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Results and discussion

The comparison between the photolysis rate of CF₃CF₂C(O)F alone and with either c-C₆H₁₂ or (FCO)₂ was performed using the setup described above. When CF₃CF₂C(O)F alone was photolized, the products formed were C₄F₁₀, CO and CF₂O (Fig. 1), in agreement with the results of Weibel et al.³ These products could be explained taking into account that the C-C bond breaking gives CF₃CF₂ and FCO radicals:

The CF₃CF₂C(O)F and CF₂O quantification wereperformed using calibration curve for each pure substance.

Fig. 1: IR spectra obtained for the photolysis of pure CF₃CF₂COF at 254 nm.

The experiments carried out in the presence of c-C₆H₁₂ gave CF₃CF₂H andHFCO as products. In these experimental runs it was not observed C₄F₁₀ or CF₂O formation.This fact indicates that all the CF₃CF₂ and FCO radicals formed reacted with the c-C₆H₁₂ molecules:

CF₃CF₂ + c-C₆H₁₂ ® CF₃CF₂H + c-C₆H₁₁ (2)

FCO + c-C₆H₁₂ ® HFCO + c-C₆H₁₁ (3)

When comparing the CF₃CF₂C(O)F photolysis rate alone and in the presence of c-C₆H₁₂(Fig. 2), it can be seen that the CF₃CF₂C(O)F amount reacted is greater in the second case. This observation suggests the CF₃CF₂ and FCO radicals react among them in a recombination reaction when there is no other gas added:

CF₃CF₂ + FCO ® CF₃CF₂C(O)F (-1)

Fig. 2: Plot of ln(moles CF₃CF₂COF) vs. photolysis time. (", with (FCO)₂ added; $, alone; %, with c-C₆H₁₂ added). The lines correspond to the least squares fitting to the experimental points

To corroborate this possibility we carried out a new CF₃CF₂C(O)F photolysis in the FCO radicals presence using a good FCO source such as oxalyl fluoride, (FCO)₂. In this case, the C₄F₁₀ formed as well as the reactant consumed are smaller than when CF₃CF₂C(O)F is photolysed alone. In Fig. 2 it can be seen the photolysis rate and in Fig. 3 are shown the spectra obtained in this new condition.

Fig. 3: IR spectra obtained for the photolysis of CF₃CF₂COF in the presence of (FCO)₂ at 254 nm.

In accordance with the results presented previously it is clear that the presence of an effective radical scavenger, able to trap radicals that participate in reactions (2) and (3), allows the reaction photolysis rate determination (1).

Two photolytic experiments were performed with initial CF₃CF₂C(O)F concentration of 4 x 10¹⁶ molecule cm^-3 in absence and in presence of c-C₆H₁₂ (90 torr). Photolysis time was 1200 s, to maintain low conversions. A comparison of the CF₃CF₂C(O)F concentration variation between the two runs is presented in Fig. 2. Note that the CF₃CF₂C(O)F disappearance is faster in the last case. From its slope (dotted line in Fig. 2), we obtain the CF₃CF₂C(O)F photolysis rate value of (1.4 ± 0.1)x 10^-4 s^-1.

The mechanism proposed after the C-C bond rupture, when the photolysis is carried out in c-C₆H₁₂ absence, is:

CF₃CF₂ + FCO ® CF₃CF₂C(O)F (-1)

CF₃CF₂ + CF₃CF₂ ® C₄F₁₀ (3)

FCO + FCO ® CF₂O +CO (4)

The k₃ and k₄ rate constants were obtained from literature. In the case of k₄, there are four values reported but only the work of Behr et al.⁵ was carried out at pressures comparable to those used in our experiments, (although independent) so we took the value (1.7 ± 0.3) x 10^-11 cm³ molecule^-1 s^-1. For k₃the value informed is 2.85 x 10^-12 cm³ molecule^-1 s^-1.⁶

With the mechanism just proposed, we simulated the time variation of the CF₃CF₂C(O)F and CF₂Oconcentration. The experimental and simulation results are shown in Fig. 4. The best fiitting of the k_-1 value gives (6.8 ± 0.8)x 10^-12 cm³ molecule^-1 s^-1. This value of the recombination rate constant also explains why, in the CF₃CF₂C(O)F photolysis in presence of oxalylfluoride, the C₄F₁₀ concentration drops so efficiently and the CF₃CF₂C(O)F concentration decreases much less than when it is photolysed alone.

Fig. 4: Fitting to the actual concentration of species ($, CF₃CF₂COF; Å, CF₂O) when the value assumed by k_-1 in the simulation is 6.8 ± 0.8)x 10^-12 cm³ molecule^-1 s^-1.