The Selective Role of Long‐Range Forces in the Stereodynamics of Ion–Molecule Reactions: The He⁺+Methyl Formate Case From Guided‐Ion‐Beam Experiments

Abstract

Long‐range intermolecular forces play a crucial role in controlling the outcome of ion–molecule chemical reactions, such as those determining the disappearance of organic or inorganic “complex” molecules recently detected in various regions of the interstellar medium due to collisions with abundant interstellar atomic ions (e.g. H+ and He+). Theoretical treatments, for example, based on simple capture models, are nowadays often adopted to evaluate the collision‐energy dependence of reactive cross sections and the temperature dependent rate coefficients of many ion–molecule reactions. The obtained results are widely used for the modelling of phenomena occurring in different natural environments or technological applications such as astrophysical and laboratory plasmas. Herein it is demonstrated, through a combined experimental and theoretical investigation on a prototype ion–molecule reaction (He++methyl formate), that the dynamics, investigated in detail, shows some intriguing features that can lead to rate coefficients at odds with the expectations (e.g. Arrhenius versus anti‐Arrhenius behaviour). Therefore, this study casts light on some new and general guidelines to be properly taken into account for a suitable evaluation of rate coefficients of ion–molecule reactions.

Destruction of dimethyl ether and methyl formate by collisions with He+

“…The difference between our measured BRs (Cernuto et al 2017(Cernuto et al , 2018 and those reported in the KIDA and UDfA databases is striking, as shown in the Tables 1 and 2. The uncertainties on our experimental BRs derive from error propagation in the cross section measurements.…”

“…From the discussion in §3, it is clear that the new rate coefficients, based on the experimental results by Cernuto et al (2017Cernuto et al ( , 2018, differ from the those reported in both KIDA and UDfA databases in terms of their absolute values, their temperature dependence (which is positive rather than negative) as well as in the product BRs. An interpolation of the new rate coefficients (and extrapolated values at very low temperatures) gives the following parameters for the global destruction rate coefficients of DME and MF through reaction with He + : α = 1.4 × 10 −9 , β=+0.295, γ=0.00 for DME and α = 1.4 × 10 −9 , β=+0.241, γ=0.00 for MF.…”

“…In conclusions, we emphasise the importance to have reliable information on the destruction pathways of iCOMs by the most abundant ions in molecular gas: H + 3 , HCO + and He + . This can be obtained with experimental works, as in the case reported here (Cernuto et al 2017(Cernuto et al , 2018 coupled with theoretical modelling, or by ab initio quantum chemistry calculations.…”

“…The present article has been stimulated by recent measurements performed by some of the present authors under high resolution conditions of the absolute total cross-section for the reaction of He + with DME and MF in a wide range of collision energies (Cernuto et al 2017(Cernuto et al , 2018. The results of this study are summarized in Fig.…”

“…However, at 10 K, it is necessary to decrease E min to 0.01 meV to have a relative error within 0.1%. Therefore, we set E max to 10 000 meV and E min to 0.01 meV in order to compute the rate coefficients with the cross-sections from our model (Cernuto et al 2017(Cernuto et al , 2018: see Section 3.1). However, since our cross-sections would strictly apply only up to energies larger than about 10-30 meV, we also calculated the rate coefficients over the 0.01 to 10 meV range using two different models for the cross-section over this range.…”