Ab initio study of exchange coupling for the consistent understanding of the magnetic ordering at room temperature in V[TCNE] x

We report quantum chemical calculations providing the exchange coupling constants of the V[TCNE]₂ model system, describing the amorphous room temperature molecular magnet V[TCNE] _x (TCNE = tetracyanoethylene, x ~ 2). The geometry is optimized for the ideal lattice using density functional theory (DFT) calculations with periodic boundary conditions. Broken-symmetry DFT calculations indicate antiparallel spin alignment resulting in ferrimagnetic ordering, but heavily overestimate the value of the exchange coupling. Better estimates of the exchange coupling parameters between the V(II) ion and the [TCNE]^? anionic radical are obtained by means of multiconfigurational calculations performed on smaller molecular models cut from the optimized crystal lattice. Complete active space self-consistent field and multireference second-order perturbation theory calculations provide the sign and the strength of the nearest-neighbor as well as next-nearest-neighbor interactions along all three crystallographic directions. We are able to explain also intuitively the mechanism for antiferromagnetic spin coupling in terms of the superexchange pathways, discussing the role of the main four types of contributions to superexchange. Moreover, we clarify the influence of the transition metal ion on the strength of the exchange interaction and on the critical temperature for long-range ferrimagnetic ordering.     

Current trends in lead discovery: are we looking for the appropriate properties?

The new drug discovery paradigm is based on high-throughput technologies, both with respect to synthesis and screening. The progression HTS hits lead series candidate drug marketed drug appears to indicate that the probability of reaching launched status is one in a million. This has shifted the focus from good quality candidate drugs to good quality leads. We examined the current trends in lead discovery by comparing MW (molecular weight), LogP (octanol/water partition coefficient, estimated by Kowwin [17]) and LogSw (intrinsic water solubility, estimated by Wskowwin [18]) for the following categories: 62 leads and 75 drugs [11]; compounds in the development phase (I, II, III and launched), as indexed in MDDR; and compounds indexed in medicinal chemistry journals [ref. 20], categorized according to their biological activity. Comparing the distribution of the above properties, the 62 lead structures show the lowest median with respect to MW (smaller) and LogP (less hydrophobic), and the highest median with respect to LogSw (more soluble). By contrast, over 50% of the medicinal chemistry compounds with activities above 1 nanomolar have MW > 425, LogP > 4.25 and LogSw < -4.75, indicating that the reported active compounds are larger, more hydrophobic and less soluble when compared to time-tested quality leads. In the MDDR set, a progressive constraint to reduce MW and LogP, and to increase LogSw, can be observed when examining trends in the developmental sequence: phase I, II, III and launched drugs. These trends indicate that other properties besides binding affinity, e.g., solubility and hydrophobicity, need to be considered when choosing the appropriate leads.     

A comparison of some integral methods

The integral methods proposed to compute the kinetic parameters of heterogeneous reactions under non-isothermal conditions are usually worked by the help of the least squares method and the obtained correlation coefficient is taken as a criterion to choose the best integral method.An analysis of several experimental data by mean of three different integral methods was performed by us and the results pointed out that this criterion, by itself, is not enough to provide reliable information on the kinetic parameters.It appears, thus, that the use of an integral method or another is a simple matter of researcher's choice.

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Zusammenfassung Integrationsmethoden zur Berechnung kinetischer Parameter von heterogenen Reaktionen unter nicht isothermen Bedingungen werden im allgemeinen nach der Methode der kleinsten Quadrate erarbeitet und der ermittelte Korrelationskoeffizient dient als ein Kriterium für die Auswahl der besten Integrationsmethode.Mittels drei verschiedenen Integrationsmethoden wurde eine Analyse verschiedener experimenteller Daten durchgeführt. Die Ergebnisse zeigen, daß dieses Kriterium allein nicht ausreicht, um ausreichende Informationen über die kinetischen Parameter zu liefern.Es scheint deshalb, daß die Verwendung der einen oder anderen Integrationsmethode einfach eine Wahl des Anwenders darstellt.

