Novel oligo-m-carboranylenemethylenesilanes

A method has been developed for the obtaining of new oligo(dimethyl)silanes comprising 1,7-bis(methyl)-m-carboranyl units and bearing the terminal chlorosilyl or methoxysilyl groups. Reactions of such oligomers with an excess of benzylmagnesium chloride or 1-chloromagnesiummethyl-m-carborane lead to oligo-m-carboranylenemethylenesilanes containing the terminal benzylsilyl or m-carboranylmethylenesilyl groups. The novel oligomers are highly viscous liquids possessing a high heat resistance and also a resistance towards the treatment with a strong alkaline reagent.

     

Synthesis of new alkoxysilyl-substituted carboranes

New 1,7-bis(trialkoxysilylpropyl)-m-carboranedicarboxamides were synthesized by the reaction of 3-aminopropyltrialkoxysilanes with either m-carboranedicarboxylic acid chloride in the presence of Et₃N or m-carboranedicarboxylic acid in the presence of N,N´-carbonyl-diimidazole. The synthesized compounds are colorless liquids soluble in benzene, toluene, tetrahydrofuran, diethyl ether, and chloroform and insoluble in water. Structures of the synthesized compounds were established by IR and NMR spectroscopy and confirmed by elemental analysis. The synthesized compounds are interesting starting materials for produc-tion of heat-resistant silicon carborane polymers and metal-carborane-silicon polymers.     

Nonequilibrium Fermi golden rule for electronic transitions through conical intersections

We consider photoinduced electronic transitions through conical intersections in large molecules. Starting from the linear vibronic model Hamiltonian and treating linear diabatic couplings within the second order cumulant expansion, we have developed a simple analytical expression for the time evolution of electronic populations at finite temperature. The derived expression can be seen as a nonequilibrium generalization of the Fermi golden rule due to a nonequilibrium character of the initial photoinduced nuclear distribution. All parameters in our model are obtained from electronic structure calculations followed by a diabatization procedure. The results of our model are found to agree well with those of quantum dynamics for a test set of systems: fulvene molecule, 2,6-bis(methylene) adamantyl cation, and its dimethyl derivative.     

Terahertz spectroscopy of enantiopure and racemic polycrystalline valine

Experimental and computational THz (or far-infrared) spectra of polycrystalline valine samples are reported. The experimental spectra have been measured using THz time-domain spectroscopy. Spectra of the pure enantiomers, both D and L, as well as the dl racemate have been taken at room temperature and low temperature (78 K). The spectra of the pure D and L enantiomers are essentially identical, and they are markedly different from the DL racemate. In addition, a temperature-dependent study of L-valine was undertaken in which the absorption maxima were found to red shift as a function of increasing temperature. The vibrational absorption spectra (frequencies and intensities) were calculated using the harmonic approximation with the Perdew-Burke-Ernzerhof (PBE) functional, localized atomic orbital basis sets, and periodic boundary conditions. The calculated and experimental spectra are in good qualitative agreement. A general method of quantifying the degree to which a calculated mode is intermolecular versus intramolecular is demonstrated, with the intermolecular motions further separated into translational versus rotational/librational motion. This allows straightforward comparison of spectra calculated using different basis sets or other constraints.     

Models for capacitated lot-sizing problem with backlogging,setup carryover and crossover

Setup operations are significant in some production environments. It is mandatory that their production plans consider some features, as setup state conservation across periods through setup carryover and crossover. The modelling of setup crossover allows more flexible decisions and is essential for problems with long setup times. This paper proposes two models for the capacitated lot-sizing problem with backlogging and setup carryover and crossover. The first is in line with other models from the literature, whereas the second considers a disaggregated setup variable, which tracks the starting and completion times of the setup operation. This innovative approach permits a more compact formulation. Computational results show that the proposed models have outperformed other state-of-the-art formulation.     

Influence of the exchange screening parameter on the performance of screened hybrid functionals

This work reexamines the effect of the exchange screening parameter omega on the performance of the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional. We show that variation of the screening parameter influences solid band gaps the most. Other properties such as molecular thermochemistry or lattice constants of solids change little with omega. We recommend a new version of HSE with the screening parameter omega=0.11 bohr(-1) for further use. Compared to the original implementation, the new parametrization yields better thermochemical results and preserves the good accuracy for band gaps and lattice constants in solids.