TEA CO2 laser-induced reaction of CH3NO2 with CF2HCl: A mechanistic study

Dissociation of nitromethane has been observed when a mixture of CF₂HCl and CH₃NO₂ is irradiated using pulsed TEA CO₂ laser at 9R (24) line (1081 cm^-1), which is strongly absorbed by CF₂HCl but not by CH₃NO₂. Under low laser fluence conditions, only nitromethane dissociates, whereas at high fluence CF₂HCl also undergoes dissociation, showing that dissociation occurs via the vibrational energy transfer processes from the TEA CO₂ laser-excited CF₂HCl to CH₃NO₂. Time-resolved infrared fluorescence from vibrationally excited CF₂HCl and CH₃NO₂ molecules as well as UV absorption of CF₂ radicals are carried out to elucidate the dynamics of excitation/dissociation and the chemical reactions of the dissociation products.     

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.     

Comparative performance of CF3I,CD3I and CH3I in an atomic iodine photodissociation laser

We have compared the performance of CF₃I, CD₃I, and CH₃I in an atomic iodine photodissociation laser over the pressure range 1–200 torr. At pressures below 5 torr, CD₃I produces larger energy outputs, while above 5 torr CF₃I gives superior performance. The crossing of the laser energy output versus pressure curves is explained on the basis of collisional quenching of I(²P_{$\text{1}\text{2}$})(≡I^*) by undissociated alkyl iodide.     

Time-resolved tunable diode laser detection of products of CF2HCl IRMPD: A linestrength measurement for CF2

Tunable diode laser transient detection of CF₂ C₂F₄, and HCl following infrared multiphoton dissociation (IRMPD) of CF₂HCl has been achieved. Quantification of the HCl and C₂F₄ leads to the calculation of an infrared absorption linestrength and the ν₁ bandstrength for CF₂ (X̃¹A₁). In addition, the rate coefficient for recombination of CF₂ was found to be (1.4± 0.4) × 10¹⁰ cm³ mol⁻¹ s⁻¹.     

The reaction of photochemically generated CF3O radicals with CO

CF₃O₂CF₃ was photolyzed at 254 nm in the presence of CO in 760 torr N₂ or air at 296 K in a static reactor. In N₂, the products CF₃OC(O)C(O)OCF₃ and CF₃OC(O)O₂C(O)OCF₃ were detected by FTIR spectroscopy. In air, the only observed products were CF₂O and CO₂ and a chain process, initiated by CF₃O, was invoked for the conversion of CO to CO₂. From both product studies, a mechanism for the CF₃O initiated oxidation of CO was derived, involving the addition reaction CF₃O₂ + CO → CF₃OC(O). The rate constant for the reaction CF₃O + CO at 296 K at a total pressure of 760 torr air was determined to be k(CF₃O + CO) = (5.0 ± 0.9) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹. © 1997 John Wiley & Sons, Inc.     

Photodissociation of CF3NO: observation of NO (υ=1)

That excitation of CF₃NO with wavelengths between 580 and 660 nm yields CF₃ + NO has been shown by two direct techniques. In the first, the CF₃ and NO radicals have been scavenged by their reaction with Cl₂ to yield CF₃Cl and NOCl. as detected by both infrared and mass spectrometry. In the second technique, NO (υ=1) vibrational fluorescence has been observed following tunable dye laser excitation of CF₃NO. The rate of vibrational relaxation of NO (υ=1) in collisions with CF₃NO has been found to be (2.19 ± 0.19) × 10³ s^?1 torr ^?1.     

