Reaction of N⁺ Ion with H₂, HD, and D₂ at Low Temperatures: Experimental Study of the Pathway to Deuterated Nitrogen-containing Molecules in the Interstellar Medium

Abstract

This work is motivated by many observations of nitrogen hydrides including their isotopologues in the interstellar space. We studied the formation of NH+ and ND+ ions in the nearly thermoneutral hydrogen abstraction reactions of N+ ions with H2, HD, and D2 at temperatures from 300 K down to 15 K using a variable-temperature 22-pole radio frequency ion trap. For the reaction of N+ with HD, the branching ratios for production of ND+ and NH+ ions were also determined. The activation energies of all four reaction channels were determined from the temperature dependencies of the measured reaction rate coefficients. Under the assumption of no energy barriers on the reaction paths, we derive the vibrationless energy change (i.e., the difference of equilibrium Born–Oppenheimer potential energies of products and reactants) in the reactions as ΔE e = (103 ± 3) meV.

Enthalpy of the N+ + H2 → NH+ + H Reaction—Experimental Study of the Reverse Process

“…A notable result was obtained by Tosi et al (1994), who observed that the change of the energy threshold due to deuteration cannot be explained by the different zero-point energies of the deuterated species, suggesting that the energy threshold is not purely due to reaction enthalpy and an energy barrier is involved. On the other hand, several studies (Adams & Smith 1985;Marquette et al 1988;Plašil et al 2022), have observed isotope effects consistent with the hypothesis of no barrier. The recent study by Plašil et al (2022) shows that the available data are compatible with the hypothesis of no reaction barrier when only low-temperature thermal reaction rate coefficients with known error estimates are taken into account.…”

“…On the other hand, several studies (Adams & Smith 1985;Marquette et al 1988;Plašil et al 2022), have observed isotope effects consistent with the hypothesis of no barrier. The recent study by Plašil et al (2022) shows that the available data are compatible with the hypothesis of no reaction barrier when only low-temperature thermal reaction rate coefficients with known error estimates are taken into account. The isotopic variation of activation energies in accordance with the hypothesis of the absence of energy barriers is a strong indication in favor of this hypothesis.…”

“…, with activation energy E A ≈ 19 meV (Marquette et al 1985;Plašil et al 2022). The presence of activation energy indicates that the reaction cross section has a certain threshold energy E T (Menzinger & Wolfgang 1969).…”

“…However, neither the activation energies nor the threshold energies of cross sections can be directly identified with the reaction enthalpy, as the data can also be explained by the presence of energy barriers. To elucidate the issue, the variation of the activation energy with the internal excitation and isotopic composition of reactants has been studied in a number of experiments (Marquette et al 1985(Marquette et al , 1988Gerlich 1993;Zymak et al 2013;Plašil et al 2022) and some have also deduced the threshold energy (Ervin & Armentrout 1987;Marquette et al 1988;Gerlich 1993;Tosi et al 1994). A notable result was obtained by Tosi et al (1994), who observed that the change of the energy threshold due to deuteration cannot be explained by the different zero-point energies of the deuterated species, suggesting that the energy threshold is not purely due to reaction enthalpy and an energy barrier is involved.…”

Isotope Exchange Reaction of OH− Anion with HD at Temperatures from 15 K up to 300 K: Ion Trap Study

“…Anions formed in the SIS are periodically extracted and mass-selected by a quadrupole mass filter. Massselected anions are injected via a quadrupole bender into the 22PT ion trap (Gerlich et al 2013;Zymak et al 2013;Plašil et al 2022).…”