Insertion Mechanism of N2 and O2 into Tn(n = 8, 10, 12)-Silsesquioxane Framework

A comparison of the chemical bonding and reactivity of Si8H8O12 and Ge8H8O12: A theoretical study

We have analyzed the chemical bonding and reactivity in the cubic molecule octahydridosilsesquioxane, Si₈H₈O₁₂, and its counterpart Ge₈H₈O₁₂ by means of ab initio quantum chemical methods and group theory. Density functional theory and MP2 methods combined with the basis sets 6-311+G(d) and 6-311++G(2d,p) were used for geometry optimization and vibrational frequency analysis. The geometries of Si₈H₈O₁₂ and Ge₈H₈O₁₂ are unstable under O_h symmetry and distort to the rare T_h molecular symmetry. The energy gained from this pseudo-Jahn-Teller distortion ranges from 0.78 to 6.14 kcal mol^-1 depending on methodological treatment. The Fukui functions and the molecular electrostatic potential were both used as DFT-based reactivity descriptors. Our study shows that Si₈H₈O₁₂ and Ge₈H₈O₁₂ are both hard amphoteric molecules. The cavity within each cage is acidic and able to encapsulate hard small bases such as F^-. The exterior of the cages is basic and can form stable exohedral complexes with hard acids, as in the case of H⁺. The insertion of F^- in Si₈H₈O₁₂ and Ge₈H₈O₁₂ cages gives the most stable endohedral complexes of the series studied, characterized by formation energies of -3.50 and -3.45 eV at CAM-B3LYP/6-311+G(d) and -3.61 and -3.68 eV at the MP2/6-311++G(d,p) level, respectively. The calculated formation energies of the exohedral and endohedral complexes align with the DFT reactivity descriptor analysis.

Jahn-Teller distortion in polyoligomeric silsesquioxane (POSS) cations

We investigated the symmetry breaking mechanism in cubic octa-tert-butyl silsesquioxane and octachloro silsesquioxane monocations (Si8O12(C(CH3)3)8(+) and Si8O12Cl8(+)) using density functional theory (DFT) and group theory. Under Oh symmetry, these ions possess (2)T2g and (2)Eg electronic states and undergo different symmetry breaking mechanisms. The ground states of Si8O12(C(CH3)3)8(+) and Si8O12Cl8(+) belong to the C3v and D4h point groups and are characterized by Jahn-Teller stabilization energies of 3959 and 1328 cm(-1), respectively, at the B3LYP/def2-SVP level of theory. The symmetry distortion mechanism in Si8O12Cl8(+) is Jahn-Teller type, whereas in Si8O12(C(CH3)3)8(+) the distortion is a combination of both Jahn-Teller and pseudo-Jahn-Teller effects. The distortion force acting in Si8O12(C(CH3)3)8(+) is mainly localized on one Si-(tert-butyl) group, while in Si8O12Cl8(+) it is distributed over the oxygen atoms. The main distortion forces acting on the Si8O12 core arise from the coupling between the electronic state and the vibrational modes, identified as 9t2g + 1eg + 3a2u for the Si8O12(C(CH3)3)8(+) and 1eg + 2eg for Si8O12Cl8(+).

Interaction of molecular nitrogen and oxygen with extraframework cations in zeolites with double six-membered rings of oxygen-bridged silicon and aluminum atoms: a DFT study

The interaction of N(2) and O(2) with extraframework cations of zeolite frameworks was studied by DFT, using the B3LYP method. The extraframework cation sites located in the vicinity of the double six-member rings (D6R) of FAU zeolites (SI, SI', SIII') were considered and clusters with composition (M(n)(+))(2/)(n)()H(12)Si(10)Al(2)O(18), M = Li(+), Na(+), K(+), Ca(2+), were selected to represent the adsorption centers. The cation sites SII in the center of single six-membered rings (S6R) were modeled by [M(I)H(12)Si(4)Al(2)O(6)](-) and M(II)H(12)Si(4)Al(2)O(6) clusters. The adsorption energy of N(2) and O(2) is the highest for Li(+) cations at the SIII' cation sites, while for the SI' and SII sites the adsorption energies decrease in the order Ca(2+) > Na(+) > Li(+). The calculated small N(2) adsorption energy for Li(+) cations at SII sites suggests that these sites do not take part in the sorption process in agreement with results of NMR studies and Monte Carlo simulations. The N(2) adsorption complexes with the extraframework cations are linear, while those of O(2) are bent regardless of the extraframework cation location. The SIII' cation sites are the most favorable ones with respect to N(2) adsorption capacity and N(2)/O(2) selectivity; the SII sites are less selective and the SI sites are not accessible.

Ab Initio Analysis of the Structural Properties of Alkyl-Substituted Polyhedral Oligomeric Silsesquioxanes

Ab initio quantum mechanical calculations have been performed to establish the potentials for alkyl-substituted polyhedral oligomeric silsesquioxane (POSS) monomers RxH8-x(SiO1.5)8. More specifically, we have examined the unsubstituted POSS (SiO1.5H)8 cage as well as linear and cyclic alkyl-substituted cages where one of the terminating hydrogen atoms is replaced by a hydrocarbon group, that is, R1H7(SiO1.5)8. The results for the minimum-energy configurations indicate that the presence of the linear hydrocarbon chains and cyclic intermediates have very little effect on the structure of the POSS cage. Although the POSS monomeric cage does influence the partial charges of the first few carbon atoms covalently bound to the POSS monomer, its effect on the structural properties of the alkyl chain is small. Differences arise, however, for cyclic alkyl substitutents bound to the POSS cage due to the repulsive interactions between the POSS cage and bulkier cyclic intermediates that result upon rotation of the Si-C-C-C dihedral angles. The interatomic potentials for these rotational, or torsional, terms need to be modified slightly in order to appropriately simulate sterically hindered substitutents on the cage. Our results suggest that combining an atomistic force field independently developed to describe silsesquioxanes with an independent atomistic model developed to describe hydrocarbon chains can be used in classical molecular simulation studies of most alkyl-silsesquioxanes. This avoids the need to develop specific force fields for each substituted POSS cage studied and opens up the possibility of using molecular simulation to probe the thermodynamic and structural properties of these unique nanoscale building blocks.

Cages, Baskets, Ladders, and Tubes: Conformational Studies of Polyhedral Oligomeric Silsesquioxanes

The conformational flexibility of a series of cage, basket, ladder, and tube polyhedral oligomeric silsesquioxanes (POSS) has been examined using the Low Mode:Monte Carlo conformational search method in conjunction with the MM3/GBSA(CHCl3) surface. An ensemble of low energy structures was generated and used to explore the molecular shape and flexibility of each system. The results indicate that, except for the ladder molecule, the incompletely condensed systems that are studied are relatively rigid. Even in cases where the molecule is able to adopt numerous low energy conformations, the overall shape remains cage-like and the conformations differ only by small angles or substituent orientations. The ladder molecule is the most flexible and this ensemble clusters into two families: one that is cage-like and the other that is more open and ladder-like. The conformational flexibilities in the gas and solvent phases, as approximated using the GBSA continuum solvent model, are very similar.