Study on the feasibility of obtaining highly enriched Carbon-13 in one- and two- stage modes of laser separation process

The feasibility of obtaining highly enriched (¹³C & 80%) carbon-13 by isotope-selective IR multiphoton dissociation (MPD) of Freon 22 (CF₂HCl) molecules in one- and two-stage modes of the process in an apparatus with the intracavity arrangement of the separation reactor, which resembled current technological facilities in its basic characteristics, was studied. The one-stage separation scheme was realized using the selective MPD of Freon 22 by which¹³C is concentrated in the dissociation product tetrafluoroethylene (C²F⁴). Carbon-13 concentrations in the product and MPD yields were measured depending on the laser radiation frequency and Freon 22 and bath gas (nitrogen) pressure. At the chosen irradiation geometry and a laser pulse repetition rate of 30 Hz,¹³C concentrations of 83 and 89% in the final product with the production rate of ∼36 and ∼7 mg¹³C/h were attained. The two-stage separation scheme was furnished on the basis of isotopically selective dissociation of the intermediate product CF²HI enriched in¹³C by its buildup process upon irradiation of a CF²HCl mixture with HI. The final product in this case was CF²H² having a¹³C concentration of 98 ± 1.5%. The production rate of the two-stage laser separation process was ∼22 mg¹³C/h.     

Kinetics and mechanism of the CF3 + NO2, reaction at T = 298 K

The rate constant for the CF₃ + NO₂ reaction (k₂) was measured at room temperature in the range of total pressures 300–600 torr. The measurements were performed using the ruby-laser-induced pulsed photodissociation of CF₃NO in the presence of NO and NO₂ in combination with time-resolved detection of the absorption of He(SINGLE BOND)Ne laser radiation by CF₃NO. The use of the CF₃ + NO reaction as a reference gives k₂ = (3.2 ± 0.7) × 10⁻¹¹ cm³/s. Analysis of the end products of the CF₃ + NO₂ reaction shows that the contribution of the association reaction channel, which leads to the formation of CF₃NO₂, is rather significant (about 30% total yield). A reaction mechanism is suggested to account for the products observed. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 203–208, 1997.     

Chlorine isotope enrichment in CO2 TEA laser photolysis of CF2Cl2

A number of lines from a CO₂ TEA laser were used to photolyze CF₂Cl₂. Enrichment of the ³⁵Cl and ³⁷Cl isotopes in the molecular chlorine formed during the photolysis was observed using a mass spectrometer. Maximum enrichment was about 1.8. The dependence of enrichment on wavelength, reactant concentration, inert gas pressure, and the presence of SiF₄ is reported. Of particular interest is the persistence of significant enrichment at pressures up to several hundred torr (≈ 10⁵ Pa). Aside from the practical significance of this enrichment at high pressures, it suggests that there are important contributions from isotopically specific interactions after the laser pulse.     

Rates of processes initiated by pulsed laser production of F atoms in the presence of HCl,CH4, and CF3H

Time-resolved vibrational chemiluminescence from HF has been recorded following the production of F atoms by the pulsed laser photolysis (λ = 266 nm) of F₂ in the presence of HCl, CH₄, and CF₃H. In the first two cases, experiments have been conducted by observing emission from HF(ν = 3) at four temperatures from 295 to 139 K. Rate constants have been determined over this range of temperature for the reactions of F atoms with HCl and CH₄ and of CH₃ radicals with F₂, and for the relaxation of HF(ν = 3) by HCl and CH₄. The reaction of F atoms with CF₃H is slower than those with HCl and CH₄ and measurements on the emission from HF(ν = 2) have been used to infer rate constants for reaction and relaxation only at 295 K. © 1994 John Wiley & Sons, Inc.     

Kinetics and mechanism of the reaction of CF3 radicals with NO2

The reaction of CF₃ with NO₂ was studied at 296 ± 2K using two different absolute techniques. Absolute rate constants of (1.6 ± 0.3) × 10⁻¹¹ and (2.1 _−0.3⁺⁰⁷) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ were derived by IR fluorescence and UV absorption spectroscopy, respectively. The reaction proceeds via two reaction channels: CF₃ + NO₂ → CF₂O + FNO, (70 ± 12)% and CF₃ + NO₂ → CF₃O + NO, (30 ± 12)%. An upper limit of 11% for formation of other reaction products was determined. The overall rate constant was within the uncertainty independent of total pressure between 0.4 to 760 torr. © 1996 John Wiley & Sons, Inc.     

