Chemical reactions in molecular clouds between stars

The darkness readily observed between the stars on a clear night sky is far from empty. In fact, a large variety of molecules has been detected during the past century. This despite the fact that the collision probabilities are small and temperatures are below 100 K in the clouds where stars are born. How these species are formed exactly is not fully known and therefore formation routes used in astrochemical models and reported in data bases (UdfA, OSU, KIDA) are largely based on estimates. This is where we can contribute with our rate calculations incorporating the effect of atom tunneling down to low temperatures.

Simple association reactions leading to saturated molecules cannot take place in the gas phase, but rather need a third body which is provided by ice-covered dust grains that act as an energy sink. Moreover, the ice provides a locally increased density of species where chemistry can flourish.

To perform computations taking into account the effect of the surface can be done in several ways. The simplest approximation is to realize that a surface reaction is not a symmetric process and rotation is typically hindered. Therefore, the symmetry factor does not play a role and the rotational partition function can be kept constant [1]. A next step would be to consider the effect of a small number of 'surface' molecules, i.e., to make use of small oligomer structures. In this way it is also  possible to obtain a first approximation of the influence of the binding mode on the value of rate constant [2]. However, using a combined approach of Quantum Mechanics / Molecular Mechanics the effect of a full surface can also be studied [3].

The rate constants obtained are not only useful for astrochemical models, but can also help elucidate experiments in the field of surface astrochemistry. In particular kinetic isotope effects (KIEs) can be determined and compared to those obtained experimentally. Reactions where deuterium atoms are involved are, for instance, expected to take place slower than the analogous reaction with H atoms. Experimentally, the effect of diffusion is included in the KIE, whereas our KIE's concern the reaction only [4,1].

However, the influence of tunneling can also be seen in the case of gas-phase reactions of which the reaction rate increases towards lower temperature [5] and the addition of hydrogen to benzene within the framework of H2 formation in space [6].

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