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Silicon is the most abundant third-row element in space and plays a major role in interstellar dust as silicate particles. Furthermore, silicon is important for the formation of silicate dust in oxygen-rich AGB stars and in supernova remnants. The formation and growth of such particles from the gas phase is a complex subject, believed to be initiated by molecules such as SiO and SiS. In this work we perform high-level electronic structure calculations on the SiSOH system, which allows us to shed light on the reactivity of several silicon and sulfur bearing species, such as SiO, SiS, SO, OH, HSiS, HSO, HSiO, and OSiS. We reveal several new neutral–neutral reaction pathways for formation and destruction that are predicted to occur without a potential energy barrier and thus be possible even in the coldest regions of the interstellar medium. We have found a new neutral–neutral destruction route for SiS (only destruction by atomic oxygen and carbon was previously known) and a possible explanation for the nondetection of OSiS, even though its rotational spectrum is well determined experimentally. Overall, we reveal 14 new relevant reactions and provide an estimate for their rate coefficients, which can be incorporated into astrochemical databases and models of the abundances of these molecules in space.

In this work, we explore the possibility of using computationally inexpensive electronic structure methods, such as semiempirical and DFTB calculations, for the search of the global minimum (GM) structure of chemical systems. The basic prerequisite that these inexpensive methods will need to fulfill is that their lowest energy structures can be used as starting point for a subsequent local optimization at a benchmark level that will yield its GM. If this is possible, one could bypass the global optimization at the expensive method, which is currently impossible except for very small molecules. Specifically, we test our methods with clusters of second row elements including systems of several bonding types, such as alkali, metal, and covalent clusters. The results reveal that the DFTB3 method yields reasonable results and is a potential candidate for this type of applications. Even though the DFTB2 approach using standard parameters is proven to yield poor results, we show that a re-parametrization of only its repulsive part is enough to achieve excellent results, even when applied to larger systems outside the training set.

In catalysis, an accurate structural elucidation of molecules, atomic clusters, nanoparticles and solid surfaces is required to understand chemical processes. Therefore, an efficient and automatic structure determination for these systems is of great benefit since it requires a global search within huge chemical spaces. In this work, we propose a new active learning (AL) method intended for atomic clusters that uses different supervised machine learning techniques and their uncertainties to decide the promising non-observed (virtual) structures to be evaluated from quantum calculations. The method was developed for structural elucidation of (I) clusters where all atomic coordinates are allowed to change in a continuous chemical space and (II) doped ones where the atoms exchange in a sufficiently rigid structure, thus, a discrete search space where all cluster descriptors are known. Particularly for case I, a genetic algorithm operator was used to create the unknown virtual structures (and then their descriptors) from the observed ones to improve the performance of the AL search. The proposed AL was applied to the global optimization of heteronuclear (Al4Si7 and 4Al@Si11) and homonuclear (Na20) clusters using self-consistent charge density-functional tight-binding (SCC-DFTB), where a new repulsion parameter was developed to reproduce isomers evaluated from high-level calculations. The performance of the Gaussian process and artificial neural network algorithms was evaluated together with several uncertainty quantification methods: from Gaussian process, K-fold cross-validation and nonparametric bootstrap (BS) resampling. The efficiency of the AL was compared to conventional global optimization methods, such as random search and a genetic algorithm. The results show that the AL can find efficiently the global minimum of atomic clusters.

Phosphorus is essential for life, forming the backbones of DNA and RNA, fueling metabolism through ATP, supporting cell membranes, and contributing to bone structure. Despite its importance, the mechanisms by which phosphorus became available on early Earth remain unclear. Several phosphorus-bearing species (PBSs), including carbon monophosphide (CP), HCP, PH3, PN, and PO, have been detected in different astrophysical environments. However, phosphorus astrochemistry remains challenging due to observational limitations and uncertainties regarding the sources and sinks of its main reservoirs. The gas-phase reactivity of phosphorus species is also poorly understood, which may contribute to the nondetection of molecules such as phosphinidene (PH), which may be efficiently removed from the gas phase through chemical reactions. In this work, we investigate the formation of CP in interstellar environments through the C(3P) + PH(3Σ−) → CP(2Σ+) + H(2S) and P(4S) + CH(2Π) → CP(2Σ+) + H(2S) reactions. High-level ab initio calculations were performed to determine reaction mechanisms, rate coefficients, and temperature dependences, providing data relevant for astrochemical models. Our results show that the C(3P) + PH(3Σ−) →CP(2Σ+) + H(2S) reaction is fast and proceeds without a barrier along the entrance channel, and its rate coefficient can be expressed as 3.18×10−10(T/300)0.05exp(−3.00/T) , indicating its relevance for PH destruction and CP formation even in cold regions of space. The P(4S) + CH(2Π) → CP(2Σ+) + H(2S) reaction is also barrierless and exothermic, but with a smaller rate coefficient of 1.55 × 10−11(T/300)0.11. Rate coefficients for four additional CPH reactions are also provided.

