The selective role of long range forces in the stereodynamics of ion-molecule reactions: The He+ + methyl formate case from guided ion beam experiments

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, e.g. based on simple capture models, are nowadays often adopted to evaluate the collision energy dependence of reactive cross-sections (hereafter indicated as ) and the temperature dependent rate coecients of many ion-molecule reactions. The obtained results are widely used for the modelling of phenomena occurring in dierent natural environments or technological applications such as astrophysical and laboratory plasmas. Here we demonstrate, 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 coecients at odds with the expectations (e.g. Arrhenius vs anti-Arrhenius behaviours). Therefore, this study casts light on some new and general guidelines to be properly taken into account for a suitable evaluation of rate coecients of ion-molecule reactions.