Search results for "DIR"
showing 10 items of 10242 documents
Electronic Structures and Spectroscopic Properties of 6π-Electron Ring Molecules and Ions E2N2 and E42+ (E = S, Se, Te)
2004
The electronic structures and molecular properties of square-planar 6π-electron ring molecules and ions E2N2 and E42+ (E = S, Se, Te) were studied using various ab initio methods and density functionals. All species were found to contain singlet diradical character in their electronic structures. Detailed analysis of the CAS wave function of S2N2 in terms of different valence bond structures gives the largest weight for a Lewis-type singlet diradical VB structure in which the two unpaired electrons reside on nitrogen atoms, though the relative importance of the different VB structures is highly dependent on the level of theory. The diradical character in both E2N2 and E42+ was found to incr…
Molecule Isomerism Modulates the Diradical Properties of Stable Singlet Diradicaloids
2019
Inclusion of quinoidal cores in conjugated hydrocarbons is a common strategy to modulate the properties of diradicaloids formed by aromaticity recovery within the quinoidal unit. Here we describe an alternative approach of tuning of diradical properties in indenoindenodibenzothiophenes upon anti → syn isomerism of the benzothiophene motif. This alters the relationship of the S atom with the radical center from linear to cross conjugation yet retains the same 2,6-naphtho conjugation pattern of the rearomatized core. We conduct a full comparison between the anti and syn derivatives based on structural, spectroscopic, theoretical, and magnetic measurements, showing that these systems are stabl…
A Tale of Two Isomers: Enhanced Antiaromaticity/Diradical Character versus Deleterious Ring‐Opening of Benzofuran‐fused s ‐Indacenes and Dicyclopenta…
2021
We examine the effects of fusing two benzofurans to s-indacene (indacenodibenzofurans, IDBFs) and dicyclopenta[b,g]naphthalene (indenoindenodibenzofurans, IIDBFs) to control the strong antiaromaticity and diradical character of these core units. Synthesis via 3-functionalized benzofuran yields syn-IDBF and syn-IIDBF. syn-IDBF possesses a high degree of paratropicity, exceeding that of the parent hydrocarbon, which in turn results in strong diradical character for syn-IIDBF. In the case of the anti-isomers, synthesized via 2-substituted benzofurans, these effects are decreased; however, both derivatives undergo an unexpected ring-opening reaction during the final dearomatization step. All th…
Treatment of scalar-relativistic effects on nuclear magnetic shieldings using a spin-free exact-two-component approach.
2013
A cost-effective treatment of scalar-relativistic effects on nuclear magnetic shieldings based on the spin-free exact-two-component theory in its one-electron variant (SFX2C-1e) is presented. The SFX2C-1e scheme gains its computational efficiency, in comparison to the four-component approach, from a focus on spin-free contributions and from the elimination of the small component. For the calculation of nuclear magnetic shieldings, the separation of spin-free and spin-dependent terms in the parent four-component theory is carried out here for the matrix representation of the Dirac equation in terms of a restricted-magnetically balanced gauge-including atomic orbital basis. The resulting spin…
Atomic structure solution of the complex quasicrystal approximant Al77Rh15Ru8 from electron diffraction data
2014
The crystal structure of the novel Al77Rh15Ru8phase (which is an approximant of decagonal quasicrystals) was determined using modern direct methods (MDM) applied to automated electron diffraction tomography (ADT) data. The Al77Rh15Ru8E-phase is orthorhombic [Pbma,a= 23.40 (5),b= 16.20 (4) andc= 20.00 (5) Å] and has one of the most complicated intermetallic structures solved solely by electron diffraction methods. Its structural model consists of 78 unique atomic positions in the unit cell (19 Rh/Ru and 59 Al). Precession electron diffraction (PED) patterns and high-resolution electron microscopy (HRTEM) images were used for the validation of the proposed atomic model. The structure of the E…
Site-directed mutagenesis of odorant-binding proteins
2020
Modifying the affinity of odorant-binding proteins (OBPs) to small ligands by replacement of specific residues in the binding pocket may lead to several technological applications. Thanks to their compact and stable structures, OBPs are currently regarded as the best candidates to be used in biosensing elements for odorants and volatiles detection. The wide and rich information on the structure of these proteins both in their apo-forms and in complexes with specific ligands provides guidelines to design reliable mutants to monitor specific targets. The same engineered proteins may also find applications in the slow release of pheromones and other chemicals in the environment, as well as in …
Site-Specific Information on Membrane Protein Folding by Electron Spin Echo Envelope Modulation Spectroscopy
2010
Compared to folding of soluble proteins, folding of membrane proteins is complicated by the fact that it requires an amphiphilic environment. Few existing techniques can provide structurally resolved information on folding kinetics. For the major plant light harvesting complex LHCII, it is demonstrated that changes in water accessibility of a particular amino acid residue can be followed during folding by measuring the hyperfine interaction of spin labels with deuterium nuclei of heavy water. The incorporation of residue 196 into the hydrophobic core of a detergent micelle was investigated. The technique provides a time constant that is similar to the one found with fluorescence spectroscop…
Strategies to decrease water drainage and nitrate emission from soilless cultures of greenhouse tomato
2010
In the spring-summer season of 2005 and 2006, we explored the influence of three fertigation strategies (A–C) on the water and nitrogen use efficiency of semi-closed rockwool culture of greenhouse tomato conducted using saline water (NaCl concentration of 9.5 mol m−3). The strategies under comparison were the following: (A) crop water uptake was compensated by refilling the mixing tank with nutrient solution at full strength (with the concentrations of macronutrients equal or close to the corresponding mean uptake concentrations as determined in previous studies) and the recirculating nutrient solution was flushed out whenever its electrical conductivity (EC) surpassed 4.5 dS m−1 due to the…
A statistical study of the additivities of substituent effects in the13C NMR chemical shifts of hydroxy- and chloro-substituted benzenes
1980
The 13C NMR spectra of six hydroxybenzenes, all chlorobenzenes, all chlorophenols and eight chlorocatechols are measured and assigned. The additivity of the substituent effects and the usefulness of some corrective parameters are studied with regression analysis. The order of the chemical shifts is most efficiently predicted by the simplest substituent effect model, containing only the direct effects of the substituents, although the 95% confidence limits of the calculated shifts are as high as 5.6 ppm. If the chemical shifts need to be predicted within the measuring errors (approximately 0.05–0.10 ppm, in the present data), the number of necessary corrections is very impractical. The corre…
Direct Evidence of the Amino Acid Side Chain and Backbone Contributions to Protein Anharmonicity
2010
Elastic incoherent neutron scattering has been used to study the temperature dependence of the mean-square displacements of nonexchangeable hydrogen atoms in powders of a series of homomeric polypeptides (polyglycine, polyalanine, polyphenylalanine and polyisoleucine) in comparison with myoglobin at the same hydration level (h = 0.2). The aim of the work was to measure the dynamic behavior of different amino acid residues separately and assess the contribution of each type of side chain to the anharmonic dynamics of proteins. The results provide direct experimental evidence that the first anharmonic activation, at approximately 150 K, is largely due to methyl group rotations entering the ti…