Analysis of the Ion–Molecule Reactions in Gaseous H2, D2, and HD by Ion Cyclotron Resonance Techniques

Abstract

Using recently developed ion cyclotron resonance (ICR) techniques, the thermal-energy rate constants of the reactions H2++H2→H3++H, D2++D2→D3++D, H2D++D↗HD++HD↘HD2++H, have been measured. The values of k1 = 2.11, k2 = 1.60, k3a = 0.75, and k3b = 1.05 × 10−9cc molecule−1·sec−1 are in good agreement with the predictions of the Langevin theory and agree in general with other data where available. The energy dependencies of (R1)–(R3) were studied by a variety of double-resonance techniques. An analysis of the energy dependence implies that a dual reaction mechanism is operative in (R1)–(R3). At low energies (KE < 1 eV) both a complex formation and a stripping mechanism occur while at higher energies the stripping mechanism becomes dominant. The relative energy dependencies of (R3a) and (R3b) are interesting in that the ratio [H2D+]/[HD2+] considerably increases as the reactant-ion energy increases (at low energies). The implication is that as the stripping mechanism becomes more dominant the importance of the displacement of the center of mass from the center of the HD bond becomes significant. This variation of [H2D+]/[HD2+] with HD+ energy is used to calibrate the ion energy in the ICR cell by comparison with the tandem-mass-spectrometer data of Futrell and Abramson. Pulsed-ion-ejection studies of an 1:1 H2, D2 mixture indicate the reactions H2++D2↗D2++H2↘HD++HD do not take place at thermal energies for any value of the impact parameter. Also, using pulsed-ion-ejection methods, the relative thermal rate constants for Reactions (R5), H2D++D↗H2++D2↘HD2++H, k5a / k5b = 0.55, are considerably lower than the HD system, k3a / k3b = 0.73, indicating a proton is preferentially transferred in (R3).