Gas-chromatographic identification of products formed in iodination of methyl phenols by retention indices

Iodination reaction followed by conversion of iodine-substituted methylphenols to the corresponding trifl uoroacetates was suggested for improving the sensitivity of the gas-chromatographic determination of phenol and its methyl-substituted derivatives (al isomers of mono- and diethylphenols, 2,3,5-, 2,3,6-, and 3,4,5-trimethylphenols) in aqueous media. Acylation products of iodo methylphenols (104 compounds) were identified by linear-logarithmic retention indices on a standard nonpolar polydimethylsiloxane stationary phase, and the pattern of their variation with the number and nature of substituents were characterized. A procedure for identification of methyl-substituted phenols in water in their gas-chromatographic determination with an electron-capture detector was developed.     

A nitroxide diradical containing a ferrocen-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler

An Aza-Wittig reaction of 1,1′-bis(triphenylphosphoranylidenamino)-ferrocene with 3-isocyanato-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-oxyl and subsequent intramolecular [2+2]-cycloaddition of the bis(carbodiimide) intermediate was used for the synthesis of a 1,3-diazetidine-2,4-diimine moiety bearing two radical groups and bridging the cyclopentadienyl (Cp) rings. According to single-crystal X-ray data, the planar 2,4-diimino-1,3-diazetidine moiety adopts a perpendicular orientation with respect to the eclipsed Cp rings. One of the radical groups lies almost within the plane of the four-membered 1,3-diazetidine ring, while the other has a conformationally preferred orientation with dihedral angles ±62.3°. The two Cp rings are constricted by the 1,3-diazetidine-2,4-diimine moiety so that the angle between their planes is 11.0°. The synthesized diradical contains ferrocene and nitroxide redox signaling units that can be oxidized step-by-step at E_1/2 = 0.26 and 0.48 V (vs Fc/Fc⁺), respectively. Electron spin resonance spectroscopy revealed a moderate exchange interaction (|J| ～ a_N) between the two nitroxide radical moieties and the following values of zero-field splitting parameters: ∣D∣ = 3.5 mT and E/D = 0 were obtained. These data were in agreement with density functional theory calculations. The newly developed approach to multispin systems may be interesting for the construction of weakly coupled rigid polyradicals for quantum technologies, the molecular design of magnets, and the creation of ferrocene-based electron-paramagnetic-resonance–active chemical sensors.     

Protein and ligand preparation: parameters, protocols, and influence on virtual screening enrichments

Structure-based virtual screening plays an important role in drug discovery and complements other screening approaches. In general, protein crystal structures are prepared prior to docking in order to add hydrogen atoms, optimize hydrogen bonds, remove atomic clashes, and perform other operations that are not part of the x-ray crystal structure refinement process. In addition, ligands must be prepared to create 3-dimensional geometries, assign proper bond orders, and generate accessible tautomer and ionization states prior to virtual screening. While the prerequisite for proper system preparation is generally accepted in the field, an extensive study of the preparation steps and their effect on virtual screening enrichments has not been performed. In this work, we systematically explore each of the steps involved in preparing a system for virtual screening. We first explore a large number of parameters using the Glide validation set of 36 crystal structures and 1,000 decoys. We then apply a subset of protocols to the DUD database. We show that database enrichment is improved with proper preparation and that neglecting certain steps of the preparation process produces a systematic degradation in enrichments, which can be large for some targets. We provide examples illustrating the structural changes introduced by the preparation that impact database enrichment. While the work presented here was performed with the Protein Preparation Wizard and Glide, the insights and guidance are expected to be generalizable to structure-based virtual screening with other docking methods.     

Magnetic Resonance and Mössbauer Studies of Superparamagnetic γ‐Fe2O3 Nanoparticles Encapsulated into Liquid‐Crystalline Poly(propylene imine) Dendrimers

