The Investigation of Dissociation of Trifluoroacetyl Fluoride in the Field of Pulsed Laser

CF3COF dissociation channels in the pressure region from 0.2 to 4 Torr under the pulsed CO2 laser radiation are investigated. The dissociation products COF2, C2F 6, and CF4 are identified according to the infrared spectra, and their relative output is measured over the whole range of the initial substance pressures. A scheme of laserochemical reactions describing the trifluoroacetyl fluoride dissociation channels is presented. The correctness of the chosen Scheme is confirmed on the basis of the elementary theory of active collisions. The measurements of CF3COF dissociation selectivity for 180 and 13C are given.

The interaction of complicated organic molecules with the infrared laser radiation has been investigated by numerous authors. Initial interest was aroused in particular by the demonstration that the process was isotopically selective. However, in spite of a sufficiently large amount of experimental and theoretical papers, it is not always possible to describe unambiguously the dissociation channels. The investigation of the dissociation of new compounds makes a certain contribution into this problem.
The substance may stay in the reactor for several hours, which results in the high stability of the measurements.
CFaCOF irradiation at room temperature was carried out by a pulsed CO2 laser with pre-ionization at the cathode. The duration of the laser pulse at its half-height was 120-150 ns, the pulse energy was 6 J at the wavelength R(16) 1075.99cm -, and 3J at the wavelength R(40) 1090 cm-. The area of the laser beam at the inlet and outlet of the experimental reactor was approximately the same, equal to 250 mm2. The experimental reactor represents a cylindrical cell 100 mm long and 30 mm in diameter made of stainless steel. The inlet and outlet windows are closed with plane-parallel NaC1 plates, and the seal is made of fluoroplastic with an indium spacer. The reactor was prepassivated with fluorine. The absorption spectra have been recorded by the spectrophotometer IKS-29 over the range from 4000 to 400 cm-. To record the absorption spectra of the dissociation products at the pressure below 4 Torr, the number of experiments was increased with decreasing pressure of the working substance introduced per one experiment. The dissociation products were frozen out into the measuring reactor, and the recording of the IR absorption spectra was carried To carry out the quantitative analysis of the dissociation products, the intensities of pure COF2 and C2F4 absorption as the functions of the pressure in the experimental reactor were measured. Figure   shows the experimentally observed dependences. The intensities of the spectral lines of all dissociation products are normalized to the same scale of relative units. On the basis of the obtained data, we can come to the following conclusions: in the pressure region below 0.6 Torr, the reaction product is mainly C2F4, while the amount of C2F6 and COF2 is small. At the pressure above 1 Torr, the production rates of all the substances become equal and, in any case, remain constant up to 4 Torr (the pressures above 4 Torr were not investigated). Besides, at the pressure of 1 Torr, CF4 appears among the products. The obtained data have allowed us to suggest that the formation of CF3COF dissociation products proceeds according to the following mechanism: i.e., the initial molecule decomposition may proceed by two channels. Then the following reactions occur: The interaction of COF and CF3 radicals with the initial molecule may serve as a possible channel of C2F6 and COF2 formation. The increase of CF3COF pressure should lead to the dependence observed in the experiment (see Figure 1). To obtain the answer concerning the role of such processes, the buffer gas (He or Xe) was added to CF3COF.
The increase of the buffer pressure up to 40 Torr did not lead to any qualitative changes of the dependence obtained before. Therefore, we may conclude that the role of such secondary processes is slight. To confirm the scheme presented above, we apply the elementary theory of active collisions. It is well-known that this theory is based on the supposition that bimolecular reactions are realized at the collision of two molecules. The reaction rate is proportional to the cbllision number in a unit volume during a unit of time. The molecule decomposition proceeds through an activated complex representing a certain instantaneous state of the active molecule, the excitation energy being concentrated at certain degrees of freedom and, finally, leading to the dissociation. The above schematic representation of the reaction confirms the validity of the above scheme (1)-(4).
The production rates of all the products become equal at the pressure of 0.5 Torr. In the general case, the expression of the reaction rate is the following: in the first order w2 k.[A] [M] in the second order CF3COF decomposition is experimentally observed by the output of laserochemical reaction products C2F4, COF 2 and C2F6 Now we show that the final products can unambiguously define the initial compound decomposition in the case under consideration. We derive the production of CEF6 out of CFaCOF on the basis of the notion of two subsequent second order reactions: g A*+M D+E g D+D-"-N The kinetics of the process under consideration will be described by the following equations: The absence of CF4 among the products at low pressures of the initial substance confirms the above supposition. CF4 formation at CF3COF pressure above 1 Torr may be, generally, described through several mechanisms. However, the experimental data do not allow us to describe the observed situation unambiguously.
It should be noted that CF4 formation is observed at the dissociation of numerous molecules (the pressure exceeding several Torr) containing CF3 groups (CF3COCF3, CF3OF etc.). Therefore, we may believe that the mechanism should be of a sufficiently general character, i.e., it should be described by an equation of the type of Eq. (7).
A pure CF3COF of natural isotopic composition was used (at pressure 1 Torr) for preparing the dissociation products isotopically modified in 13C and 180. Spectral dependence of the degree of COF2 enrichment in the lasing frequency range from 980 to 1072 cm -1 was determined. The best results at the P(30) 1037.4cm -1 line were as follows: 6% for 13C and 0.53% for 180 which corresponds to a selectivity 10 for both the isotopes. Isotopic analyses were done on the mass spectrometer MI-1201 on the mass lines 66, 67, 68 (COF-) and 116, 117, 118 (CF3COF+). The C2F6 enrichment in 3C, measured in terms of C2Fions at pressures 1 Torr represents the enrichment for COF2. According to the dissociation mechanism proposed and Eqs. (1) and (2) the highest enrichment in 13C should be expected in tetrafluoroethylene resulting from the monomolecular decay of the initial material. The C2F4 produced directly in the mass spectrometer from the residual CFaCOF however prevents from isolating the mass lines of laser dissociation products. CONCLUSIONS It is shown that the CF3COF dissociation depending on its pressure in the field of a pulsed CO2 laser light can be described by a reaction as a result of active collisions of particles having some adequate energy. Low dissociation threshold and selectivity ---10 with the dissociation yield -8% allow trifluoroacetyl fluoride to be considered as rather a promising material for oxygen isotopes separation.