds as the first step. CF₂Br₂ and CF₂BrCl show a two‐step reduction in accord with the C–X bond energies (C–F > C–Cl > C–Br) and the “Perfluoro‐effect”. The electrolysis of CF₃Br in different SSE‐systems with sacrificial zinc or cadmium anodes has been reinvestigated with our experimental set‐up to elucidate the influence of the experimental conditions on the type and ratio of the products. The observed products CF₃MBr·42L and (CF₃)₂M·42L (M = Zn, Cd; L = DMF or AN) are the same as in the previous investigations, but are obtained in different ratios, as a rule caused by a parallel chemical corrosion of the respective anodes. By using aluminium as sacrificial anode no CF₃Al compounds are formed. The CF₃ species generated by electroreduction of CF₃Br react with the solvents via hydrogen abstraction and formation of CF₃H. The current yield with respect to the dissolution of the Al anode reaches 120 % indicating a considerable chemical corrosion in addition to the anodic oxidation. This result enabled a one‐pot trifluoromethylation reaction of NMP as organic carbonyl substrate and solvent with CF₃Br and aluminium powder (ratio 3 : 2) at higher temperatures (> 70 °C). The complete reaction of CF₃Br to give CF₃H and 1‐methyl‐2‐trifluoromethyl‐4,5‐dihydropyrrol allowed the isolation of the latter by vacuum condensation and distillation in 45 % yield, rel. to the CF₃Br used. Gallium and indium were also applied as sacrificial anodes in combination with CF₃Br as substrate. In both cases, anodic current yields of about 280 % indicated an extreme chemical corrosion together with cathodic metal depositions corresponding to the cathodic current yield. These deposits – in contrast to those of Zn and Cd – do not react with CF₃Br in Grignard‐type conversions to CF₃Ga and CF₃In compounds. So, the observed products (CF₃)_nMBr_3–n·L (M = Ga, In; n 1‐3; L = DMF, NMP) are obviously formed by chemical corrosion of the electro‐activated anodes. Finally, electrochemical and chemical trifluoromethylations were successfully carried out, using R₃SiCl (R = Me, Vi, Ph), Me₃M′Cl (M′ = Ge, Sn) and aluminium anodes or Al‐powder. The products were characterized either after isolation or in the product solutions by NMR‐spectroscopic investigations.     

Carbon-13 Separation by IRMPD of mixtures of C2F6 and Br2

The infrared multiple-photon decomposition of mixtures of C₂F₆ and Br₂ has been examined as functions of various experimental parameters. Carbon-13 was found to be enriched in the main product CF₃Br; the maximum enrichment factor was 35. The combination of this process with the IRMPD of CF₃Br provides a closed chemical cycle for efficient carbon isotope separation     

IR predissociation of halogenated methane van der Waals complexes

The infrared dissociation spectra of several halogenated methanes (CHCl₂F, CH₃F, CF₃I and CF₃Br) have been studied. Bonded and free C–F stretch excitations have been observed.H-bridges forming the bonding in CHCl₂F or CH₃F dimers lead to very large red-shifts in the dissociation spectrum with respect to the gas-phase absorptions. Double resonance techniques have been employed to study the homogeneity of the spectral features.     

Infrared predissociation of SiF4 and CF3Br clusters in a molecular-beam experiment

IR predissociation spectra of SiF₄ and CF₃Br clusters have been studied by attenuating a molecular beam with a line-tunable cw CO₂ laser. T     

C?X⋅⋅⋅O Halogen Bonding: Interactions of Trifluoromethyl Halides with Dimethyl Ether

The formation of weakly bound molecular complexes between dimethyl ether (DME) and the trifluoromethyl halides CF₃Cl, CF₃Br and CF₃I dissolved in liquid argon and in liquid krypton is investigated, using Raman and FTIR spectroscopy. For all halides evidence is found for the formation of C? X???O halogen‐bonded 1:1 complexes. At higher concentrations of CF₃Br, a weak absorption due to a 1:2 complex is also observed. Using spectra recorded at temperatures between 87 and 125 K, the complexation enthalpies for the complexes are determined to be ?6.8(3) kJ mol^?1 (DME?CF₃Cl), ?10.2(1) kJ mol^?1 (DME?CF₃Br), ?15.5(1) kJ mol^?1 (DME?CF₃I), and ?17.8(5) kJ mol^?1 [DME(?CF₃Br)₂]. Structural and spectral information on the complexes is obtained from ab initio calculations at the MP2/ 6‐311++G(d,p) and MP2/6‐311++G(d,p)+LanL2DZ* levels. By applying Monte Carlo free energy perturbation calculations to account for the solvent influences, and statistical thermodynamics to estimate the zero‐point vibrational and thermal influences, the ab initio complexation energies are converted into complexation enthalpies for the solutions in liquid argon. The resulting values are compared with the experimental data deduced from the cryosolutions.     

ESR spectra and structure of the CF3Cl−, CF3Br−, and CF3I− radical anions

The anisotropic ESR spectra of the CF₃X^? radical anions (X = Cl, Br, I) have been observed following γ irradiation at 77 K of solid solutions containing up to 5 mole % of the CF₃X parent compound in tetramethylsilane (TMS), neopentane, and 2-methyltetrahydrofuran. The resolution of the line components in the TMS matrix allowed the spectra to be analyzed in detail, the parallel and perpendicular features showing clear evidence for axially symmetric hyperfine interactions with three equivalent fluorines and the unique halogen. On this basis, a matrix diagonalization program was used to calculate the line positions and the best-fit ESR parameters obtained. Confirmation of the CF₃X^? identification was achieved through chemical studies which showed that a similar ESR spectrum was generated by electron attachment to the parent molecule in a photoionization experiment. Also, the spectrum of the CF₃ radical was observed to grow in during the decay of the CF₃X^? spectrum in neopentane above 100 K. The spin density distributions calculated from the ESR parameters of these congeneric radical anions suggest that the unpaired electron resides in an a₁ (σ^*) antibonding orbital which is composed largely of the p orbitals from carbon and the unique halogen which lie along the C_3v symmetry axis of the radical anion. Consistent with this proposal, the spin densities in the s and p orbitals of the unique halogen increase along the series Cl, Br, I, which is the order expected for the effect of decreasing halogen electronegativity.     

Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br

An opto-thermal molecular beam study has been carried out to investigate the multiple-photon laser excitation of SF₆ and CF₃Br. The molecular beam was produced by means of a supersonic expansion through a nozzle at variable temperature. The opto-thermal signal was measured by means of a high-sensitivity superconducting bolometer. The multiple-photon excitation of SF₆ has been measured as a function of the initial ro-vibrational population of the molecule. The experimental results have been compared with both previously published data of molecular beam and gas cell experiments and theoretical calculations. A satisfactory agreement has been found between some of our experimental results and the theoretical spectra obtained by means of the heat-bath feed-back model.     

The IR multiphoton decomposition of CF3I. The effect of inert and reactive gases

The multiphoton decomposition of CF₃I with a pulsed CO₂ laser has been studied at incident fluences of 0.6 and 1.2 J/cm². The effect of pressure on the reaction probability for dissociation of CF₃I was measured in the presence of added isobutane, Ar and CO₂. In the experiments with isobutane, the CF₃ radicals generated by the decomposition of excited CF₃I react to yield CF₃H in competition with the recombination to C₂F₆. The laser absorption cross section was also measured as a function of fluence at a pressure of 0.1 torr of CF₃I and with 0.5–2.0 torr of added isobutane. The experimental results were modeled with a master equation in order to obtain information on the energy transferred by collisions of excited CF₃I with the bath molecules. An energy dependent value of 〈ΔE〉_d produces the best fit to the experimental data. Integration of the rate equations to account for the fractional product yield, [CF₃I]/[C₂F₆], allowed for the calculation of the specific rate constant for hydrogen abstraction from isobutane by CF₃ radicals. The value obtained is dependent on the total pressure and higher than expected at room temperature. From these results, an effective temperature for the reaction mixture was calculated. © 1994 John Wiley & Sons, Inc.     

Fluorine core esca linewidths in CH3F and CF4

Ab initio calculations within the Hartree-Fock formation have been carried out on potential energy surfaces of the ground and the F_1s hole states of CH₃F and CF₄ in order to investigate linewidths of their ESCA spectra. The calculations show that potential energy surfaces of both hole states have dissociative character and can be approximated by straight lines in the region of interest. A simple formula for the ESCA fwhm linewidth is derived which yields results in good agreement with experiment. Theoretically derived relaxation and Koopmans' energies have been investigated as a function of geometry.     

On the effect of low-energy ion bombardment on polystyrene and polymethylmethacrylate etch rates in rf plasmas

The effect of low-energy ion bombardment on CD₄/O₂ and CF₃X (X=F, Cl, Br) plasma etching has been assessed by applying controlled rf bias voltages on polystyrene (PS) and polymethylmethacrylate (PMMA) samples. In both cases ion bombardment has been found to have a chemical effect. In the case of CF₄/O₂ discharges, ion bombardment has been found to change the relative etching efficiency of different mixtures. In the case of CF₃X plasmas, ion bombardment has been found to alter PMMA and PS etch rates in a different way. In particular, the ratios between CF₄ and CF₃X (X=Cl, Br) etch rates of PS have been found to decrease with increasing bias voltage. This effect has been tentatively attributed to an ion bombardment-induced enhancement of the reaction between the aromatic ring and halogen molecules formed in CF₃Cl and CF₃Br discharges.     

Schwingungsspektren und Normalkoordinatenanalyse von (CF3)2PX und (CF3)2AsX (X = F,Cl, Br,J)

Vibrational Spectra and Normal Coordinate Analysis of (CF₃)₂PX and (CF₃)₂AsX (X = F, Cl, Br, I) Gas phase i. r. spectra and liquid-phase Raman spectra of (CF₃)₂AsX are reported. The i. r. investigation of the corresponding phosphorus compounds is complited. The spectra of both the phosphorus and the arsenic compounds were assigned on the basis of a normal coordinate analysis using a transferred force field.     

Chemical effects of radical vibrational energy content on the abstraction reaction: CF3 + Br2 → CF3Br + Br

We have studied the effect of vibrational mode activation in the CF₃ radical on the bromine abstraction reaction; CF₃ + Br₂ → CF₃Br + Br. Excess vibrational energy resides in the symmetric modes of the radical after 248 nm photolysis of the parent molecule, CF₃I. Our data indicate that the hot radicals react no faster than thermalized CF₃, and may actually have a lower cross-section for reaction. Dynamical factors that result in poor coupling of the vibrational energy to the reaction coordinate, as well as other similar considerations, could be responsible for the experimental observations. In addition, we have made an independent determination of the rate for the bromine abstraction reaction of (1.08 ± .13) × 10¹² s^?1 cm³ mol^?1.     

Physikalische und spektroskopische daten von trifluormethylselenol und derivaten

The characterization of the compounds CF₃SeX (X = H, Cl, Br, CN, CF₃, SeCF₃) is completed by the report of melting points, boiling points, enthalpies of vaporization and entropies of vaporization. Ultraviolet and mass spectra are presented and discussed. An improved synthesis for CF₃SeH is reported.