Silicate dust grains are thought to be essential in catalysing the formation of H₂. Ultrasmall silicates (diameter ≤1.5 nm) are fundamental intermediates in silicate dust formation in stellar outflows, and are ubiquitous in the interstellar medium. To investigate the catalytic formation and dissociation of H₂ on such nanosilicates, we have performed ab initio quantum chemical calculations of hydrogen interacting with a stable 21 atom nanosilicate cluster having the stoichiometry of forsterite, (MgO)₆(SiO₂)₃. Due to its small size and high percentage of surface atoms, our particle inherently does not exhibit the bulk forsterite crystal structure and possesses a range of chemisorption and physisorption sites, presumably similar to those that larger amorphous silicates would offer. We find a number of exothermic H₂ formation routes and pathways for H₂ catalytic dissociation on the nanosilicate. In particular, we discover some H₂ formation routes that are energetically more favourable than that reported for the forsterite (010) surface. Further, we find a linear correlation between the dissociative chemisorption of two H atoms and the dissociation transition state, suggestive of a general Brønsted–Evans–Polanyi relation for H₂ dissociation on bare silicates independent of dust grain size and/or crystallinity.

We extend the prediction of vibrational spectra to large sized polycyclic aromatic hydrocarbon (PAH) molecules comprising up to ∼1500 carbon atoms by evaluating the efficiency of several computational chemistry methodologies. We employ classical mechanics methods (Amber and Gaff) with improved atomic point charges, semi-empirical (PM3, and density functional tight binding), and density functional theory (B3LYP) and conduct global optimizations and frequency calculations in order to investigate the impact of PAH size on the vibrational band positions. We primarily focus on the following mid-infrared emission bands 3.3, 6.2, 7.7, 8.6, 11.3, 12.7, and 17.0 μm. We developed a general Frequency Scaling Function (⁠|$\mathcal {FSF}$|⁠) to shift the bands and to provide a systematic comparison versus the three methods for each PAH. We first validate this procedure on IR scaled spectra from the NASA Ames PAH Database, and extend it to new large PAHs. We show that when the |$\mathcal {FSF}$| is applied to the Amber and Gaff IR spectra, an agreement between the normal mode peak positions with those inferred from the B3LYP/4-31G model chemistry is achieved. As calculations become time intensive for large sized molecules N_c > 450, this proposed methodology has advantages. The |$\mathcal {FSF}$| has enabled extending the investigations to large PAHs where we clearly see the emergence of the 17.0 μm feature, and the weakening of the 3.3 μm one. We finally investigate the trends in the 3.3 μm/17.0 μm PAH band ratio as a function of PAH size and its response following the exposure to fields of varying radiation intensities.

H₂O/D₂O negative ion time-of-flight mass spectra from electron transfer processes at different collision energies with neutral potassium yield OH^-/OD^-, O^-, and H^-/D^-. The branching ratios show a relevant energy dependence with an important isotope effect in D₂O. Electronic state spectroscopy of water has been further investigated by recording potassium cation energy loss spectra in the forward scattering direction at an impact energy of 205 eV (lab frame), with quantum chemical calculations for the lowest-lying unoccupied molecular orbitals in the presence of a potassium atom supporting most of the experimental findings. The DO-D bond dissociation energy has been determined for the first time to be 5.41 ± 0.10 eV. The collision dynamics revealed the character of the singly excited (1b₂^-1) molecular orbital and doubly excited states in such K-H₂O and K-D₂O collisions.

A recent suggestion that acetamide, CH₃C(O)NH₂, could be readily formed on water-ice grains by the acid induced addition of water across the C≡N bond has now been shown to be credible. Computational modeling of the reaction between R-CN (R = H, CH₃) and a cluster of 32 molecules of water and one H₃O⁺ proceeds catalytically to form first a hydroxy imine R-C(OH)═NH and second an amide R-C(O)NH₂. Quantum mechanical tunneling, computed from small-curvature estimates, plays a key role in the rates of these reactions. This work represents the first reasonable effort to show, in general, how amides can be formed from nitriles and water, which are abundant substrates, reacting on a water-ice cluster containing catalytic amounts of hydrons in the interstellar medium with consequential implications toward the origins of life.