Unbiased auxiliary basis sets for accurate two-electron integral approximations

We propose Cholesky decomposition (CD) of the atomic two-electron integral matrix as a robust and general technique for generating auxiliary basis sets for the density fitting approximation. The atomic CD (aCD) auxiliary basis set is calculated on the fly and is not biased toward a particular quantum chemical method. Moreover, the accuracy of the aCD basis set can be controlled with a single parameter.     

Effects of Substrate Loading on Enzymatic Hydrolysis and Viscosity of Pretreated Barley Straw

In this study, the applicability of a “fed-batch” strategy, that is, sequential loading of substrate or substrate plus enzymes during enzymatic hydrolysis was evaluated for hydrolysis of steam-pretreated barley straw. The specific aims were to achieve hydrolysis of high substrate levels, low viscosity during hydrolysis, and high glucose concentrations. An enzyme system comprising Celluclast and Novozyme 188, a commercial cellulase product derived from Trichoderma reesei and a β-glucosidase derived from Aspergillus niger, respectively, was used for the enzymatic hydrolysis. The highest final glucose concentration, 78 g/l, after 72 h of reaction, was obtained with an initial, full substrate loading of 15% dry matter weight/weight (w/w DM). Conversely, the glucose yields, in grams per gram of DM, were highest at lower substrate concentrations, with the highest glucose yield being 0.53 g/g DM for the reaction with a substrate loading of 5% w/w DM after 72 h. The reactions subjected to gradual loading of substrate or substrate plus enzymes to increase the substrate levels from 5 to 15% w/w DM, consistently provided lower concentrations of glucose after 72 h of reaction; however, the initial rates of conversion varied in the different reactions. Rapid cellulose degradation was accompanied by rapid decreases in viscosity before addition of extra substrate, but when extra substrate or substrate plus enzymes were added, the viscosities of the slurries increased and the hydrolytic efficiencies decreased temporarily.     

Rupturing polymeric micelles with cyclodextrins

Small-angle neutron scattering has been used to investigate the associative structures formed by triblock copolymers of poly(ethylene oxide) (PEO)-polypropylene oxide (PPO)-poly(ethylene oxide) (PEO) (also known as Pluronics) and to monitor the structural changes occurring upon complexation with heptakis(2,6-di-O-methyl)-beta-cyclodextrin (hbeta-CD) over the temperature range from 5 to 70 degrees C. At low temperature, the Pluronics are dispersed as unimers. Close to ambient temperature, the hydrophobicity of PPO causes the aggregation of the polymers into spherical micelles with core sizes between 40 and 50 A and a high inclusion of solvent. The aggregation number increases with temperature as the hydrophobicity of the core is gradually enhanced. hbeta-CD spontaneously forms pseudopolyrotaxanes with the triblock copolymers either when in their unimer form or micellized. The complexation results in an increase in the effective critical micellar concentration. It is suggested that the cyclodextrins thread onto the polymer backbone to localize preferentially on the central PPO block, therefore improving its water solubility. At temperatures where the polymers exist in micellar form, complexation with hbeta-CD gives rise to a complete disruption of the aggregates. These processes are highly temperature-dependent. Above 50 degrees C, the break-up of the aggregates is inhibited, and large-scale aggregation is observed.     

Applied topology optimization of vibro-acoustic hearing instrument models

Designing hearing instruments remains an acoustic challenge as users request small designs for comfortable wear and cosmetic appeal and at the same time require sufficient amplification from the device. First, to ensure proper amplification in the device, a critical design challenge in the hearing instrument is to minimize the feedback between the outputs (generated sound and vibrations) from the receiver looping back into the microphones. Secondly, the feedback signal is minimized using time consuming trial-and-error design procedures for physical prototypes and virtual models using finite element analysis. In the present work it is demonstrated that structural topology optimization of vibro-acoustic finite element models can be used to both sufficiently minimize the feedback signal and to reduce the time consuming trial-and-error design approach. The structural topology optimization of a vibro-acoustic finite element model is shown for an industrial full scale model hearing instrument.     

Nanoparticle plasmon resonances in the near-static limit

Localized surface plasmon resonances of metal nanoparticles of arbitrary shape are analyzed in the near-static limit with retardation included to the second order. Starting from the electrostatic approximation, the second-order correction to the resonant dielectric constant is expressed by means of a triple surface integral. For arbitrary nanoparticles with cylindrical symmetry we show how the triple surface integral can be significantly simplified, resulting in a computationally efficient scheme for evaluation of nanoparticle plasmon eigenresonances in the near-static limit. The approach allows for calculation of both dipolar and higher-order resonances.     

Amino triazolo diazepines (Ata) as constrained histidine mimics

Two synthetic routes for the synthesis of amino-triazolodiazepine (Ata) scaffolds are presented. The scope of both of these proceeding through key intra- and intermolecular Huisgen cycloaddition reactions is discussed. The replacement of the His-Pro dipeptide segment in angiotensin IV by the dipeptide mimetic Ata-Gly and subsequent biological evaluation in two inhibitory enzyme assays validated the use of the Ata moiety as a His mimic given the equipotency of both peptidic analogs.     

A linear single-molecule magnet based on [Ru(III)(CN)6]3-

We report the synthesis, structure and magnetic properties of the first molecular cluster and single-molecule magnet to incorporate [Ru(III)(CN)(6)](3-). Frequency-domain Fourier-transform THz-EPR (FDFT THz-EPR) and magnetic susceptibility measurements indicate strongly anisotropic Mn-Ru exchange interactions.     

THIATION OF CARBONYL COMPOUNDS WITH THE DIMER OF P-METHOXY-P-THIONOPHOSPHINSULFIDE

Abstract

Recent studies on thiation of carbonyl compounds with organic P,S-compounds have shown that p-methoxyphenyl-thionophosphinsulfide (1) is the most effective thiation reagent for ketones, carboxamides, esters, and thioloesters hitherto known.     

Adsorption and surface diffusion of silicon growth species in silicon carbide chemical vapour deposition processes studied by quantum-chemical computations

The effect chlorine addition to the gas mixture has on the surface chemistry in the chemical vapour deposition (CVD) process for silicon carbide (SiC) epitaxial layers is studied by quantum-chemical calculations of the adsorption and diffusion of SiH₂ and SiCl₂ on the (000-1) 4H–SiC surface. SiH₂ was found to bind more strongly to the surface than SiCl₂ by approximately 100 kJ mol^?1 and to have a 50 kJ mol^?1 lower energy barrier for diffusion on the fully hydrogen-terminated surface. On a bare SiC surface, without hydrogen termination, the SiCl₂ molecule has a somewhat lower energy barrier for diffusion. SiCl₂ is found to require a higher activation energy for desorption once chemisorbed, compared to the SiH₂ molecule. Gibbs free energy calculations also indicate that the SiC surface may not be fully hydrogen terminated at CVD conditions since missing neighbouring pair of surface hydrogens is found to be a likely type of defect on a hydrogen-terminated SiC surface.