About ageing and fading of Cr-39 PADC track detectors used as air radon concentration measurement devices

PADC detectors are widely used as air radon concentration measurement devices and the typical procedure that a Radon Service Laboratory uses to manage the detectors provides a calibration phase followed by the field measurement. The calibration is performed in a reference radon concentration atmosphere, using high radon concentration values in order to achieve typical exposure values of few MBq h m^?3 with an exposure time of few days. On the other hand the field measurement is characterized by long term exposures lasting up to six months and by radon concentrations that are quite lower than the ones used in the calibration.The aim of this study is to check whether the calibration procedure is actually representative of a field measurement, or, in other words, whether and how much ageing or fading can affect the calibration factor.We found that the ageing and fading effect can produce a decrease in the detector sensitivity leading to an underestimation of the radon concentration up to 40% for exposure lasting few months. An important issue is that both ageing and fading can be ascribed to a decrease in the track etching velocity V_t.In the paper we also provide an algorithm to compensate for the sensitivity variation due to fading/ageing effect.     

Novel polymer-supported β-diketonate nickel catalysts for α-olefin activation

Styrene–divinylbenzene resins were used for the synthesis of different polymer-bound β-diketones, obtained by anchoring the chelating group through either the central or the lateral position. The heterogenized diketone ligand was subsequently reacted with Ni(COD)₂ analogously to the corresponding homogeneous catalysts active in α-olefin oligomerization. The heterogenized catalysts showed a good activity only when the central position of the chelate moiety was free. Heterogenization caused a significant change of selectivity: olefin oligomerization was accompanied by the formation of a large amount of polymeric products. This behavior is discussed in terms of steric effects caused by the bulky polymeric ligand. © 1998 John Wiley & Sons, Ltd.     

N‐Arylated 3,5‐Dihydro‐4H‐dinaphtho[2,1‐c:1′,2′‐e]azepines: Axially Chiral Donors with High Helical Twisting Powers for Nonplanar Push–Pull Chromophores

Axially chiral, N‐arylated 3,5‐dihydro‐4H‐dinaphtho[2,1‐c:1′,2′‐e]azepines have been prepared by short synthetic protocols from enantiopure 1,1′‐bi(2,2′‐naphthol) (BINOL) and anilines. Alkynes substituted with two N‐phenyldinaphthazepine donors readily undergo a formal [2+2] cycloaddition, followed by retro‐electrocyclization, with tetracyanoethene (TCNE) to yield donor‐substituted 1,1,4,4‐tetracyanobuta‐1,3‐dienes (TCBDs) featuring intense intramolecular charge‐transfer (CT) interactions. A dicyanovinyl derivative substituted with one N‐phenyldinaphthazepine donor was obtained by a “one‐pot” oxidation/Knoevenagel condensation from the corresponding propargylic alcohol. Comparative electrochemical, X‐ray crystallographic, and UV/Vis studies show that the electron‐donor qualities of N‐phenyldinaphthazepine are similar to those of N,N‐dimethylanilino residues. The circular dichroism (CD) spectrum of a push–pull chromophore incorporating the chiral donor moiety features Cotton effects of exceptional intensity. With their elongated shape and the rigidity of the chiral N‐aryldinaphthazepine donors, these chromophores are effective inducers of twist distortion in nematic liquid crystals (LCs). Thus, a series of the dinaphthazepine derivatives was used as dopants in the nematic LC E7 (Merck) and high helical twisting powers (β) of the order of hundreds of μm^?1 were measured. Theoretical calculations were employed to elucidate the relation between the structure of the dopants and their helical twisting power. For the derivatives with two dinaphthazepine moieties, a strong dependence of the β‐values on the structure and conformation of the linker between them was found.     