The removal of emergent contaminants via adsorption on granular activated carbon, prepared from Macauba palm, has been studied, contributing to the recovery of the residual biomass, endocarp, obtained in the Macauba palm oil extraction process. The material was characterized by different techniques, such as Raman spectroscopy, thermal analysis, adsorption/desorption of N 2 , zeta potential, and scanning electron microscopy. The N 2 adsorption studies showed that the material presents wide micropores and narrow mesopores, and has a surface area of 907.0 m 2  g −1 . Its maximum adsorption capacity towards the three main emerging contaminants (bisphenol A, ethinylestradiol, and amoxicillin) is much higher than that obtained with benchmark adsorbents (0.148, 0.104, and 0.072 mmol g −1 , respectively). The influence of temperature and pH on the adsorption was also analyzed, allowing an improved description of the adsorption mechanism and showing very promising results.

Aromatic molecules represent fundamental building blocks in prebiotic chemistry and are contemplated as vital precursors to DNA and RNA nitrogen bases. However, despite the identification of some 300 molecules in extraterrestrial environments, the pathways to pyridine (C 5 H 5 N), pyridinyl (C 5 H 4 N·) and (iso)quinoline (C 9 H 7 N)—the simplest representative mono- and bicyclic aromatic molecules carrying nitrogen—are elusive. Here we afford compelling evidence on the gas-phase formation of methylene amidogen (H 2 CN·) and cyanomethyl (H 2 CCN·) radicals via molecular beam studies and electronic structure calculations. The modelling of the chemistries of the Taurus molecular cloud (TMC-1) and Titan’s atmosphere contemplates a complex chain of reactions synthesizing pyridine, pyridinyl and (iso)quinoline from H 2 CN· and H 2 CCN· at levels of up to 75%. This study affords unique entry points to precursors of DNA and RNA nitrogen bases in hydrocarbon-rich extraterrestrial environments thus changing the way we think about the origin of prebiotic molecules in our Galaxy. A pathway towards the formation of pyridine and (iso)quinoline, precursors to DNA and RNA, is revealed for conditions appropriate to Saturn’s moon Titan and the Taurus molecular cloud, providing insight into the synthesis of prebiotic molecules in space.

Phosphorus is a key and vital element for a diverse set of important biological molecules, being indispensable for life as we know. A deeper comprehension of its role in astrochemistry and atmospheric chemistry may aid in finding answers to how this element became available on Earth. The PO molecule is one of the main reservoirs of phosphorus in the interstellar medium (ISM), and a better understanding of the mechanisms and rate coefficients for its formation in the ISM is important for modelling its abundances. In this work, we perform multireference configuration interaction calculations on the formation of PO via the P ( 4 S ) + O 2 ( 3 Σ - ) reaction, analyzing its potential energy surface and rate coefficients for the global reaction on both doublet and quartet states. We also perform DFT (M06-2X) and CCSD(T) calculations, in order to compare the results. We found that the OPO system possesses a high multiconfigurational character, making DFT and CCSD methodologies not suitable for its potential energy landscape calculation. The rate coefficients have been calculated using the master equation system solver (MESS) package, and the results compared to recent experimental data. It is shown that the quartet state contributes for temperatures higher than 700K. The computed rate coefficient can be described by a modified Arrhenius equation [ α ( T / 300 ) β exp ( - γ / T ) ] with α = 1.44 × 10 - 12 cm 3 s - 1 , β = + 1.66 and γ = 704 K.

Mo-doped graphitic carbon nitride (Mo/g-C3N4) was successfully prepared by thermal condensation of a Mo/melamine precursor in a semi-closed alumina crucible at 550 °C without atmosphere. Thermogravimetric analysis (TGA) was used to evaluate the polymerization route of the precursor, and theoretical and experimental investigations revealed that the Mo species were likely dispersed and anchored to the pyridinic groups of g-C3N4. As a result, the obtained Mo/g-C3N4 displayed exceptional catalytic activity in the oxidative desulfurization of dibenzothiophene (DBT) with H2O2. The effects of catalyst dosage, O/S ratio, and temperature on the catalytic properties of Mo/g-C3N4 were investigated. The kinetic studies revealed a pseudo-first-order kinetic process for DBT oxidation with an apparent activation energy of 43.6 kJ mol−1. Experimental and theoretical evaluation of the Mo/g-C3N4 stability suggests that catalytically active Mo species are progressively leached from the g-C3N4 structure. The decrease in the Mo–N bond order after forming reactive peroxo-Mo(VI) groups was associated with catalyst deactivation.

The gas-phase mechanisms of UF 4 and UF 5 conversion into UF 6 at different electronic states have been investigated using density functional theory (DFT) calculations, including relativistic effects and without the frozen core approximation. New geometries and electronic states of the isolated molecules were obtained, and their reactions were predicted to occur without a potential energy barrier. Our calculations show that UF 4 +F 2 collision will most likely produce UF 5 +F in the gas phase. Relevant surface crossings between different electronic states are also obtained and their roles in collision dynamics are discussed.