A quantum chemical study of silsesquioxanes: H8Si8O12, Me8Si8O12, H@Me8Si8O12, He@Me8Si8O 12 + , and He@Me8Si8O12

Quantum chemical PBE0 and B3LYP/cc-pVTZ, PBE0, B3LYP, RHF and MP2/6-31G(d,p) methods are employed to calculate the structural parameters of octa(silsesquioxane) H₈Si₈O₁₂ and octa(methylsilsesquioxane) Me₈Si₈O₁₂. These molecules and complexes H@Me₈Si₈O₁₂, He@Me₈Si₈O ₁₂ ⁺ , and He@Me₈Si₈O₁₂ have highly symmetric (O _h ) equilibrium configurations. With the use of the PBE0 method and a cc-pVTZ multicenter basis set common for the complex and its components coincidence is achieved between the calculated polarizability of a free He atom and the experimental value of 0.21 ų and the polarizability depression of 0.17 ų was found for He@Me₈Si₈O₁₂. In order to avoid the false conclusion about molecular symmetry the calculations of the structure of silsesquioxanes must be performed with sufficiently high accuracy (Int = ultrafine and Opt = tight in the use of the GAUSSIAN program).     

A quantum chemical study of the electronic structure of substituted germocanes

The PRIRODA (riDFT method, BLYP functional, hf.bas basis set) and Gaussian 98 (HF method, 6-311G(d,p) basis set) programs are used to calculate the spatial and electronic structures of a number of molecules of substituted germocanes with a general formula of R,Rs’Ge(XCH₂CH₂)₂Y (where X = C, O, S, and Y = N, O, S). With the use of the AIM method the topological characteristics of Ge—Y donation bonds are calculated in these molecules. An analysis of the obtained data shows that up to the values of Ge—Y interatomic distances of ~2.7 Å these bonds can be considered as the intermediate type bonds. At Ge—Y distances of ~3.0 Å these bonds become ionic, therefore the Coulomb interaction between oppositely charged Ge and Y atoms mainly contributes to Ge—Y bonding.     

The structure of the Si9H12 cluster: A coupled cluster and multi-reference perturbation theory study

Full geometry optimizations using both singles and doubles coupled cluster theory with perturbative triple excitations, CCSD(T), and second order multi-reference perturbation theory, MRMP2, have been employed to predict the structure of Si9H12, a cluster commonly used in calculations to represent the Si(100) surface. Both levels of theory predict the structure of this cluster to be symmetric (not buckled), and no evidence for a buckled (asymmetric) structure is found at either level of theory.     

Normal coordinate analysis of H8Si8O12

Normal coordinate analysis of the fundamental vibrations of H₈Si₈O₁₂ has been carried out. Because of the octahedral symmetry, the 78 vibrational degrees of freedom lead to 33 different vibrations, six of which are infrared active, 13 are Raman active and 14 are inactive. From the internal coordinates one gets 116 symmetry coordinates. We describe a straightforward method for determining the internal symmetry coordinates of any molecular system. Internal coordinates, symmetry force constants, the full set of orthonormal symmetry coordinates as well as the 38 redundant orthonormal symmetry coordinates of H₈Si₈O₁₂ are tabulated. The potential energy distribution analysis shows that most of the fundamental vibrations can be very well interpreted in terms of the internal vibrations ν(SiH), ν(SiO), δ(SiH), δ(OSiO) and δ(SiOSi) which makes it easy to compare them with vibrations observed in other silsesquioxanes and similar silicon compounds.     

Pd-katalysierter Deuterium-Austausch am Octa(silsesquioxan)H8Si8O12 zu D8Si8O12

Pd-Catalyzed Deuterium Exchange of Octa(silsesquioxane) H₈Si₈O₁₂ to D₈Si₈O₁₂ Deuterium exchange on octa(silsesquioxane) H₈Si₈O₁₂ to D₈Si₁₈O₁₂ catalyzed by palladium on carbon is reported.     

Electronic structure and reactivity of guanylthiourea: A quantum chemical study

Electronic structure analysis of guanylthiourea (GTU) and its isomers has been carried out using quantum chemical methods. Two major tautomeric classes (thione and thiol) have been identified on the potential energy (PE) surface. In both the cases conjugation of pi‐electrons and intramolecular H‐bonds have been found to play a stabilizing role. Various isomers of GTU on its PE surface have been analyzed in two different groups (thione and thiol). The interconversion from the most stable thione conformer ( GTU‐1 ) to the most stable thiol conformer ( GTU‐t1 ) was found to take place via bimolecular process which involves protonation at sulfur atom of GTU‐1 followed by subsequent C? N bond rotation and deprotonation. The detailed analysis of the protonation has been carried out in gas phase and aqueous phase (using CPMC model). Sulfur atom (S1) was found to be the preferred protonation site (over N4) in GTU‐1 in gas phase whereas N4 was found to be the preferred site of protonation in aqueous medium. The mechanism of S‐alkylation reaction in GTU has also been studied. The formation of alkylated analogs of thiol isomers (alkylated guanylthiourea) is believed to take place via bimolecular process which involves alkyl cation attack at S atom followed by C? N bond rotation and deprotonation. The reactive intermediate RS(NH₂)C? N? C(NH₂)₂⁺ belongs to the newly identified ^⊕N(←L)₂ class of species and provides the necessary dynamism for easy conversion of thione to thiol. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Monosubstitution von Octa(hydridosilasesquioxan) H8Si8O12 zu R′H7Si8O12 mittels Hydrosilylierung Kurzmitteilung

