Kinetics, equilibria and diffusion of ions produced in N2, CO and CO2, studied as a function of temperature using a high-pressure pulsed mass spectrometer source

Abstract

Gas phase ion–molecule association reactions of the type A⁺+ A [graphic omitted] A˙ A⁺ have been studied as a function of temperature in a pulsed electron beam high-pressure mass spectrometer source. Unlike previous studies of this type, the effect of ionic diffusion on the measured pseudo-first-order ion decay is measured and accounted for when estimating k_f. Experiments were performed in the ranges 67–399 N m^–2 and 365–530 K. At 300 K values of k_f were found to be 4.52 × 10^–29 and 1.33 × 10^–28 molecules^–2 cm⁶ s^–1 for N^+˙₄ formation in N₂ and (CO)^+˙₂ formation in CO, respectively. When expressed in the form k_f=CT^m, the values of m were found to be –3.8 ± 0.3 and –3.3 ± 0.2, a factor of two larger than the most recent literature values in each case.

The equilibrium CO^+˙₂+ 2CO₂rlarr;(CO₂)^+˙₂+ CO₂ was also observed for which the values of ΔH=–66.1 ± 2.9 kJ mol^–1 and ΔS=–95.4 ± 4.6 J K^–1 mol^–1 were found, in good agreement with other literature values, leading to ΔH_f[(CO₂)^+˙₂]= 475.3 ± 2.5 kJ mol^–1.

The diffusion coefficients of N^+˙₂ in N₂ and CO⁺˙ in CO were found to be almost identical, as were those of N^+˙₄ and (CO)^+˙₂ over the range of temperatures studied. The fact that the dimer ions have larger coefficients than the monomer is consistent with the role played by resonance charge transfer in the transport of ions. Temperature dependences are, on the whole, lower than expected from theory.