Gaseous CF4 Research Articles

Effect of resonance dipole-dipole interaction on the infrared spectra of adsorbed CF4. Experimental investigation and theoretical modeling

Adsorption of CF4 on ZnO has been studied by means of FTIR spectroscopy at 55–300K. The contour of the band of the ν3 vibration in the spectra obtained is shown to depend crucially on surface coverage, due to strong resonance dipole-dipole interaction in the layer of adsorbed molecules. At high surface coverage the band is split into two components at 1320 and 1245cm−1, about 75cm−1 apart. The shape of the low frequency component is complicated by an “Evans hole” at 1258cm−1. At lowest coverage only a weak band of ν3 vibration was seen at 1270–1272cm−1; about 10cm−1 below the corresponding wavenumber for gaseous CF4. Preliminary adsorption of CO does not affect significantly the spectrum of adsorbed CF4, leading only to slight decrease of the distance between the two components. Experimentally observed band contours were compared with those calculated theoretically using a model developed earlier for the spectra of interacting molecules in liquid phase, or dissolved in liquefied noble gases, and updated for the case of 2D systems. The calculated spectra reproduce well the splitting of the ν3 band for a monolayer, as well as the coverage and temperature dependence of the IR spectra.

Quantum-chemical simulation of the gas-phase molecular, thermodynamic, and kinetic parameters of CF, CF2, and CF3 radicals and CF4, C2F2, C2F4, and C2F6 molecules

Enthalpy of formation of gaseous CF, CF2, CF3, CF4, C2F2, and C2F4 molecules in the ground state have been simulated via the ab initio method in the CCSD(T) approximation with extrapolation to full basis and correlation energy. Equilibrium geometry, frequency of normal vibrations, and other parameters of these molecules have been determined; their thermodynamic functions at 0–2000 K were calculated from these data. Constants of the C2F6←→ 2CF3, C2F2←→ 2CF, C2F4←→ 2CF2, CF4←→ CF3 + F, CF4←→ CF2 + F2, and CF3←→ CF2 + F equilibriums have been calculated, and the data on the rate constants in the high-pressure limit have been obtained.

Kinetic analysis of the non-isothermal decomposition of carbon monofluoride

Abstract The kinetic mechanism of carbon monofluoride ((CF)n) decomposition and the products generated in this reaction were studied. Given the non-isothermal differential scanning calorimetry and thermogravimetric analysis curves, nucleation mechanisms for (CF)n decomposition were proposed using model-free method. According to the extent of conversion of the reaction, the kinetic process could be described by the two-dimensional Avrami–Erofeev equation f(α) = 2(1 − α)[−ln(1 − α)]1/2, one-dimensional Avrami–Erofeev equation f(α) = 1 − α and two-parameter Sestak–Berggren equation f(α) = αm(1 − α)n (m = −0.294, n = 1.428), respectively. Due to the heterogeneity of the thermal decomposition process of (CF)n, the apparent activation energy of the thermal decomposition varied with the extent of conversion. In addition, the final decomposition products of (CF)n were confirmed to be gaseous CF4 and C2F6, and amorphous carbon via Raman spectroscopy and gas chromatography, and the volume ratio of CF4/C2F6 was found to be approximately 12.

Ab Initio calculation of molecular, thermodynamic, and kinetic parameters of CF, CF2, CF3, and C2F5 radicals and CF4, CF3I, C2F4, and C2F6 molecules in gas

Enthalpies of formation of ground states of the gaseous particles CF, CF2, C2F5, CF4, CF3I, C2F4, and C2F6 were calculated by ab initio method in the CCSD(T) approximation with extrapolation to the full basis and regard to the correlation energy. Their equilibrium geometrics, frequencies of normal vibrations, and other values were found by the B3LYP/aug-cc-pvdz method, from which thermodynamic functions within the range of 0–6000 K were calculated. Equilibrium constants were calculated from these functions, and then the information on the rate constants in the limit of high pressures was obtained.