Monosubstitution of Octa(hydridosilasesquioxane) H₈Si₈O₁₂ to R′H₇Si₈O₁₂ by Hydrosilylation Hydrosilylation of hex-1-ene and of styrene by octa(hydridosilasesquioxanes) catalyzed by hexachloroplatinic acid leads to the first monosubstituted octasilasesquioxane R′H₇Si₈O₁₂ molecules.     

Ab initio quantum chemical calculations of a cluster C8H12

Using unrestricted Hartree–Fock–Roothaan approximation and density functional theory on 6‐31G* basis, ab initio calculation cluster C₈H₁₂ has been carried out. It was shown as a result of calculations that a ground state for the conformation given is a septet state. This cluster can be used for investigation of the kinematic mechanism of magnetic exchange and magnetic ordering in polyradicals. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

A dynamical study of the Si(+) + H(2)O reaction

A dynamical study of the Si(+) + H(2)O reaction has been carried out by means of a quasiclassical trajectory method that decomposes the reaction into a capture step, for which an accurate analytical potential is employed, and an unimolecular step, in which the evolution of the collision complex is studied through a direct dynamics BHandHLYP/6-31G(d,p) method. The capture rate coefficient has been computed for thermal conditions corresponding to temperatures ranging from 50 to 1000 K. It is concluded that the main reason why the reaction rate is about 10 times smaller than the capture rate (at T = 298 K) is the topology of the potential energy surface of the ground state. It is also concluded that the ratio between the rates of product and reactant generation from the collision complex decreases quite steeply with increasing temperature, and therefore, the reaction rate decreases even more sharply. Exciting the stretching normal modes of water substantially increases that ratio, and moderate rotational excitation does not appear to have a relevant effect. The collision complex is always initially SiOH(2)(+), but in some trajectories, it becomes HSiOH(+), which generates the products, although the former species is the main intermediate.     

3-硝基-1, 2, 4-三唑-5-酮钾配合物的制备、晶体结构和量子化学研究

通过3-硝基-1, 2, 4-三唑-5-酮(NTO)与氢氧化钾水溶液反应,制备了标题配合物, 并用TG, 元素分析, 红外光谱分析对它进行了表征。其结构用单晶分析法测定, 所得晶体学参数为a=0.6408(1),b=0.8218(1), c=1.2626(1)nm, β=100.63ⅲ(1), V=0.6535(1)nm^3,Z=4, Dc=1.892g.cm^-^3, μ=0.785mm^-^1, F(000)=376; 晶体属单斜晶系, 空间群为P21/n, 最终偏离因子R为0.0246。用EHMO计算表明, 标题化合物主要是靠静电引力形成的配合物, 中心原子K与H2O的配位较K与NTO环的结合弱, 预示热解优先脱水。     

A quantum chemical study of comparison of various propylene epoxidation mechanisms using H2O2 and TS-1 Catalyst

We present a comparison of the following prominent propylene epoxidation mechanisms using H2O2/TS-1 at a consistent density functional theory (DFT) method: (1) the Sinclair and Catlow mechanism on tripodal site through Ti-OOH species, (2) the Vayssilov and van Santen mechanism on tetrapodal site without Ti-OOH formation, (3) the Munakata et al. mechanism involving peroxy (Ti-O-O-Si) species, (4) the defect site mechanism with a partial silanol nest, and (5) the defect site mechanism with a full silanol nest. We have reproduced the previously published (ethylene epoxidation) pathways (1-3) for propylene epoxidation using larger and SiH3-terminated cluster models of the T-6 crystallographic site of TS-1. Mechanism 5 is a new mechanism reported here for the first time. The use of a consistent level of theory for all the pathways allows for the first time a meaningful comparison of the energetics representing the aforementioned pathways. We have rigorously identified the important reaction intermediates and transition states and carried out a detailed thermochemical analysis at 298.15 K and 1 atm. On the basis of the Gibbs free energy of activation, the Sinclair and Catlow mechanism (Delta G(act) = 7.9 kcal/mol) is the energetically most favorable mechanism, which is, however, likely to operate on the external surface of TS-1 due to the tripodal nature of the Ti site in their model. The newly reported defect site mechanism (with a full silanol nest) is a competitive propylene epoxidation mechanism. There are two main steps: (1) hydroperoxy formation (Delta G(act) = 8.9 kcal/mol) and (2) propylene epoxidation (Delta G(act) = 4.6 kcal/mol). This mechanism is likely to represent the chemistry occurring inside the TS-1 pores in the liquid-phase epoxidation (H2O2/TS-1) process and could operate in direct gas-phase epoxidation (H2/O2/Au/TS-1) as well. If only the propylene epoxidation step is considered, then the Munakata peroxo intermediate (Si-O-O-Ti) is the most reactive intermediate, which can epoxidize propylene with a negligible activation barrier. However, formation of the Munakata intermediate is a very activated step (Delta G(act) = 19.8 kcal/mol). We also explain the trends in the activation barriers in different mechanisms using geometric and electronic features such as orientation of adsorbed H2O2 and propylene, hydrogen bonding, O1-Ti bond distance in the Ti-O1-O2-H intermediate, and O1-O2 stretching in the transition state. Implications of different Ti site models are also discussed in light of the nature of external/internal and nondefect/defect sites of TS-1.     