The fluorine atom initiated oxidation of CF3CFH2 (HFC-134a) studied by FTIR spectroscopy

Experiments have been carried out on the oxidation of CF₃CFH₂ (HFC-134a). Reaction was initiated by continuous photolysis of F₂ in the near-ultraviolet. The F atoms produced abstracted a hydrogen atom from CF₃CFH₂ initiating oxidation in gas mixtures containing O₂ and made up to a total pressure of 700 torr with N₂. Product yields were measured using Fourier-transform infrared (FTIR) spectroscopy. Experiments were performed with several different partial pressures of O₂ present, and at three temperatures; 298, 323, and 357 K. The major products were HC(O)F, CF₃C(O)F, and CF₃O₃CF₃, consistent with H atom abstraction by O₂ and CC bond scission being the dominant loss processes for CF₃CFHO radicals: CF₃CFHO+0₂ → CF₃C(O)F+HO₂ (4a) CF₃CFHO+M → CF₃+HC(O)F+M (4b) The following expression was derived for the ratio of rate constants for these reactions: k_4a/k_4b=(3.8±1.6)×10⁻²⁴ exp[(2400±500)/T]cm³ molecule⁻¹ (viii) The main fate of the CF₃ radicals was formation of CF₃O₃CF₃ and small amounts of CF₃OH were detected. The results of the present experiments in which F atoms were used to initiate reaction are in good agreement with those of previous studies in which Cl atoms were employed to initiate the oxidation of HFC-134a. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 541–554, 1998     

Infrared photochemistry of bis(trifluoromethyl) peroxide

Bis(trifluoromethyl) peroxide is readily dissociated by multiple infrared photon excitation at CO₂ laser wavelengths. The primary dissociation product is CF₃O; approximately 85% of the nascent radicals are further dissociated in the laser field to form CF₂O and F. The F atoms then react with the remaining CF₃O to produce CF₃OF. The formation of CF₃OF is strongly inhibited by addition of HI, which reacts preferentially with the F atoms.     

Vacuum ultraviolet (147 nm) photolysis of 1,2-fluorochloroethane

1,2-Fluorochloroethane was photolyzed at 147 nm in the pressure range of 3.8–20.9 torr. The effects of added NO, H₂S, and large pressures of CF₄ were also investigated. The exponential extinction coefficient at 147 nm and 296°K was found to be 147 ± 4 atm^?1 · cm^?1. The photochemistry in some respects is similar to that of ethyl chloride. The primary processes again appear to involve at least two excited states. One of these yields ethylene by FCl elimination (Φ ? 0.3) and has a lifetime of ～3.2 × 10^?10 sec, with respect to an internal conversion to the vibrationally excited ground state or, more probably, a collisionally induced crossover to a state decomposing mainly by carbon? halogen bond fission. The molecular elimination of HCl, H₂, and small amounts of HF also occurs but not apparently from the same state as does FCl. The quantum yields of products with radical precursors, however, are not large, and hence little, if any, of the FCl and probably none of HCl, H₂, and HF subsequently dissociates. The vinyl fluoride and vinyl chloride, accompanying the elimination of HF and HCl, are postulated as possible sources of the secondary production of acetylene.     

Photolysis of 1,1,1-difluorochloroethane (Freon 142) at 147 nm

1,1,1-Difluorochloroethane (Freon 142) was photolyzed at 147 nm in the pressure range of 3.6–20.6 torr. The effects of added NO, H₂S, and CF₄ were investigated. The extinction coefficient at 147 nm and 296°K was determined to be 64 ± 8 atm^?1 · cm^?1. The molecule photodecomposes largely by α,β elimination of HCl to give 1,1-difluoroethylene (Φ = 0.74 ± 0.06). There is no observable elimination of HF, but there is strong evidence for the elimination of the elements of FCl though the relative importance of this process is minor, as are contributions from carbon? carbon and carbon? halogen bond fission. The 1,1-difluoroethylene formed is undoubtedly vibratonally excited and is the source of a pressure-dependent small yield of fluoroacetylene. Over the pressure range studied there is no evidence that the major primary process itself is affected by changes in total pressure as is the case in the 147-nm photolysis of ethyl chloride.     

