The kinetics of the decompositions of the proton bound dimers of 1,4-dimethylpyridine and dimethyl methylphosphonate from atmospheric pressure ion mobility spectra

The rate constants for the dissociations, A2H+ → AH+ + A, of the symmetrical proton bound dimers of 2,4-dimethylpyridine and dimethyl methylphosphonate have been determined using an ion mobility spectrometer operating with air as drift gas at ambient pressure. Reaction time was varied by varying the drift electric field. The rate constants were derived from the mobility spectra by determining the rate at which ions decomposed in the drift region. Arrhenius plots with a drift gas containing water vapor at 5 ppmv gave the following activation energies and pre-exponential factors: 2,4-dimethylpyridine, 94 ± 2 kJ mol−1, log A (s−1) = 15.9 ± 0.4; dimethyl methylphosphonate, 127 ± 3 kJ mol−1, log A (s−1) = 15.6 ± 0.3. The enthalpy changes for the decompositions calculated from the activation energies are in accord with literature values for symmetrical proton bound dimers of oxygen and nitrogen bases. The results for dimethyl methylphosphonate were obtained over the temperature range 478–497 K and are practically independent of water concentration (5–2000 ppmv). The activation energy for 2,4-dimethylpyridine, obtained over the temperature range 340–359 K, decreased to 31 kJ mol−1 in the presence of 2.0 × 103 ppmv of water. At the low temperature, a displacement reaction involving water may account for the decrease. The reduced mobilities of the protonated molecules and the proton bound dimers have been determined over a wide temperature range. While the values for the dimers are essentially independent of the water concentration in the drift gas, those of the protonated molecules show a strong dependence.