H/D exchange in reactions of OH⁻ with D₂ and of OD⁻ with H₂ at low temperatures

Abstract

Using a cryogenic linear 22-pole rf ion trap, rate coefficients for H/D exchange reactions of OH(-) with D2 (1) and OD(-) with H2 (2) have been measured at temperatures between 11 K and 300 K with normal hydrogen. Below 60 K, we obtained k1 = 5.5 × 10(-10) cm(3) s(-1) for the exoergic . Upon increasing the temperature above 60 K, the data decrease with a power law, k1(T) ∼T(-2.7), reaching ≈1 × 10(-10) cm(3) s(-1) at 200 K. This observation is tentatively explained with a decrease of the lifetime of the intermediate complex as well as with the assumption that scrambling of the three hydrogen atoms is restricted by the topology of the potential energy surface. The rate coefficient for the endoergic increases with temperature from 12 K up to 300 K, following the Arrhenius equation, k2 = 7.5 × 10(-11) exp(-92 K/T) cm(3) s(-1) over two orders of magnitude. The fitted activation energy, EA-Exp = 7.9 meV, is in perfect accordance with the endothermicity of 24.0 meV, if one accounts for the thermal population of the rotational states of both reactants. The low mean activation energy in comparison with the enthalpy change in the reaction is mainly due to the rotational energy of 14.7 meV contributed by ortho-H2 (J = 1). Nonetheless, one should not ignore the reactivity of pure para-H2 because, according to our model, it already reaches 43% of that of ortho-H2 at 100 K.

Effect of rotational excitation of H2 on isotopic exchange reaction with OD− at low temperatures

“…Nevertheless, the activation energy o E A of the reaction with ortho-H 2 is in good agreement with the Born-Oppenheimer estimate of 9.3 meV. It is also in agreement with the activation energy of reaction with normal H 2 , E A-exp = (7.9 ± 0.3) meV determined by Mulin et al (2015), which confirms that reaction with ortho-H 2 was the dominant process in that experiment.…”

“…The nuclear spin states I = 0 (para) and 1 (ortho) are linked to the even and odd rotational states, respectively. The 0 K reaction enthalpy of 24 meV was calculated in the Born-Oppenheimer approximation, as discussed in detail by Mulin et al (2015). At meV accuracy, the energetics are influenced by isotopic electronic shifts of the potential energy surfaces of OD − and OH − , as shown for several other isotope exchange reactions with H 2 and D 2 (Kleinman & Wolfsberg 1973, 1974Adohi-Krou et al 2004).…”

“…We analyzed our data using the simple statistical model described by Mulin et al (2015), which takes into account any nonequilibrium population of H 2 rotational states. Generally, the rate coefficient k 1 of reaction (1) can be expressed as a weighted sum k Σ of state-specific rate coefficients k J H 2 J OD − , averaged over the rotational states J H 2 and J OD − of H 2 and OD − .…”

Incomplete rotational cooling in a 22-pole ion trap

“…We employ HD because in contrast to normal H 2 only the rotational ground state is populated at the temperatures of the experiment. We also moved to deuterated hydroxyl anions, because for OD − the isotopic exchange reaction with HD is suppressed [3]. The two systems, OH − /He and OD − /HD, are described by ion-to-neutral mass ratios of 4.25 and 6, respectively.…”

“…This has opened up a diverse field of research on rate coefficients of ion-molecule reactions of astrochemical relevance [2,3], cross sections for photodetachment of interstellar ions [4,5] and high resolution molecular spectroscopy [6,7,8,9]. As sensitive spectroscopic probes laser-induced reactions [10], desorption of a tagged atom [11,12], and photodetachment [9] have been implemented.…”