Unusual Chemical Processes in Interstellar Chemistry: Past and Present

Abstract

The chemistry that occurs in interstellar clouds consists of both gas-phase processes and reactions on the surfaces of dust grains, the latter particularly on and in water-dominated ice mantles in cold clouds. Some of these processes, especially at low temperature, are very unusual by terrestrial standards. For example, in the gas-phase, two-body association reactions form a metastable species known as a complex, which is then stabilized by the emission of radiation under low-density conditions, especially at low temperatures. In the solid phase, it has been thought that the major process for surface reactions is diffusive in nature, occurring when two species undergoing random walks collide with each other on a surface that has both potential wells and intermediate barriers. There is experimental evidence for this process, although very few rates at low interstellar temperatures are well measured. Moreover, since dust particles are discrete, modeling has to take account that reactant pairs are on the same grain, a problem that can be treated using stochastic approaches. In addition, it has been shown more recently that surface reactions can occur more rapidly if they undergo any of a number of non-diffusive processes including so-called three-body mechanisms. There is some experimental support for this hypothesis. These and other unusual gaseous and solid-state processes will be discussed from the theoretical and experimental points of view, and their possible role in the synthesis of organic molecules in interstellar clouds explained. In addition, their historical development will be reviewed.

The Formation of Monosubstituted Cyclopropenylidene Derivatives in the Interstellar Medium via Neutral–Neutral Reaction Pathways

“…In cases where these two processes could feasibly compete, if the dissociation lacks an exit barrier, radiative association becomes too slow in comparison. It follows that in cases where a leaving group may struggle to dissociate from an intermediate, raising the energy of that intermediate will discourage radiative association and therefore promote the success of the reaction (Herbst 2001(Herbst , 2021.…”

Valence-shell ionization of acetyl cyanide: simulation of the photoelectron and infra-red spectra

“…We thus recomputed (i) the components of the normal coordinates in the sub-space formed by the three hydrogens and (ii) derived the Franck-Condon factors with the new hydrogen cartesian coordinates. For each mode, nine new components are thus introduced into the normal modes of the 3a 00(À1) final state where the conditions of normalization through eqn (6) are fulfilled with the new set of a l,j components (see eqn (5)). As a results, with the transformed normal modes induced by the methyl rotation (601) the projection of o 0 0 18 onto the initial o 18 torsion mode is now estimated to be 0.78 instead of 0.23 while the projection value onto the out-of-plane C-CN bending mode is still equal to 0.12.…”

“…In particular, the composition of the molecular gases in ISM continually evolves due to interactions with electromagnetic fields, producing highly reactive neutral or ionized atomic and molecular fragments. [3][4][5][6][7] Most of these atomic or molecular identifications can be accomplished by the comparison of recorded spectra on earth to the observations of the interstellar medium. Consequently, the determination of the physical/chemical properties as well as the understanding of the various reaction paths of these atomic/chemical species in space 8,9 require laboratory measurements assisted by numerical models.…”

Chemical evolution in planet-forming regions with growing grains

“…They only considered hydrogenation reactions for ices, however, and not two-body ice reactions via the Langmuir-Hinchelwood-mechanism (see e.g. Hasegawa & Herbst 1993;Herbst 2021). Krijt et al ( 2020) used a dynamical model setup for grain evolution, including settling from upper disk layers to the midplane, grain growth, and grain drift.…”