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Theoretical calculations based on the density functional theory, using the PBE functional with the D3 dispersion correction under periodic boundary conditions, have been employed aiming to investigate the properties of α-, β-, and γ-glycine. Structural parameters have been predicted with a maximum error of 1.42% for lattice parameters and 2.53% for the unit-cell volume, for the α phase. Band structure calculations suggest the band gap values of 4.80, 5.01, and 5.23 eV for the α, β, and γ phases, respectively. Quasi-harmonic calculations have been performed and the Gibbs free energy function has been calculated in a wide range of temperature and pressures, suggesting the stability ordering γ > α > β, at room temperature, and the γ to α-glycine phase transition temperature of 442.55 K, at 1 bar, in agreement with the experimental findings. Moreover, a deviation from the experimental value of only 0.44 J mol–1 K–1 is observed for the predicted S(α→γ) at 298.15 K. Finally, calculated sublimation enthalpies of 140.58, 138.09, and 141.70 kJ mol–1 (α, β, and γ-glycine, respectively), at 298.15 K and 1 bar, have also shown good agreement with the experimental values.

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Complex organic molecules from extraterrestrial source are expected to have contributed to the Early Earth chemistry. Methylamine (CH3NH2) has already been observed in the interstellar medium (ISM) and is generally related to the formation of glycine, although the latter has not been identified in the ISM yet. In this work, a chemical model for CH3NH2 was investigated, comprising twenty-eight reactions and including reactions involving NH3 and HOOC, aiming to understand the main routes for formation and decomposition of methylamine and also to infer about the chemical behavior of glycine in the ISM. Calculations were performed at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level and rate coefficients were calculated adopting the Canonical Variational Transition State Theory (CVTST), in the temperature range from 100 to 4000 K, including tunneling effects. Starting from HCN, the preferred pathway for methylamine formation is through consecutive hydrogenation steps, forming CH2N, CH2NH and CH2NH2 intermediates. Considering the decomposition, dissociation into CH3 and NH2 is the most favorable step. NH3 and HCN are common compounds in interstellar ice analogues and react producing NH2 and CH2N through NH2NCH2 and H2NCH2N intermediates. The latter is proposed here and spectroscopic data for any future experimental investigation are given. Finally, an extension to the ISM glycine chemistry is explored and routes to its formation, from the simplest compounds found in interstellar ices, are proposed.

The doping of graphitic and nanocarbon structures with nonmetal atoms allows for the tuning of surface electronic properties and the generation of new active sites, which can then be exploited for several catalytic applications. In this work, we investigate the direct conversion of methane into H2 and C2H x over Klein-type zigzag graphene edges doped with nitrogen, boron, phosphorus and silicon. We combine Density Functional Theory (DFT) and microkinetic modeling to systematically investigate the reaction network and determine the most efficient edge decoration. Among the four edge-decorated nanocarbons (EDNCs) investigated, N-EDNC presented an outstanding performance for H2 production at temperatures over 900 K, followed by P-EDNC, Si-EDNC and B-EDNC. The DFT and microkinetic analysis of the enhanced desorption rate of atomic hydrogen reveal the presence of an Eley–Rideal mechanism, in which P-EDNC showed higher activity for H2 production in this scenario. Coke deposition resistance in the temperature range between 900 and 1500 K was evaluated, and we compared the selectivity toward H2 and C2H4 production. The N-EDNC and P-EDNC active sites showed strong resistance to carbon poisoning, whereas Si-EDNC showed higher propensity to regenerate its active sites at temperatures over 1100 K. This work shows that decorated EDNCs are promising metal-free catalysts for methane conversion into H2 and short-length alkenes.

The experimental data of sublimation properties is limited to important materials, encouraging the development and assessment of theoretical models. Here, such properties were evaluated from periodic density functional theory calculations for three glycine polymorphs (α, β, and γ-glycine) along with the quasi-harmonic approximation for the determination of thermodynamic properties. Careful treatment of cohesive properties was performed, which are shown to be fundamental in the glycine dynamic sublimation process due to its zwitterionic nature in the crystalline environment in contrast with the molecular form in the gas phase. Computational limitations are addressed, and a detailed treatment of vibrational modes of crystalline and vapor phases is discussed. Also, the agreement with the fundamental physical–chemical background on the glycine sublimation process, obtained from different computational methodologies, is discussed. The uncertainties of sublimation properties were evaluated. The maximum absolute deviation of the sublimation temperature, from the experimental data for α-glycine, within the pressure range from 0.1 to 1 Pa, was −5.31 K. Our findings corroborate the experimental evidence for the preferential recrystallization of gas-phase glycine into the metastable β phase.

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