Geometric changes around an N atom due to a urethane‐type bis(tert‐butoxycarbonyl) substituent

Two crystal structures of urethane‐protected derivatives of aspartic acid dimethyl ester are presented, namely dimethyl (2S)‐2‐[(tert‐butoxycarbonyl)amino]butanedioate, C₁₁H₁₉NO₆, and dimethyl (2S)‐2‐{bis[(tert‐butoxycarbonyl]amino}butanedioate, C₁₆H₂₇NO₈. The geometry at the N atom is discussed and compared with similar structures. The analysis of singly and doubly N‐substituted derivatives reveals an elongation of all bonds involving the N atom and conformational changes of the amino acid side chain due to steric interactions with two bulky substituents on the amino group.     

Spatial smoothing for localized correlated sources – Its effect on different localization methods in the nearfield

The spatial smoothing (SS) technique has been proved to be effective in decorrelating coherent signals by restoring the rank of the signal covariance matrix R. Averaging the covariance matrices of subarrays of the original array, is a technique which increases the rank of the smoothed matrix R_SS. Algorithms like MUSIC or Capon, which rely on the use of the signal covariance matrix R and fail in the case of correlated sources, can be applied to scenarios with correlated sources after spatial smoothing.However, SS is most practically applied to uniformly spaced arrays or to arrays which have a translational symmetry.In addition the formulation is strictly applicable only to such farfield conditions, where the incoming waves are plane waves and the steering vectors to the sources of the different subarrays are identical. These conditions are not fulfilled in the nearfield.Spatial smoothing is now applied with an acoustic camera in the nearfield and it is shown that up to some limits this technique is applicable. Effects/limitations are studied using simulation and measurements with several Beamforming algorithms (MUSIC, Capon and Orthogonal Beamforming) are carried out.The results demonstrate the benefits of SS even in the nearfield up to some limits, which are given through the distance of the different subarrays in comparison to the spatial resolution of the Beamforming algorithm. Especially at lower frequencies SS in connection with MUSIC- or Capon-Beamforming give better resolution in comparison to D + S Beamforming.     

Ion acceleration from aluminium plasma generated by a femtosecond laser in different conditions

Non-equilibrium plasma was obtained by irradiating Al foils in vacuum with a femtosecond (fs) laser at intensities of the order of 10¹⁸ W/cm². Protons and other light ions were accelerated in the forward direction by using the target-normal-sheath acceleration regime. Time-of-flight technique was employed to measure the ions' kinetic energy using SiC detectors placed at known distances and angles. The ion acceleration was monitored under different conditions of laser focal position, laser pulse energy, and laser contrast. The target was irradiated using different thicknesses and anti-reflecting graphene films. By optimizing the laser parameters, irradiation conditions, and target properties, it was possible to accelerate up to 2.3 MeV per charge state, as will be presented and discussed.     

Influence of Polymer Solvents on the Properties of Halloysite-Modified Polyethersulfone Membranes Prepared by Wet Phase Inversion

Ultrafiltration polyethersulfone (PES) membranes were prepared by wet phase inversion. Commercial halloysite nanotubes (HNTs) in the quantities of 0.5 wt% vs. PES (15 wt%) were introduced into the casting solution containing the polymer and different solvents: N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), or 1-methyl-2-pyrrolidinone (NMP). The type of solvent influenced the membranes’ morphology and topography, as well as permeability, separation characteristics, and antifouling and antibacterial properties. The membranes prepared using DMA exhibited the loosest cross-section structure with the thinnest skin and the roughest surface, while the densest and smoothest were the DMF-based membranes. The advanced contact angles were visibly lower in the case of the membranes prepared using DMF compared to the other solvents. The highest water permeability was observed for the DMA-based membranes, however, the most significant effect of the modification with HNTs was found for the NMP-based series. Regardless of the solvent, the introduction of HNTs resulted in an improvement of the separation properties of membranes. A noticeable enhancement of antifouling performance upon application of HNTs was found only in the case of DMF-based membranes. The study of the antibacterial properties showed that the increase in surface roughness had a positive effect on the inhibition of E. coli growth.     

Influence of Fluorine Substitution on Nonbonding Interactions in Selected Para-Halogeno Anilines

A series of 4-halogeno aniline derivatives was studied employing combined theoretical and experimental methods (i. e. crystal structure analysis and vibrational spectroscopies). This simplified model system was selected to shed light on the impact of fluorine substitution on the formation of noncovalent interactions such as halogen bonds (XBs) and hydrogen bonds (HBs), which are key interactions in fluorinated/halogenated drug-protein complex formation. Comparative analysis of three previously reported and five newly determined crystal structures indicated that, in most cases, 2-fluoro and 2,6-difluoro substitution of 4-X anilines increases the ability of adjacent amine to form strong N−H⋅⋅⋅N HBs. Additionally, fluorine substituents in the difluorinated derivatives are competitive and attractive HB and XB acceptors and increase the probability of halogen-halogen contacts. A peculiar observation was made for 4-iodoaniline and 2,6-difluoro-4-iodoaniline, which form distinct interaction patterns compared to the corresponding 4-Cl and 4-Br analogs. The observed intramolecular N−H⋅⋅⋅F interactions lead to additional NH bands in the FT-IR spectra.     

Effect of water deposition on the surface dynamics of mesopores in MCM-41

The surface acoustic waves in empty cylindrical pores in the amorphous silica MCM-41 as well as in the same pores partially filled with water are studied with the use of a continuum model. The model is shown to be adequate to predict dispersion relations, cut-off wave vectors and the Airy phases for the secular surface waves of the lowest azimuthal indices n. Quantitative predictions are presented both in the liquid and in the polycrystalline solid phase of water. Two sagittal surface waves exist when water is in the liquid phase. The phase transition to the solid phase (ice) results in the disappearance of the high-frequency mode. All the effects occur in the Terahertz frequency region.     

Obtaining the two-body density matrix in the density matrix renormalization group method

We present an approach that allows to produce the two-body density matrix during the density matrix renormalization group (DMRG) run without an additional increase in the current disk and memory requirements. The computational cost of producing the two-body density matrix is proportional to O(M3k2+M2k4). The method is based on the assumption that different elements of the two-body density matrix can be calculated during different steps of a sweep. Hence, it is desirable that the wave function at the convergence does not change during a sweep. We discuss the theoretical structure of the wave function ansatz used in DMRG, concluding that during the one-site DMRG procedure, the energy and the wave function are converging monotonically at every step of the sweep. Thus, the one-site algorithm provides an opportunity to obtain the two-body density matrix free from the N-representability problem. We explain the problem of local minima that may be encountered in the DMRG calculations. We discuss theoretically why and when the one- and two-site DMRG procedures may get stuck in a metastable solution, and we list practical solutions helping the minimization to avoid the local minima.