Ab-initio calculations on the 1-imidazolyl radical

Ab-initio calculations on the 1-imidazolyl radical with a minimal basis set were performed. The ground state symmetry is calculated to be ²B₁(π); the first excited state is ²A₂(π) and lies at 0.359 eV. The σ-state shows a double minimum potential having its well 0.626 eV above the C_2v ground state minimum. The height of the inner hill, peaking at C_2v symmetry, is 0.612 eV; the state is of B₂ character. The reason for the distortion is discussed.     

Optosensing properties of fac-Re(CO)(3)(dpknph)Cl (dpknph=di-2-pyridyl ketone p-nitrophenyl hydrazone)

Optical and thermodynamic measurements on fac-Re(CO)(3)(dpknph)Cl in polar non aqueous solvents revealed the existence of two interlocked conformational forms for fac-Re(CO)(3)(dpknph)Cl. The equilibrium distribution of the low (alpha-) and high (beta-) energy conformations is solvent dependent, controlled by the dipole moment of the solvent molecules and their orientation around the total dipole of fac-Re(CO)(3)(dpknph)Cl. The interplay between the alpha- and beta-conformations of fac-Re(CO)(3)(dpknph)Cl, allowed calculations of their extinction coefficients, by forcing the equilibrium to shift to one conformation, using chemical stimuli. In DMSO and DMF extinction coefficients of 87 000+/-2000 and 35 000+/-2000 M(-1) cm(-1) were calculated for the beta- and alpha-conformations of fac-Re(CO)(3)(dpknph)Cl at lambda(max.), respectively. Thermo-optical measurements on fac-Re(CO)(3)(dpknph)Cl, allowed calculations of the activation parameters for the interconversion between the alpha- and beta-conformations of fac-Re(CO)(3)(dpknph)Cl. In DMSO and DMF changes in enthalpy (DeltaH(?)) of -11.2+/-1.3 and 10.9+/-0.5 kJmol(-1), entropy (DeltaS(?)) of -12.7+/-4.3 and 29.4+/-1.7 JK(-1) mol(-1), and free energy (DeltaG(?)) of -7.5+/-0.2 and+2.2+/-0.2 kJmol(-1) and hence equilibrium constants of 20.9+/-1.7 and 0.4+/-0.1 were calculated for fac-Re(CO)(3)(dpknph) at 295 K. The high values for the extinction coefficients and low values for the activation parameters for the interconversion between the alpha- and beta-conformations of fac-Re(CO)(3)(dpknph)Cl, in polar non aqueous solvents allowed the use of these systems as molecular sensors to probe their structural relaxation and interactions with their surroundings. These systems (fac-Re(CO)(3)(dpknph)Cl and surrounding solvent molecules) optically sense chemical and physical stimuli and their sensing power depends on the intensity and nature of these stimuli, i.e. the systems exhibit a high degree of sensitivity and selectivity.     

Solution of the excluded volume problem for biaxial particles

Abstract

We report the solution of the excluded volume problem for a pair of biaxial hard molecules; namely, sphero-platelets. As an application of this result we study the isotropic to nematic liquid crystal transition for a fluid composed of these particles in the Onsager limit (length δ breadth or width). We show that the range of stability of the isotropic phase decreases with increasing particle biaxiality.     

Reversing Dane’s strategy: a new, concise, enantioselective synthesis of the steroid nucleus

Reversing the polarity of the cycloaddition partners in Dane’s steroid synthesis provides ready access to enantiomerically pure, functionally-rich steroidal structures.     

Quantitative analysis of penicillins in porcine tissues, milk and animal feed using derivatisation with piperidine and stable isotope dilution liquid chromatography tandem mass spectrometry

Penicillins are used universally in both human and veterinary medicine. The European Union (EU) has established maximum residue levels (MRLs) for most ß-lactam antibiotics in milk and animal tissues and included them in the National Residue Monitoring Programs. In this study, a novel method is described for the determination and confirmation of eight penicillins in porcine tissues, milk and animal feed by liquid chromatography–tandem mass spectrometry (LC–MS/MS). To prevent degradation of penicillin residues during workup, a derivatisation procedure was developed, by which penicillins were converted to stable piperidine derivatives. Deuterated piperidine derivatives were synthesised for all relevant penicillins, enabling the use of isotope dilution for accurate quantification. Penicillin residues were derivatised in the crude extract with piperidine and isolated using solid-phase extraction. The penicillin piperidine derivatives were determined by LC–MS/MS. The method was validated at the current MRLs, which range from 25–300 µg kg^?1 in muscle and kidney to 4–30 µg kg^?1 in milk as well as at the target value of 100 µg kg^?1 chosen for animal feed, according to the EU requirements for a quantitative confirmatory method. Accuracy ranged from 94–113% (muscle), 83–111% (kidney) and 87–103% (milk) to 88–116% (animal feed). Intra-day precision (relative standard deviation (RSD)_r) ranged from 5–13% (muscle, n?=?18), 4–17% (kidney, n?=?7) and 5–18% (milk, n?=?7) to 11–32% (animal feed, n?=?18). Inter-day precision (RSD_RL, n?=?18) ranged from 6–23% (muscle) to 11–36% (animal feed). From the results, it was concluded that the method was fit for purpose at the target MRLs in animal tissue and target levels for animal feed.     

Spherical and Clockwise Spherical Graphs

The main subject of our study are spherical (weakly spherical) graphs, i.e. connected graphs fulfilling the condition that in each interval to each vertex there is exactly one (at least one, respectively) antipodal vertex. Our analysis concerns properties of these graphs especially in connection with convexity and also with hypercube graphs. We deal e.g. with the problem under what conditions all intervals of a spherical graph induce hypercubes and find a new characterization of hypercubes: G is a hypercube if and only if G is spherical and bipartite.     

Characterization of linear polyurethanes

Methods are described for the qualitative analysis of linear polyurethanes of unknown composition. Based on the type of polymer, determined from the I.R.-spectrum of a polymer film, the products obtained on alkaline hydrolysis under pressure in a Parr bomb are separated by means of ion-exchangers, the course of the analysis being dependent on the type of polymer to be examined. Identification of the isolated hydrolysis products is accomplished by I.R. analysis or thin-layer chromatography.