Trifluoroselenoacetic acid, CF3C(O)SeH: Preparation and Properties

The hitherto unknown trifluoroselenoacetic acid was prepared through the reaction of trifluoroacetic acid with Woollins' reagent. The compound was fully characterized by mass spectrometry, (1)H, (19)F, (77)Se, and (13)C NMR, UV-visible, IR and Raman spectroscopy, and the boiling point at 46 °C was estimated from the vapor pressure curve. An IR matrix isolation study revealed the presence of two different syn-anti and anti-syn conformers. The IR spectra of the two stereoisomers have been assigned, aided by DFT, and ab initio calculations. The UV photolysis of Ar matrix isolated CF(3)C(O)SeH yielded CO, OCSe, CF(3)SeH, and CHF(3). Apart from CF(3)SeH, these products were also obtained by vacuum flash-pyrolysis (310 °C) of gaseous CF(3)C(O)SeH. Instead of CF(3)SeH, CF(2)Se, and HF were detected among the pyrolysis products. The different decomposition pathways of CF(3)C(O)SeH are discussed.

Vibrational Spectroscopy at High Pressures in CF4: Implications to the Phase Diagram

The molecular analogue of methane CF4 is the most fundamental saturated perfluorocarbon, exhibiting complex optical behavior that is highly unusual for such a simple molecular system. We present Raman measurements in solid CF4 over a wide range in pressure from 1.6 to over 30 GPa at room temperature. The Raman spectra exhibit polarization-dependent intensity variations and history-dependent absence or presence of high pressure modes. Our results compellingly demonstrate that previously identified phase transitions in CF4 based on Raman signatures need to be reconsidered. Though our data suggest possible new high-pressure transitions, we do not identify new phases because of spectral complexity. Finally, we used the measured longitudinal and transverse optical mode splitting to estimate the dipole moment derivative at high pressures and find it close to that of gaseous CF4.

Intramolecular fluorine migration via four-member cyclic transition states

Gaseous CF(3)(+) interchanges F(+) for O with simple carbonyl compounds. CF(3)(+) reacts with propionaldehyde in the gas phase to produce (CH(3))(2)CF(+) via two competing pathways. Starting with 1-(13)C-propionaldehyde, the major pathway (80%) produces (CH(3))(2)CF(+) with the carbon label in one of the methyl groups. The minor pathway (20%) produces (CH(3))(2)CF(+) with the carbon label in the central position. The relative proportions of these two pathways are measured by (19)F NMR analysis of the neutral CH(3)CF=CH(2) produced by deprotonation of (CH(3))(2)CF(+) at <10(-)(3) Torr in an electron bombardment flow (EBFlow) reactor. Formation of alkene in which carbon is directly bonded to fluorine means that (in the minor product, at least) an F(+) for O transposition occurs via adduct formation followed by 1,3-atom transfer and then isomerization of CH(3)CH(2)CHF(+) to the more stable (CH(3))(2)CF(+). Use of CF(4) as a chemical ionization (CI) reagent gas leads to CF(3)(+) adduct ions for a variety of ketones, in addition to isoelectronic transposition of F(+) for O. Metastable ion decompositions of the adduct ions yield the metathesis products. Decompositions of fluorocycloalkyl cations formed in this manner give evidence for the same kinds of rearrangements as take place in CH(3)CH(2)CHF(+). Density functional calculations confirm that F(+) for O metathesis takes place via addition of CF(3)(+) to the carbonyl oxygen followed by transposition via a four-member cyclic transition state. A computational survey of the effects of different substituents in a series of aldehydes and acyclic ketones reveals no systematic variation of the energy of the transition state as a function of thermochemistry, but the Hammond postulate does appear to be obeyed in terms of progress along the reaction coordinate. Bond lengths corresponding to the central barrier correlate with overall thermochemistry of the F(+) for O interchange, but in a sense opposite to what might have been expected: the transition state becomes more product-like as the metathesis becomes increasingly exothermic. This reversal of the naive interpretation of the Hammond postulate is accounted for by the relative positions of the potential energy wells that precede and follow the central barrier.

Thermophysical Properties of Gaseous CF4 and C2F6 from Speed-of-Sound Measurements

A cylindrical resonator was employed to measure the sound speeds in gaseous CF4 and C2F6. The CF4 measurements span the temperature range 300 to 475 K, while the C2F6 measurements range from 210 to 475 K. For both gases, the pressure range was 0.1 MPa to the lesser of 1.5 MPa or 80% of the sample’s vapor pressure. Typically, the speeds of sound have a relative uncertainty of less than 0.01 % and the ideal-gas heat capacities derived from them have a relative uncertainty of less than 0.1%. The heat capacities agree with those determined from spectroscopic data. The sound speeds were fitted with the virial equation of state to obtain the temperature-dependent density virial coefficients. Two models for the virial coefficients were employed, one based on square-well potentials and the second based on a Kihara spherical-core potential. The resulting virial equations reproduce the sound-speed measurements to within 0.005 % and yield densities with relative uncertainties of 0.1% or less. The viscosity calculated from the Kihara potential is 2 to 11% less than the measured viscosity.

