Electron attachment to halomethanes at high temperature: $CH2Cl2$⁠, $CF2Cl2$⁠, $CH3Cl$⁠, and $CF3Cl$ attachment rate constants up to 1100 K

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 $CH2Cl2$ (495–973 K), $CF2Cl2$ (291–1105 K), and $CF3Cl$ (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: $CH2Cl2$ $(1.8×10−13cm3s−1)$⁠, $CH3Cl$ $(1.1×10−17cm3s−1)$⁠, and $CF3Cl$ $(4.2×10−14cm3s−1)$⁠, all of which are difficult to measure directly. In the case of $CF2Cl2$⁠, the room temperature rate constant was sufficiently large to be measured $(1.6×10−9cm3s−1)$⁠. The Arrhenius plots yield activation energies for the attachment reactions: $390±50meV$ $(CH2Cl2)$⁠, $124±20meV$ $(CF2Cl2)$⁠, $670±70meV$ $(CH3Cl)$⁠, and $406±50meV$ $(CF3Cl)$⁠. 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.