Interstellar ice-cold chemistry invoking hot reactions

Summary

In the cold interstellar medium temperatures are so low that exothermic reactions fully dominate the chemistry. Not only small molecules, such as water and methanol are known to be formed there, but also simple sugars and amino acids have been proposed to find their origins on the ice-covered dust grains in dense molecular clouds. Typically, the reactions of interest release much more energy (>15 kcal/mol) than the binding energy of the formed products on the ice mantle (<10 kcal/mol). In principle this opens up the possibility for products to diffuse on or evaporate from the surface, but experimentally it is hard to constrain this in a quantitative manner. Consequently, the mechanisms underlying energy and heat transfer in ices are hardly understood.
Here, I propose to use a computational approach using multi-scale (QM/MM) molecular dynamics to study energy dissipation in ices. This allows a quantum mechanical (QM) description of the chemical bond formation of new species on a surface, while capturing the properties of the ice mantle with molecular mechanics (MM) force fields.
Interstellar ices are most likely composed of amorphous structures incorporating several molecules. I will therefore compare the energy dissipation following several prototypical reactions between crystalline and amorphous, pure and mixed, as well as hydrogen-bonded and apolar ices. This explicitly includes a collaboration with experimentalists. I aim to obtain insight into local hot spot temperature, dissipative timescales, and the percentage of desorption after reaction. This information can then subsequently be parametrized such that it can be implemented into coarse-grained astrochemical models, which currently invoke the use of severe approximations when it comes to heat transfer after reaction.

Details

Project number

722.017.008

Main applicant

Dr. A.L.M. Lamberts

Affiliated with

Universit├Ąt Stuttgart

Duration

01/01/2018 to 31/12/2020