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.
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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