Atomistic Simulations of CO Vibrations in Ices Relevant to Astrochemistry

The experimental absorption band of carbon monoxide (CO) in mixed ices has been extensively studied in the past. The astrophysical interest in this band is related to its characteristic shape, which appears to depend on the surrounding ice structure. Herein, molecular dynamics simulations are carried out to analyze the relationship between the structure of the ice and the infrared (IR) spectrum of embedded CO molecules at different concentrations. Instead of conventional force fields, anharmonic potentials are used for the bonded interactions. The electrostatic interactions are more accurately described by means of fluctuating atomic multipole moments (up to quadrupole). The experimentally observed splitting of the CO absorption band (gas phase: 2143 cm^?1) into a blue‐ (2152 cm^?1) and a red‐shifted (2138 cm^?1) signal is also found in the simulations. Complementary atomistic simulations allow us to relate the spectra with the structural features. The distinction between interstitial and substitutional CO molecules as the origin of this splitting is found to be qualitatively correct. However, at increasing CO concentrations, additional effects—such as mutual interactions between CO molecules—become important, and the simplistic picture needs to be revised.     

Capture of asymmetric top dipolar molecules by ions: Rate constants for capture of H2O, HDO, and D2O by arbitrary ions

The capture of rotationally state-selected and unselected asymmetric top polar molecules by ions is investigated. Analytical expressions (for all rotational states up to j = 2) of capture rate constants in the perturbed-rotor second-order limit are derived for application to low temperature conditions. Approximate analytical representations over wider temperature ranges are also given for rotationally unselected molecules. The capture of H₂O, D₂O, and HDO by arbitrary ions is chosen for demonstration of the approach. Capture rate constants for the about 60 reactions of H₂O with ions listed in the UMIST 2006 data base for astrochemistry are calculated, compared with experimental data, and represented in the format k_cap(T) ≈ c₁ + c₂(T/300 K)^−1/2. The parameters c₁ and c₂ can be predicted in a very simple way. The approach allows one to identify capture-controlled mechanisms and/or to trace experimental artifacts. The approach applies equally well to the capture of symmetric top and linear dipole molecules by arbitrary ions.     

A combined experimental and computational study on the ionization energies of the cyclic and linear C3H isomers.

For the first time, two hydrogen-deficient hydrocarbon radicals are generated in situ via laser ablation of graphite and seeding the ablated species in acetylene gas, which acts as a carrier and reactant simultaneously. By recording photoionization efficiency curves (PIE) and simulating the experimental spectrum with computed Franck-Condon (FC) factors, we can reproduce the general pattern of the PIE curve of m/z=37. We recover ionization energies of 9.15 eV and 9.76 eV for the linear and cyclic isomers, respectively. Our combined experimental and theoretical studies provide an unprecedented, versatile pathway to investigate the ionization energies of even more complex hydrocarbon radicals in situ, which are difficult to prepare by classical synthesis, in future experiments.     

H2 Formation on Cosmic Grain Siliceous Surfaces Grafted with Fe+: A Silsesquioxanes‐Based Computational Model

Cosmic siliceous dust grains are involved in the synthesis of H₂ in the inter‐stellar medium. In this work, the dust grain siliceous surface is represented by a hydrogen Fe‐metalla‐silsesquioxane model of general formula: [Fe(H₇Si₇O_12?n)(OH)_n]⁺ (n=0,1,2) where Fe⁺ behaves like a single‐site heterogeneous catalyst grafted on a siliceous surface synthesizing H₂ from H. A computational analysis is performed using two levels of theory (B3LYP‐D3BJ and MP2‐F12) to quantify the thermodynamic driving force of the reaction: [Fe‐T7H₇]⁺+4H→[Fe‐T7H₇(OH)₂]⁺+H₂. The general outcomes are: 1) H₂ synthesis is thermodynamically strongly favored; 2) Fe‐H / Fe‐H₂ barrier‐less formation potential; 3) chemisorbed H‐Fe leads to facile H₂ synthesis at 20≤T≤100 K; 4) relative spin energetics and thermodynamic quantities between the B3LYP‐D3BJ and MP2‐F12 levels of theory are in qualitative agreement. The metalla‐silsesquioxane model shows how Fe⁺ fixed on a siliceous surface can potentially catalyze H₂ formation in space.