We present the first results of electron magnetic resonance (EMR) and Mössbauer spectroscopy studies of γ‐Fe₂O₃ nanoparticles (NPs) incorporated into liquid‐crystalline, second‐generation dendrimers. The mean size of NPs formed in the dendrimers was around 2.5 nm. A temperature‐driven transition from superparamagnetic to ferrimagnetic resonance was observed for the sample. Low‐temperature blocking of the NP magnetic moments has been clearly evidenced in the integrated EMR line intensity and the blocking temperature was about 60 K. The physical parameters of magnetic NPs (magnetic moment, effective magnetic anisotropy) have been determined from analyses of the EMR data. The effective magnetic anisotropy constant is enhanced relative to bulk γ‐Fe₂O₃ and this enhanced value is associated with the influence of the surface and shape effects. The angular dependence of the EMR signal position for the field‐freezing sample from liquid‐crystalline phase showed that NPs possessed uniaxial anisotropy, in contrast to bulk γ‐Fe₂O₃. Mössbauer spectroscopy determined that fabricated NPs consisted of an α‐Fe core and a γ‐Fe₂O₃ shell.     

Quantification of chloroanilines in drinking water by gas chromatography as bromo derivatives

A procedure is developed for determination of trace amounts of simple aniline, 2- and 4-chloro-anilines, 2,4- and 2,6-dichloroanilines, 2,4,5- and 2,4,6-trichloroanilines in drinking water; it includes the formation of bromo derivatives, solvent extraction with toluene, and gas chromatography monitoring with electron-capture detection. The conditions of bromination for chloroanilines in aqueous solutions are refined; the bromination agent in use is bromine water in the presence of glycine. The analytical range makes 0.01–10 μg/L; relative standard deviation, 0.01–0.08; detection limits, 0.002–0.007 μg/L; and the duration of analysis, 50 min.     

Specific features of the absorption and phase velocity of ultrasound near the low-temperature phase transition induced by 3<Emphasis Type="Italic">d</Emphasis> impurities in a ZnSe crystal

This paper reports on the results of temperature investigations into the absorption and velocity of ultrasound in ZnSe: Ni and ZnSe: Cr crystals in the frequency range 33–268 MHz. The frequency dependence of the absorption at the maximum is analyzed, and the energy of the excited state of the Ni²⁺ ions is calculated. The dynamic contribution to the effective elastic modulus is determined, and the results obtained are used to construct the temperature dependences of the relaxed and unrelaxed elastic moduli.     

C(sp2)‐Coupled Nitronyl Nitroxide and Iminonitroxide Diradicals

Spin‐labelled compounds are widely used in chemistry, physics, biology and the materials sciences but the synthesis of stable high‐spin organic molecules is still a challenge. We succeeded in synthesising heteroatom analogues of the 1,1,2,3,3‐pentamethylenepropane (PMP) diradicals with two nitronyl nitroxide ( DR¹ ) and with two iminonitroxide ( DR² ) fragments linked through the C(sp²) atom of the nitrone group. According to magnetic susceptibility measurements, EPR data and ab initio calculations at the (8,6)CASSCF and (8,6)NEVPT2 levels, DR¹ and DR² have singlet ground states. The singlet–triplet energy splitting (2J) is low (J/k=?7.4 for DR¹ and ?6.0 K for DR² ), which comes from the disjoint nature of these diradicals. The reaction of [Cu(hfac)₂] with DR¹ gives rise to different heterospin complexes in which the diradical acts as a rigid ligand, retaining its initial conformation. For the [{Cu(hfac)₂}₂( DR¹ )(H₂O)] complex, sufficiently strong ferromagnetic interactions (J₁/k=42.7 and J₂/k=14.1 K) between two coordinating Cu^II ions and DR¹ were revealed. In [{Cu(hfac)₂}₂( DR¹ )(H₂O)][Cu(hfac)₂(H₂O)], the very strong and antiferromagnetic (J/k=?416.1 K) exchange interaction between one of the coordinating Cu^II ions and DR¹ is caused by the very short equatorial Cu?O bond length (1.962 Å).     

Extractive concentration of halogen-substituted phenols in their gas-chromatographic determination in aqueous media

Method for recovery of chlorophenols from natural and drinking water for their subsequent gas-chromatographic determination was considered. Processes of microliquid extraction of bromo derivatives of chlorophenols and the method for liquid extraction with intermediate concentration of these compounds in an alkaline solution were studied.     

A modification of the Fourier-Motzkin algorithm for constructing a triangulation and star development

The notion of a star development of a simplicial complex is introduced, and an algorithm is offered that simultaneously constructs a triangulation and a star development of the simplicial complex of the faces of the triangulation for a point configuration in general position.