Chemiluminescence in the Reaction of Mn Atoms with CF4

Translational excitation functions have been determined for production of several MnF* statesb5Π, c5Σ+, d5Π, and (most probably) e5Σ+in the reaction of a laser-ablated beam of Mn atoms with gaseous CF4. Although all observed channels show high initial thresholds, ∼200−300 kJ mol-1, reaction appears to be due to excited Mn atoms rather than the ground state, a6S. The reagent species appears to be either the first or third metastable level, a6DJ or a4DJ. Analysis of the energy dependences, in terms of a multiple line-of-centers model [Levy, Res. Chem. Kinet. 1993, 1, 163], indicates that at relatively low energies, a common process is responsible for b5Π and c5Σ+ formation, involving a ∼14% forward shift in reaction transition state as collision energy increases. Quite separate processes, without transition state shifts, lead to production of MnF*(d5Π) and of MnF*(e5Σ+)/“blue” emission at relatively low energies and to enhanced c5Σ+ production at high energies. It is possible that enhanced production of MnF*(e5Σ+) and perhaps the d5Π state from ∼650−700 kJ mol-1 derives from the depletion of MnF*(b5Π, c5Σ+). Despite the undoubted negative CF4 electron affinity, it seems likely that avoided ionic−covalent curve crossings at least play a role in the b5Π/c5Σ+ production channel.

Ionic Lewis superacids in the gas phase. Part 3. Reactions of gaseous CF +3 with nitrogen bases

The results of a Fourier-transform ion cyclotron resonance mass spectrometric study of the gas-phase reactions of CF + 3 ions toward a number of amines are reported and compared with those of the preceding paper. As with alcohols, the CF + 3 ion exclusively attacks the n-center of the amine giving the corresponding excited onium intermediate that, at variance with that from alcohols, undergoes fragmentation via preferential HF elimination. This process is overwhelmed by simple electron transfer within the collision complex, when thermochemically allowed. With azetidine, pyrrolidine, and piperidine, a cycloreversion reaction in the corresponding adducts with CF + 3 is observed as well. The reactivity patterns of CF + 3 towards amines has been contrasted with that of SiF + 3. The different behavior of the two electrophiles toward n-bases is ascribed to the much larger heterolytic bond dissociation energies of SiX (X = F, O, or N) compared to those of CX. According to its reactivity properties, the CF + 3 ion can be ranked as a gaseous “Lewis superacid” but much weaker than SiF + 3.

Ionic Lewis superacids in the gas phase. Part 2. Reactions of gaseous CF +3 with oxygen bases

The reactivity of the CF + 3 cation toward some oxygen bases, i.e. H 2O, CH 3OH, CH 3CH 2OH, and FCH 2CH 2OH, has been investigated by Fourier-transform ion cyclotron resonance mass spectrometry. The CF + 3 ion, while inert toward H 2O, displays a high affinity for the n-center of the selected bases, yielding the corresponding excited onium intermediate that undergoes fragmentation by predominant O-alkyl bond cleavage, leading to CF 3OH and the alkyl cation. When the alkyl moiety is CH 3, a rapid F − transfer from the CF 3OH to CH + 3 takes place in the fragmentation complex prior to collapse into products. A comparison between the gas-phase reactivity of CF + 3 and of the SiF + 3 analogue is presented and the factors governing their different behavior toward the selected oxygen bases discussed.

Thermal electron mobility and scattering in carbon tetrafluoride gas

Electron mobility has been measured in gaseous CF4 at low electric field strengths, E/n&amp;lt;1 Td (Td=10−21 V m2/molecule), at densities 0.7&amp;lt;n/1025 molecule m−3 &amp;lt;2.3, and temperatures 121&amp;lt;T/K&amp;lt;502. At E/n&amp;lt;0.2 Td the field-independent mobility μ0 of thermal electrons was attained. In the vapor at equilibrium with the liquid nμ0 increased on increasing n from 0.7 to 2 (1025 molecule/m3). Away from the gas/liquid coexistence curve nμ0 increased slightly as the gas was heated from 130 to ∼200 K, then decreased with further heating. The experimental values of nμ0 are compared to those calculated using cross sections recently reported in the literature, and the cross section values are modified.

