Mechanosynthesis of YIG and GdIG: A Structural and Mössbauer Study

We have investigated the mechanosynthesis of gadolinium and yttrium iron garnets by high-energy ball-milling of α-Fe₂O₃ and Gd₂O₃ or α-Fe₂O₃ and Y₂O₃, respectively, followed by short thermal annealings conducted at moderate temperatures. The samples were characterized by X-ray diffraction and Mössbauer spectroscopy, in order to determine the influence of the milling time and annealing conditions on the final products. For as-milled samples of each rare-earth system, the results revealed the formation of perovskite phases, in relative amounts that depend on the milling time. The formation of garnet phases was observed in as-annealed samples treated at 1000°C for 2 h or 1100°C for 3 h, i.e., at very modest annealing requirements when compared with ordinary solid-state-reaction processes performed without previous high-energy milling. Also, the occurrence was verified of a milling time for which the relative amount of garnet phases formed by annealing was maximized. This time depends on the rare-earth composing the garnet phase and on the annealing temperature.     

Ab initio calculations of vibronic activity in phosphorescence microwave double resonance spectra of p-dichlorobenzene

The phosphorescence spectrum of p-dichlorobenzene has been calculated using multiconfiguration self-consistent-field wave functions and the quadratic response technique. Attention has been paid to the intensity distribution of the singlet–triplet (³B_1u¹A_g) transition through a number of vibronic subbands. The second order spin–orbit coupling (SOC) contribution to the spin splitting of the ³B_1u (^3*) state is found to be almost negligible, and the calculations therefore provide a good estimate for the zero-field splitting (ZFS) parameters based only on the electron spin–spin coupling expectation values. Nuclear quadrupole resonance constants for the different Cl isotopes are also calculated to accomplish the ZFS assignment. The electric dipole activity of the spin sublevels in the triplet–singlet transitions to the ground-state vibrational levels is estimated by calculations of derivatives using distorted geometries which are shifted from the equilibrium position along different vibrational modes. A vibrational analysis of the phosphorescence spectrum, based on the SOC-induced mixing of the singlet and triplet states calculated along different vibrational modes, provides reasonable agreement with experimental data.Acknowledgment O. R.-P. would like to thank the European MOLPROP network for support. The authors thank Alexander Baev for fruitful discussions. This work was supported by the Swedish Royal Academy of Science (KVA).     

Evaluation of different quantitative approaches for the determination of noneasily ionizable molecules by different atmospheric pressure interfaces used in liquid chromatography tandem mass spectrometry: Abamectin as case of study

The liquid chromatography tandem mass spectroscopy residue determination of compounds without any acidic or basic centers such as abamectin has been investigated. Several approaches regarding the interface used and adduct formation have been compared. The low acidity of the hydroxyl groups only made deprotonation feasible using the atmospheric pressure chemical ionization (APCI) interface. To obtain sufficient sensitivity for residue analysis, the Ion Sabre APCI interface was necessary. However, the sensitivity attained was lower than for monitoring adducts in positive ion mode. Using electrospray ionization, different adducts with Na+, NH4+, and Li+ were tested and compared. The best results were obtained for the ammoniated adduct in electrospray ionization (ESI) because of its high sensitivity and the presence of several product ions with similar abundance. The highest sensitivity was reached using an in-source fragment as precursor ion, leading to a limit of detection (LOD) of 2 microg/L with low relative standard deviation. The relatively high abundance of other transitions allowed abamectin confirmation at concentrations close to the LOD (6 microg/L). Alkali ions were found to be a suitable alternative to determine and confirm abamectin at residue levels. The [M + Na]+ also presented various product ions with similar abundance, which allowed confirmation at LOD levels. However, this LOD was found to be almost four times higher than with [M + NH4]+ because of the poor sensitivity of the transitions obtained. Although the use of Li+ facilitated the fragmentation of the adduct [M + Li]+, with similar sensitivity to [M + NH4]+, this fragmentation preferentially generated only one product ion, which did not allow confirmation at concentration levels lower than 15 microg/L. The use of APCI for monitoring adducts was also feasible, but with less sensitivity. The sensitivity increased with the Ion Sabre APCI, although it was still five times lower than with ESI. Other adduct formers such as Co2+ and Ni2+ also were tested with unsatisfactory results.     

