Electron Attachment to Halomethanes at High Temperature: CH2CI2, CF2CI2, CH3CI, and CF3CI Attachment Rate Constants up to 1100 K
AIR FORCE RESEARCH LAB HANSCOM AFB MA SPACE VEHICLES DIRECTORATE
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We have used a high-temperature flowing-afterglow Langmuir-probe apparatus to measure rate constants for electron attachment to halomethanes which attach electrons very inefficiently at room temperature, yielding Cl- ion product. We studied CH2C12 495-973 K, CF2C12 291-1105 K, and CF,C1 524-1004 K and include our recent measurement for CH3Cl 700-1100 K in the discussion of the electron attachment results. The measured attachment rate constants show Arrhenius behavior in the temperature ranges examined, from which estimates of rate constants at 300 K. may be made CH2Cl2 1.8X10-13 cm3 s-1, CH3Cl 1.1 X 10-17 cm3 s-1, and CF3Cl 4.2 x 10-14 cm3 s-1, all of which are difficult to measure directly. In the case of CF2Cl2, the room temperature rate constant was sufficiently large to be measured 1.6 X 10-9 cm3 s-1. The Arrhenius plots yield activation energies for the attachment reactions 390-50 meV CH2Cl2, 124-20 meV CF2Cl2, 670-70 meV CH3Cl, and 406-50 meV CF3Cl. Comparisons are made with existing data where available. G3 calculations were carried out to obtain reaction energetics. They show that the parent anions of CH2Cl2 CF2Cl2, CH3Cl, and CF3Cl are stable, though CH3Cl- exists only as an electrostatically bound complex.
- Physical Chemistry
- Nuclear Physics and Elementary Particle Physics