Vibrational analysis and ab initio molecular orbital calculations of the molecular force constants of gaseous CF 4 and SiF 4

The molecular potential constants of gaseous CF 4 and SiF 4 have been evaluated using approximate force fields and ab initio quantum mechanical methods. A recently developed three-parameter approximate force field, called the adjusted valence force field (AVFF), is found to give force constants for CF 4 and SiF 4 that are in reasonably good agreement with published values based on detailed analysis of vibrational data (including isotopic frequencies and Coriolis constants) and other three-parameter approximate force fields, such as the Urey—Bradley force field (UBFF) and the orbital valence force field (OVFF). In the ab initio studies, force constants were calculated at several basis set levels; correlation effects were included by applying second-order Möller Plesset theory. At the highest level of the theory (6–31 G* basis set), the ab initio force constants for CF 4 are in very good agreement with the most accurately derived values reported in the literature. The ab initio force constants computed for SiF 4 are in reasonable accord with accurately determined values, but the agreement is not as good as that obtained for CF 4.

Efficient vibrational excitation of gaseous CF 4 and C 2F 6 by low-energy electrons

Evidence is presented from a linewidth study of electron cyclotron resonance (ECR) absorption for efficient T → V energy transfer between low-energy electrons (<0.5 eV) and the fluorocarbons CF 4 and C 2F 6. It is suggested that this excitation proceeds by means of a direct mechanism.

The reaction occurring between gaseous CF4 and oxides dissolved in different molten fluorides. Some practical aspects

When CF4 is bubbled through molten fluorides containing dissolved oxides within the range 900–1020° C, a chemical reaction takes place which decreases the oxide concentration and yields CO2 and F− ions. The possibility of the reaction between the CF4 evolved at the anode and dissolved alumina, occurring during the anode effect in aluminium reduction cells, is discussed. This reaction provides a simple and convenient method for removing oxides and hydroxides from molten fluorides.

Recoil 18F chemistry. Superexcited molecules from non-thermal F-for-F substitution in CF 4

Nuclear recoil F-for-F substitution reactions have been studied in gaseous CF 4 over the effective pressure range 1.0–44.5 kTorr. At 1.0 kTorr 76.7% of the initially formed CF 3 18F undergoes secondary decomposition and 12.4% undergoes 20.4 eV endoergic complete dissociative atomization. An approximate phenomenological technique is described for characterizing the CF 3 18F excitation energy probability density distribution.

Determination of the equation of state of a gas from the density dependence of the light scattering intensity

A new experimental, positive test of the Einstein and Vuks theories of light scattering in a gas was made. Then the theory was applied in order to compute the equation of state on the basis of the density dependence of the measured scattering intensity in the case of gaseous CF 4.

Determination of the spontaneous raman linewidth of CF 4 by measurements of stimulated raman scattering in both transient and steady states

The spontaneous Raman linewith Δ v of the v 1 mode of gaseous CF 4 is determined from 30 to 360 amagat by comparison of stimulated Raman scattering threshold intensity measurements performed with picosecond and nanosecond laser excitation. At high densities Δ v = 0.20 cm −1 is constant. A possible explanation of this effect is given in terms of collisional narrowing

Molecular reorientation in liquid CF4

New values of the spin–rotation interaction constants for CF4 are used to obtain improved values of angular mometnum correlation times from 19F NMR spin–lattice relaxation times in liquid and gaseous CF4. Within limitations mainly imposed by uncertainties in the orientational correlation times derived from Raman line shape analysis, the J-diffusion limit of McClung’s extended diffusion model for spherical top molecules is applicable over a density range which extends from the liquid at the melting point through the critical point to the dilute gas.

Schwingungsspektren und Kiraftkonstanten symmetirischeir K]reisel—XVIII. Das Schwingungs-Rotations-Spektrum des CF 3Cl im Bereich von ν 4

The i.r. spectrum of gaseous CF 3Cl has been recorded in the ν 4 region near 1200 cm −1 with a resolution of 0.08 cm −1. The rovibrational band system has been analyzed by means of band contour simulation. It is shown that the irregular structure of the R branch arises from the two components of ν 4 + ν 6 - ν 6. A general theoretical treatment for hot bands ν i + ν j - ν j is given where ν i and ν j belong to E species of point group C 3ν.