     

Mechano-chemische Reaktionen des Polyvinylchlorids 7. Einfluß einiger Faktoren auf die Zugfestigkeit der durch Walzen von PVC-S in Anwesenheit von aromatischen Diaminen erhaltenen Produkte

Zusammenfassung Die Möglichkeit von chemischen Reaktionen des Polyvinylchlorids mit niedermolekularen Beimischungen vom Typ aromatischer Diamine während der Verarbeitung des Polymeren durch Walzen wird besprochen.Die so erhaltenen Produkte weisen eine höhere Streckfestigkeit gegenüber dem ursprünglichen Polymeren auf; diese mechanische Eigenschaft wird als Auswirkung der Reaktion betrachtet. Es wird der Einfluß einiger Parameter des Verarbeitungsvorganges (Diaurin - Menge, Dauer, Temperatur) auf die Streckfestigkeit der so erhaltenen Erzeugnisse verfolgt.     

Optimization of CAMD techniques 3. Virtual screening enrichment studies: a help or hindrance in tool selection?

Over the last few years many articles have been published in an attempt to provide performance benchmarks for virtual screening tools. While this research has imparted useful insights, the myriad variables controlling said studies place significant limits on results interpretability. Here we investigate the effects of these variables, including analysis of calculation setup variation, the effect of target choice, active/decoy set selection (with particular emphasis on the effect of analogue bias) and enrichment data interpretation. In addition the optimization of the publicly available DUD benchmark sets through analogue bias removal is discussed, as is their augmentation through the addition of large diverse data sets collated using WOMBAT.     

Obituary: Toshio Fujita,QSAR pioneer

This is the obituary for Toshio Fujita, pioneer of the quantitative structure activity relationship (QSAR) paradigm.     

Mechanochemically initiated polymerizations—5. Polymerization by vibratory milling of acrylamide and methacrylamide

Mechanochemical polymerization by vibratory milling of acrylamide and methacrylamide is discussed. The influences of duration, nature of medium and temperature are examined. The polymers are characterized by elemental analysis, i.r.-spectroscopy. X-ray and by determination of some properties. An anion-radical mechanism is proposed.     

DFT study of electronic structure and optical properties of some Ru- and Rh-based complexes for dye-sensitized solar cells

The paper reports Time Dependent Density Functional Theory (TD DFT) calculations providing the structure, electronic properties and spectra of [Ru(II)(bpy)_{3? n} (dcbpy) _n ]²⁺ and [Rh(III)(bpy)_{3? n} (dcbpy) _n ]³⁺ complexes, where bpy?=?2,2′-bipyridyl, dcbpy?=?4,4′-dicarboxy-2,2′-bipyridyl, and n?=?0,?1,?2,?3, studied as possible pigments for dye-sensitized solar cells. The role of the metallic ion and of the COOH groups on the optical properties of these complexes are compared and contrasted and their relevance as dyes for hybrid organic–inorganic photovoltaic cells is discussed. It was found that the optical spectra are strongly influenced by the metallic ion, with visible absorption bands for the Ru(II) complexes and only ultraviolet bands for the Rh(III) complexes. Upon excitation, the extra positive charge of the Rh³⁺ centre tends to draw electrons towards the metal ion, facilitating some charge transfer from the ligand to the metal, whereas in the case of the Ru²⁺ ion the electron transfer is clearly from the metal to the ligand. The carboxyl groups play an important role in strengthening the absorption bands in solution in the visible region. Of the complexes studied, the most suited as pigments for dye-sensitized solar cells are the [Ru(II)(bpy)_{3? n} (dcbpy) _n ]²⁺ complexes with n?=?1 and 2. This is based on the following arguments: (i) their intense absorption band in the visible region, (ii) the presence of the anchoring groups allowing the bonding to the TiO₂ substrate and the charge transfer, and (iii) the good energy level alignment with the conduction band edge of the semiconducting substrate and the redox level of the electrolyte.