New synthetic approaches to sugar ureas. Access to ureido-β-cyclodextrins

An efficient method for the preparation of urea-bridged cyclodextrins using triphosgene in the isocyanation step in an aqueous two-phase system is reported. Per-O-acetylated glycopyranosylamines and 2-amino-2-deoxy-α and β-d-glucose were also transformed into the corresponding isocyanates using either an aqueous two-phase or an anhydrous dichloromethane medium, and converted in situ into ureas. An alternative method for the preparation of sugar-derived ureas consisting of desulfurization of sugar thioureas with mercury oxide is also presented.     

A facile access to ureido sugars. Synthesis of urea-bridged β-cyclodextrins

The preparation of a urea-bridged β-cyclodextrin dimer and of a 6-monodeoxy-6-mono[3-(β-d-glucopyranos-2-yl)ureido]-β-cyclodextrin has been developed, using triphosgene as the isocyanation agent in an aqueous two-phase system. Per-O-acetylated β-d-gluco and mannopyranosylamines and 2-amino-2-deoxy-α- and β-d-glucose were also transformed into the corresponding isocyanates and converted in situ into ureas by coupling with aromatic and aliphatic amines.     

Unexpected reactivity of pyridinium salts toward alkynyl Fischer complexes to produce oxo‐heterocycles

The unprecedented reaction of ketone‐containing aromatic pyridinium salts 3a ‐ e and alkynyl Fischer complexes 1a ‐ f proceeds via a mild domino process to provide 4,6‐disubstituted pyran‐2‐ones 5a ‐ k and 2,3,5‐trisubstituted furans 6a ‐ h (45‐97%). According of the results of isotopic labeling experiments, a mechanism involving an initial Michael addition appears to be the key step, obtaining a mesomeric structure responsible for the formation of both products.     

Automatic generation of linear programming problems for computer aided instruction

The paper presents a method for generating random linear programming problems with a previously selected type of solution. The user can choose a problem whose solution is unbounded, bounded for minima, maxima or both, unique or multiple, with given structure, at wish. Initially, the feasible solution of the LPP is generated as the sum of a linear space, a cone, and a polytope, depending on the desired properties of the solution. With the aim of obtaining a simple set of constraints, the generators of these three structures are selected as random vectors with integer simple components, the range of which can be given. Next, an objective function that satisfies the required conditions, i.e. leads to a solution of the desired type, is obtained. The generating algorithms have been implemented in Mathematica and some illustrative examples are given to clarify the generation process. With this tool, a LPP can be generated, according to the instructor requirements, where this is a human or an expert system. They can control student progress and generate a sequence of problems covering all possible cases, in steps of increasing difficulty. Combining this tool with another (also produced by the same authors) that solves the problems and explains the whole process, step by step, a computer aided module for learning LPP, which is completely autonomous, can be easily obtained.     

Homomorphisms of matrix algebras and constructions of Butson–Hadamard matrices

An $n\times n$ matrix $H$ is Butson–Hadamard if its entries are $k$th roots of unity and it satisfies $H{H}^1=n{I}_n$. Write $\mathrm{BH}(n,k)$ for the set of such matrices.Suppose that $k=p^{\alpha }{q}^{\beta }$ where $p$ and $q$ are primes and $\alpha \ge 1$. A recent result of Östergård and Paavola uses a matrix $H\in \mathrm{BH}(n,pk)$ to construct $H^{\prime }\in \mathrm{BH}(pn,k)$. We simplify the proof of this result and remove the restriction on the number of prime divisors of $k$. More precisely, we prove that if $k=mt$, and each prime divisor of $k$ divides $t$, then we can construct a matrix $H^{\prime }\in \mathrm{BH}(mn,t)$ from any $H\in \mathrm{BH}(n,k)$.