Comparative Study of Methanol Activation by Different Small Mixed Silicon Clusters Si₂M with M = H, Li, Na, Cu, and Ag

High-accuracy quantum chemical calculations were carried out to study the mechanisms and catalytic abilities of various mixed silicon species Si₂M with M = H, Li, Na, Cu, and Ag toward the first step of methanol activation reaction. Standard heats of formation of these small triatomic Si clusters were determined. Potential-energy profiles were constructed using the coupled-cluster theory with extrapolation to complete basis set CCSD­(T)/CBS, and CCSD­(T)/aug-cc-pVTZ-PP for Si₂Cu and Si₂Ag. The most stable complexes generated by the interaction of methanol with the mixed clusters Si₂M possess low-spin states and mainly stem from an M–O connection in preference to Si–O interaction, except for the Si₂H case. In two competitive pathways including O–H and C–H bond breakings, the cleavage of the O–H bond in the presence of all clusters studied becomes predominant. Of the mixed clusters Si₂M considered, the dissociation pathways of both O–H and C–H bonds with Si₂Li turns out to have the lowest energy barriers. The most remarkable finding is the absence of the overall energy barrier for the O–H cleavage with the assistance of Si₂Li. The breaking of O–H and C–H bonds with the assistance of Si₂H, Si₂Li, and Si₂Na is kinetically preferred with respect to the Si₂Cu and Si₂Ag cases, apart from the case of Si₂Na for O–H cleavage. In comparison with other transition-metal clusters with the same size, such as Cu₃, Pt₃, and PtAu₂, the energy barriers for the O–H bond activation in the presence of small Si species, especially Si₂H and Si₂Li, are found to be lower. Consequently, these small mixed silicon clusters can be regarded as promising alternatives for the expensive metal-based catalysts currently used for methanol activation particularly and other dehydrogenation processes of organic compounds. The present study also suggests a further extensive search for other doped silicon clusters as efficient and more realistic gas-phase catalysts for important dehydrogenation processes in such a way that they can be experimentally prepared and implemented

Radical Pathways for the Prebiotic Formation of Pyrimidine Bases from Formamide

The prebiotic formation of nucleobases, the building blocks of RNA/DNA, is of current interest. Highly reactive radical species present in the atmosphere under irradiation have been suggested to be involved in the prebiotic synthesis of nucleobases from formamide (FM). We studied several free radical reaction pathways for the synthesis of pyrimidine bases (cytosine, uracil, and thymine) from FM under cold conditions. These pathways are theoretically determined using density functional theory (DFT) computations to examine their kinetic and thermodynamic feasibilities. These free radical reaction pathways share some common reaction types such as H-rearrangement, ^•H/^•OH/^•NH₂ radical loss, and intramolecular radical cyclization. The rate-determining steps in these pathways are characterized with low energy barriers. The energy barriers of the ring formation steps are in the range of 3–7 kcal/mol. Although DFT methods are known to significantly underestimate the barriers for addition of ^•H radical to neutral species, many of these reactions are highly exergonic with energy release of −15 to −52 kcal/mol and are thus favorable. Among the suggested pathways for formation of cytosine (main route, routes <b>7a</b> and <b>1a</b>), uracil (main route, routes <b>7b</b> and <b>1b</b>), and thymine (main route and route <b>26a</b>), the main routes are in general thermodynamically more exergonic and more kinetically favored than other alternative routes with lower overall energy barriers. The reaction energies released following formation of cytosine, uracil, and thymine from FM via the main radical routes amount to −59, −81, and −104 kcal/mol, respectively. Increasing temperature induces unfavorable changes in both kinetic and thermodynamic aspects of the suggested routes. However, the main routes are still more favored than the alternative pathways at the temperature up to the boiling point of FM

Structures, Thermochemical Properties, and Bonding of Mixed Alkaline-Earth-Metal Silicon Trimers Si₃M^+/0/– with M = Be, Mg, Ca

The ground state geometries, electronic structures, and thermochemical properties of binary alkaline-earth-metal silicon clusters Si₃M with M = Be, Mg, Ca in neutral, cationic, and anionic states were investigated using quantum chemical computations. Lowest-lying isomers of the clusters were determined on the basis of the composite G4 energies. Along with total atomization energies, thermochemical parameters were determined for the first time by means of the G4 and coupled-cluster theory with complete basis set CCSD­(T)/CBS approaches. The most favored equilibrium formation sequences for Si₃M clusters emerge as follows: all Si₃M^+/0/– clusters are formed by attaching the M atom into the corresponding cation, neutral and anion silicon trimer Si₃^+/0/–, except for the Si₃Mg⁺ and Si₃Ca⁺ where the metal cations are bound to the neutral Si₃. The resulting mixed tetramers exhibit geometrical and electronic features similar to those of the pure silicon tetramer Si₄^+/0/–. Electron localization function (ELF) and ring current analyses point out that the σ-aromatic character of silicon tetramer remains unchanged upon substituting one Si atom by one alkaline-earth-metal atom

Reaction Routes for Experimentally Observed Intermediates in the Prebiotic Formation of Nucleobases under High-Temperature Conditions

The prebiotic synthesis of nucleobases is of particular interest, given the experimental evidence that indicated formation of the nucleobases under abiotic conditions on the Early Earth under high-temperature conditions. Biomolecules have been formed under meteoritic impact scenarios that lead to high temperature and the generation of high energy. Free radical pathways for the formation of biomolecules are appropriate under these conditions. Density functional theory computations were used to study the free radical routes for the formation of nucleobases at the UB3LYP/6-311G­(d,p) level. We have found that both 5-aminoimidazole-4-carboxamide (AICA) and 5-(formylamino)­imidazole-4-carboxamide (fAICA) are formed first from formamide then the nucleobases are formed. Calculated results show the radical reaction routes of AICA as a precursor for guanine. Both hypoxanthine and xanthine are formed from radical pathways of fAICA. In addition, generation of imino-AICA and imino-fAICA has been shown for the first time to be needed for the production of adenine, purine, and isoguanine. Formation of hypoxanthine and adenine/purine from fAICA and imino-fAICA, respectively, is consistent with experiments performed nearly seven decades ago