Kinetics and mechanism of the pyrolysis of 1-chloro-1,1-difluoroethane in the presence of additives

The thermal dehydrochlorination CF₂ClCH₃→CF₂(DOUBLEBOND)CH₂+HCl has been studied in a static system between 597 and 664 K in the presence of CCl₄, C₂Cl₆, CF₂(DOUBLEBOND)CH₂, HCl, and CF₃CH₃. A kinetic radical and molecular reaction model has been developed. In addition to describing earlier results on the acceleration of the pyrolysis by CCl₄ and the further acceleration by HCl, this model describes quantitatively up to conversions of 20% (i) the dependence of the catalytic effect of CCl₄ at low concentrations, (ii) the stronger catalytic effect of C₂Cl₆, and (iii) the inhibitory effect of added CF₂CH₂ and CF₃CH₃ when CCl₄ is used as a catalyst. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 359–366, 1998     

Control strategies for laser separation of carbon isotopes

Laser isotope separation (LIS) by infrared laser chemistry of polyatomic molecules has come a long way since its discovery. The last decade has seen considerable efforts in scaling up of the process for light elements like carbon, oxygen and silicon. These efforts aim at ways to improve both the enrichment factor and the throughput. The achievement is quite significant especially for carbon isotope separation wherein macroscopic operating scales have been realized. We report our studies on the IR laser chemistry of two promising systems, viz. neat CF₂HCl and CF₃Br/Cl₂. We have investigated conditions for optimizing the dissociation yield and selectivity using natural samples containing l.l%C-13.Wealso highlight our current efforts for scaling up the process. These include the design aspects of a photochemical reactor with multipass refocusing Herriott optics for efficient photon utilization, development of a cryogenic distillation set up and a preparative gas chromatograph for large scale separation/collection of the isotopically enriched photoproduct in the post-irradiation stage.     

Effective IR Laser Conversion of Carbon-13 Enriched Chlorodifluoromethane to Dibromodifluoromethane

Infrared laser conversion of chlorodifluoromethane (CF₂HCl) enriched in ¹³C to 31%, to dibromodifluoromethane (CF₂Br₂) with the same enrichment has been studied. Optimum conditions have been found, and effective laser synthesis of macroscopic amounts (1 g) of CF₂Br₂ has been carried out. IR absorption spectra of synthesized CF₂Br₂ with a ¹³C concentration of 31% have been recorded. The ¹³C isotope shifts of the ν₁, ν₆, and ν₈ modes have been measured.     

Pyrolysis of 1,1‐dichloro‐1‐fluoroethane in the absence and presence of added propene or CCl4: A computer‐aided kinetic study

The thermal dehydrochlorination CCl₂FCH₃ → CClFCH₂ + HCl has been studied in a static system between 610 and 700 K at pressures ranging from 14 to 120 torr. The experiments were performed in the absence and presence of an added inhibitor (0.5 to 7 torr of C₃H₆) or catalyst (2 to 8 torr of CCl₄). The evolution of the reaction was followed by measuring the pressure rise in the quartz reaction vessel and analyzing the products by gas chromatography. All the experimental results can be explained quantitatively in terms of a reaction model both radical and molecular. The molecular dehydrochlorination has an activation energy of 57.05 kcal/mol and a preexponential factor of 10^14.02 s⁻¹. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 191–197, 2001     

Photolysis of 1,1,1-trifluoromethylazomethane as a source of methyl and trifluoromethyl radicals

The photochemistry of 1,1,1-trifluoromethylazomethane has been partially characterized. The quantum yield for N₂ formation from photolysis at 366 nm and room temperature was unity at low pressure and decreased to 0.5 at 630 torr. At room temperature the principal products were C₂H₆, C₂F₆, CH₃CF₃ (or CH₂CF₂ + HF at reduced pressures), plus substituted hydrazines, which mainly arise from addition of CF₃ to the parent followed by combination of these radicals with CH₃ or CF_3. These fluorinated methyl hydrazine products detract from the general utility of CF₃-N₂-R compounds as sources for simultaneous study of the chemistry of CF₃ and R radicals. At room temperature the hydrazine products accounted for more than 50% of the total yield; however, these products can be reduced by lowering the temperature and at 195°K their yields are negligible. The quantum yield for intramolecular (direct) formation of CH₃CF₃ + N₂ was shown to be ≤0.002.