Silicon buckyballs versus prismanes: Influence of spatial confinement on the structural properties and optical spectra of the Si18H12 and Si19H12 clusters

The genetic algorithm is combined with the density functional theory to predict how the cylindrical spatial confinement affects the structural characteristics and optical adsorption spectra of the low‐energy Si₁₈H₁₂ and Si₁₉H₁₂ isomers. Retrieved ground states (minimum energy states) of Si₁₈H₁₂ and Si₁₉H₁₂ isomers significantly differ from the earlier proposed “ultrastable” aromatic molecular systems and prismanes. According to our calculations, they are represented by the almost spherical endohedral buckyballs. In contrast to pure silicic clusters (Si₁₈ or Si₁₉), the most of Si atoms in the low‐energy Si₁₈H₁₂ and Si₁₉H₁₂ are four‐coordinated. Spatial confinement results in more oblong structures with the different optical spectra. Prismanes under confinement get some energy advantages over the spherical structures, but despite this they do not become the most energetically favorable ones. Thus, the current results warrant however further research on the spatial confinement of silicic prismanes, as our study suggests challenges in devising the method of their synthesis without additional chemical techniques.     

Spectroscopic and quantum chemical study of the structure of 4-aminopyrimidinoanthrones

4-Amino derivatives of pyrimidinoanthrone exist in the form of an aminoketone isomer in the crystalline state in neutral organic solutions, but in acid and alkaline media the tautomeric equilibrium is shifted toward formation of ionic forms of the iminohydroxyl tautomer. We present the characteristics of the IR and electronic absorption spectra and quantum chemical calculations for the neutral and ionic forms of the tautomers indicated above.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1109–1116, August, 1994.     

H8Si8O12和H8Si7TiO12 团簇的几何构型和电子结构的从头算研究

应用HF、 MP2和杂化的B3LYP方法,使用3-21G基组,对H8Si8O12 和H8Si7TiO12团簇的几何构型、总能进行了计算,并在B3LYP/3-21G的水平上对硅原子的核磁共振化学位移进行了研究,得到的几何构型,以及核磁共振化学位移与实验结果进行了比较,发现吻合得很好。计算了H8Si8O12和H8Si7TiO12团簇的Mulliken布居数的大小。并对Si原子被Ti原子取代前后的H8Si8O12体系的几何构型、 Mulliken布居数的变化进行了比较和研究。     

A reduced dimensionality quasiclassical and quantum study of the proton transfer reaction H3O(+)+H2O-->H2O+H3O(+)

We report quantum and quasiclassical calculations of proton transfer in the reaction H(3)O(+)+H(2)O in three degrees of freedom, the two OH(+) bond lengths and the OH(+)O angle. The reduced dimensional potential energy surface is obtained from the full dimensional OSS3(p) energy function of H(5)O(2) (+) [L. Ojamae, I. Shavitt, and S. J. Singer, J. Chem. Phys. 109, 5547 (1998)], with an additional long-range correction to reproduce the correct ion-molecule interaction. This surface is used to perform both quasiclassical trajectory and quantum reactive scattering calculations of the zero total angular momentum cumulative reaction probability and cross sections for initial rotational states 0, 1, and 2. Comparison of these quantities are made to assess the importance of quantum effects in this reduced dimensional reaction. Additional quasiclassical cross sections are calculated to obtain the thermal rate constant for the reaction.     

Crystal structure and quantum chemical investigation of [Cd(NTO)_4Cd(H_2O)_6]·4H_2O

The common explosives, RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (1,3,5,7- tetranitro-1,3,5,7-tetraazacyclooctane) and TNT (trinitrotoluene), were considered adequately for all weapons applications. Due to many catastrophic explosions resulting from unintentional initia-tion of impact, friction or shock, these explosives have become less attractive. TATB (1,3,5-tria- mino-2,4,6-trinitrobenzene) is noted for its insensitivity, however, it does not have the energetic performance of e…     

Electronic structure of 1,3-dinitrobenzene radical anion: A multiconfigurational quantum chemical study

The structure of 1,3-dinitrobenzene radical anion in the doublet ground and lowest excited states was studied by ab initio multiconfiguration CASSCF methods. The results of calculations suggest the existence of one symmetrical and two asymmetrical structures of the radical anion. The energies of these structures were estimated. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2645–2647, December, 2005.