Theoretical Study of Silicon Monoxide Reactions with Ammonia and Methane

High-accuracy calculations were performed to study the mechanisms of the reactions between the diatomic silicon monoxide (SiO) with NH₃ and CH₄. These reactions are relevant to the SiO-related astrochemistry and atmospheric chemistry as well as the activation of the N–H and C–H bonds by the SiO triple bond. Energetic data used in the construction of potential energy surfaces describing the SiO + NH₃/CH₄ reactions were obtained at the coupled-cluster theory with extrapolation to the complete basis set limit (CCSD­(T)/CBS) using DFT/B3LYP/aug-cc-pVTZ optimized geometries. Standard heats of formation of a series of small Si-molecules were predicted. Insertion of SiO into the N–H bond is exothermic with a small energy barrier of ∼8 kcal/mol with respect to the SiO + NH₃ reactants, whereas the C–H bond activation by SiO involves a higher energy barrier of 45 kcal/mol. Eight product channels are opened in the SiO + NH₃ reaction including dehydrations giving HNSi/HSiN and dehydrogenations. These reactions are endothermic by 16–119 kcal/mol (calculated at 298.15 K) with the CCSD­(T)/CBS energy barriers of 21–128 kcal/mol. The most stable set of products, HNSi + H₂O, was also the product of the reaction pathway having lowest energy barrier of 21 kcal/mol. Ten product channels of the SiO + CH₄ reaction including decarbonylation, dehydration, dehydrogenation, and formation of Si + CH₃OH are endothermic by 19–118 kcal/mol with the energy barriers in the range of 71–126 kcal/mol. The formation of H₂CSiO + H₂O has the lowest energy barrier of 71 kcal/mol, whereas the most stable set of products, SiH₄ + CO, is formed via a higher energy barrier of 90 kcal/mol. Accordingly, while SiO can break the N–H bond of ammonia without the assistance of other molecules, it is not able to break the C–H bond of methane

π‑Conjugated Molecules Containing Naphtho[2,3‑<i>b</i>]thiophene and Their Derivatives: Theoretical Design for Organic Semiconductors

We performed a theoretical investigation on a series of π-conjugated organic molecules containing naphtho­[2,3-<i>b</i>]­thiophene and their derivatives using density functional theory calculations. All molecules considered exhibit planar structures and aromaticity. Energy levels of frontier orbitals and reduction and oxidation potentials of these compounds predicted by our solvation model reveal good agreement with available experimental values. The UV absorption spectra point out a clear trend that maximum peaks corresponding HOMO–LUMO transitions are red-shifted: (i) from compounds containing O to those containing Se, (ii) from dimers <b>1a</b>–<b>3a</b> and <b>1b</b>–<b>3b</b> to trimers <b>4a</b>–<b>6a</b> and <b>4b</b>–<b>6b</b>, and (iii) from parent compounds <b>1a</b>–<b>6a</b> to perfluorinated derivatives <b>1b</b>–<b>6b</b>. Parent compounds <b>1a</b>–<b>6a</b> can be considered as p-type semiconducting materials with low reorganization energies, high transfer integrals, and hole mobility. Perfluorinated compounds <b>1b</b>–<b>6b</b> are suggested to be very good candidates for ambipolar semiconducting materials. Introduction of fused-ring core molecules considerably improves the charge transport characteristics of the co-oligomers <b>4a</b>–<b>6a</b> and <b>4b</b>–<b>6b</b> as compared to those of corresponding molecules <b>1a</b>–<b>3a</b> and <b>1b</b>–<b>3b</b>. Accordingly, the former have lower reorganization energies, higher electron transfer integrals, and higher rates of charge hopping

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