Anisotropic photonic crystals: Generalized plane wave method and dispersion symmetry properties

This article presents a generalized vector plane wave expansion method, applicable to isotropic and anisotropic periodic dielectric media of arbitrary geometry and dimension. The influence of anisotropic material orientation on the symmetry properties of photonic crystal dispersion surface is discussed. It is shown that the overall Brillouin zone symmetry is formed by the intersection of the photonic crystal lattice symmetry and the symmetry determined by the anisotropic material orientation. This work explains how to define the irreducible Brillouin zone of a two-dimensional anisotropic photonic crystal and demonstrates that doing it correctly allows one to avoid erroneous results, when calculating band gap diagrams of anisotropic photonic crystals. With the help of the methods presented, the possibility of controlling the band gaps of anisotropic photonic crystals by means of external electric field is shown.     

DFT computational schemes for 1H and 13C NMR chemical shifts of natural products,exemplified by strychnine

A number of computational schemes based on different Density Functional Theory (DFT) functionals in combination with a number of basis sets were tested in the calculation of ¹H and ¹³C NMR chemical shifts of strychnine, as a typical representative of the vitally important natural products, and used as a challenging benchmark and a rigorous test for such calculations. It was found that the most accurate computational scheme, as compared with experiment, was PBE0/pcSseg-4//pcseg-3 characterized by a mean absolute error of 0.07 ppm for the range of about 7 ppm for ¹H NMR chemical shifts and that of only 1.13 ppm for ¹³C NMR chemical shifts spread over the range of about 150 ppm. For more practical purposes, including investigation of larger molecules from this series, a much more economical computational scheme, PBE0/pcSseg-2//pcseg-2, characterized by almost the same accuracy and much less computational demand, was recommended.     

Computational NMR of natural products: On the way to super large molecules exemplified with alasmontamine A

¹H and ¹³C nuclear magnetic resonance (NMR) chemical shifts of a tetrakis monoterpene indole alkaloid alasmontamine A with a molecular formula of C₈₄H₉₁N₈O₁₂ have been calculated at the PBE0/pcSseg-2//pcseg-2 level of theory on M06-2X/aug-cc-pVDZ geometry. In the course of the preliminary conformational search, six true minimum energy conformers were identified that can contribute to the actual conformation of this huge alkaloid. Calculated chemical shifts generally demonstrated a good agreement with available experimental data characterized with a corrected mean absolute error of 0.10 ppm for the range of about 7 ppm for protons and 1.1 ppm for the range of about 160 ppm for carbons.     

On the accuracy of the GIAO‐DFT calculation of 15N NMR chemical shifts of the nitrogen‐containing heterocycles – a gateway to better agreement with experiment at lower computational cost

The main factors affecting the accuracy and computational cost of the gauge‐independent atomic orbital density functional theory (GIAO‐DFT) calculation of ¹⁵N NMR chemical shifts in the representative series of key nitrogen‐containing heterocycles – azoles and azines – have been systematically analyzed. In the calculation of ¹⁵N NMR chemical shifts, the best result has been achieved with the KT3 functional used in combination with Jensen's pcS‐3 basis set (GIAO‐DFT‐KT3/pcS‐3) resulting in the value of mean absolute error as small as 5 ppm for a range exceeding 270 ppm in a benchmark series of 23 compounds with an overall number of 41 different ¹⁵N NMR chemical shifts. Another essential finding is that basically, the application of the locally dense basis set approach is justified in the calculation of ¹⁵N NMR chemical shifts within the 3–4 ppm error that results in a dramatic decrease in computational cost. Based on the present data, we recommend GIAO‐DFT‐KT3/pcS‐3//pc‐2 as one of the most effective locally dense basis set schemes for the calculation of ¹⁵N NMR chemical shifts. Copyright © 2014 John Wiley & Sons, Ltd.     

Exhaustive Conformational Search for Sialyl Cation Reveals Possibility of Remote Participation of Acyl Groups

Finding convenient ways for the stereoselective α-sialylation is important due to the high practical significance of α-sialic acid-containing glycans and neoglycoconjugates. It was proposed that sialylation stereoselectivity is determined by the structure of the sialyl cation (also known in biochemistry as “sialosyl cation”), a supposed intermediate in this reaction. Here we design a new approach for studying the conformational space of highly flexible sialyl cation and find 1625 unique conformers including those stabilized by covalent remote participation (also known as long-range participation) of 4-O-acetyl (4-OAc), 5-N-trifluoroacetyl (5-NTFA), as well as 7,8,9-OAc from both α and β sides. The most energetically stable sialyl cation conformers are featured by 4-OAc participation, closely followed by 5-NTFA- and 7-OAc-stabilized conformers; unstabilized sialyl cation conformers are ∼10 kcal mol⁻¹ less stable than the 4-OAc-stabilized ones. Analysis of all the obtained conformers by means of substituents positions, side chain conformations and ring puckering led us to a new “eight-conformer hypothesis” which describes interconversions among the most important sialyl cation conformers and predicts that stronger remote participation of acyl groups favors β-anomers. Thus, selective synthesis of the desired α-sialosides requires minimization of acyl groups participation.     

Spin Transport over Huge Distances in a Magnetized 2D Electron System

Experimental results on the properties of a recently discovered new collective state, the magnetofermionic condensate, are summarized herein. Condensation occurs in a fermionic system, a quantum Hall insulator (filling factor ν = 2), as a result of the formation of a dense ensemble of long‐lived spin cyclotron magnetoexcitons, composite bosons. At temperatures below 1 K, the exciton ensemble exhibits a sharp enhancement in its response to an external electromagnetic field due to the formation of a super‐absorbing state that interacts coherently with the electromagnetic field. Simultaneously, the electrons below the Fermi level rearrange to form a new non‐equilibrium radiative recombination channel. The condensate shows a sharp decrease in viscosity and the ability to spread over macroscopically large distances, on the order of a millimeter, at a speed of $\approx {10}^3\text{cm}{\text{s}}^{?1}$. Due to this rapid long‐distance spin transfer, new opportunities in the field of spintronics have been opened up.