Pyrolysis of C2H7+ and other Ion–Molecule Reactions in Methane containing Traces of Ethane

Abstract

The major reactions in methane containing traces of ethane were studied with a pulsed electron beam high ion source pressure mass spectrometer. The CH4: C2H6 ratios were changed from 50:1 to 100 000:1 at reaction temperatures between 28–210 °C. The reaction 1: CH5+ + C2H6 = CH4 + C2H7+ still proceeded from left to right even at the highest dilution ratios. Measurement of the C2H7+/CH5+ ratio under these conditions leads to a lower limit of the proton affinity difference PA(C2H6)–PA(CH4) > 10 kcal/mol. This result is in agreement with measurements by Bohme.It was observed that C2H7+ decomposes slowly at 30 °C. The decomposition becomes more rapid at higher temperature. Measurements of the temperature dependence of the thermal reaction 2: C2H7+ + CH4 = C2H5+ + H2 + CH4, and an Arrhenius plot of k2 led to the activation energy E2 = 10.5 kcal/mol and preexponential factor A2 = 8.3 × 10−8 (cm3 molecule−1 s−1). Assuming E2 = ΔH2 one obtains ΔHf(C2H7+) = 208.5 ± 2 kcal/mol and PA(C2H7+) = 137.4 ± 2 kcal/mol. This is close to the proton affinity PA(C2H7+) = 139 ± 2 kcal/mol that can be deduced from Bohme's results.At higher dilution ratios the ion C2H5+ was observed to react not only with ethane but also with methane by reaction 6: C2H5+ + CH4 = C3H7+ + H2, k6 ≈ 1 × 10−14 cm3 molecule−1 s−1 at 86 °C. The reaction has positive temperature dependence.