Homochiral helices of oligonaphthalenes inducing opposite-handed cholesteric phases

The helical structure of the chiral nematic phases (cholesterics) obtained by doping nematic solvents with chiral non-racemic compounds is a macroscopic proof of the solute chirality. Oligonaphthalene (tetra-, hexa-, octa-) derivatives linked at the 1,4-positions have been used as chiral dopants: When the chirality axes are configurationally homogeneous (that is, all-S), the molecular structures correspond to right-handed helices. Yet, we have found series of derivatives with the surprising property that the handedness of the induced cholesteric phase alternates from positive to negative and to positive again, on passing from tetra- to hexa- and to octanaphthalene. A comparison with oligonapthalene derivatives, which do not exhibit this twisting ability, points to the importance of the substitution pattern. Both the possibility of inducing oppositely-handed cholesteric phases by homochiral helices of different length, and the role played of substituents, are confirmed by calculations performed with the surface chirality model.     

A study of the stereochemistry of 2-aryl-4,5-dimethyl-1,3-dioxolanes by cholesteric induction in nematic phases and circular dichroism spectroscopy

[Chemical reaction: See text] The circular dichroism spectra and the twisting ability of a series of 2-aryl-4,5-dimethyl-1,3-dioxolanes used as dopants in nematic solvents have been related to their absolute configuration. Whereas the circular dichroism (CD) spectra are deeply affected by the substituents present in the aromatic ring, which in several cases cause sign inversion, the helical twisting power beta is only marginally influenced. The values of beta also seem not very sensitive to the rotamer population around the aromatic ring; this indicates the predominant importance of the chiral dioxolane ring in determining the cholesteric induction. These facts can be explained by the different nature of the two observables: in CD, the chirality is read by the absorbing chromophore and is deeply influenced even by small changes of this group. In cholesteric induction we are dealing instead with chiral solute-solvent interactions that determine a twist in the solvent. In light of the present and previous results, this process seems predominantly determined by short-range interactions, which are modulated by the molecular shape. From a practical point of view, a configurational correlation using CD for the present series of compounds seems problematic, while the values of beta are nicely correlated to the absolute configurations. Calculations with the surface chirality method predict well the sign and order of magnitude of beta and their limited sensitivity to the phenyl substituents and rotamer population.     

Molecular theory of dielectric relaxation in nematic dimers

This paper reports a theory for the dielectric relaxation of dimeric mesogenic molecules in a nematic liquid crystal phase. Liquid crystal dimers consist of two mesogenic groups linked by a flexible chain. Recent experimental studies [D. A. Dunmur, G. R. Luckhurst, M. R. de la Fuente, S. Diez, and M. A. Perez Jubindo, J. Chem. Phys. 115, 8681 (2001)] of the dielectric properties of polar liquid crystal dimers have found unexpected results for both the static (low frequency) and variable frequency dielectric response of these materials. The theory developed in this paper provides a quantitative model with which to understand the observed experimental results. The mean-square dipole moments of alpha,omega-bis[(4-cyanobiphenyl-4'-yl]alkanes in a nematic phase have been calculated using both the rotational isomeric state model and a full torsional potential for the carbon-carbon bonds of the flexible chain. The orienting effect of the nematic phase is taken into account by a parametrized potential of mean torque acting on the mesogenic groups and the segments in the flexible chain. Results of calculations using the full torsional potential are in excellent agreement with experimental results for comparable systems. The probability density p(eq)(beta(A),beta(B)) for the orientation of the mesogenic groups (A,B) along the nematic director is also calculated. The resultant potential of mean torque is a surface characterized by four deep energy wells or sites equivalent to alignment of the terminal groups A and B approximately parallel and antiparallel to the director; of course, the reversal of the director leads to equivalent sites. This potential energy surface provides the basis for a kinetic model of dielectric relaxation in nematic dimers. Solution of the Fokker-Planck equation corresponding to this four-site model gives the time dependence of the site populations, and hence the time-correlation functions for the total dipole moment along the director. In this model the end-over-end rotation of the molecule, corresponding to simultaneous reversal of both mesogenic groups, is excluded because the activation energy is too large. Results are presented for a number of cases, in which a dipole is located on one or both of the mesogenic groups, and additionally where the groups differ in size. For the latter, under particular conditions, the correlation function exhibits a biexponential decay, which corresponds to two low frequency absorptions in the dielectric spectrum. This is exactly what has been observed for nonsymmetric nematic dimers having different groups terminating a flexible chain. Experimental results over a range of temperature for the nonsymmetric dimer alpha-[(4-cyanobiphenyl)-4'-yloxy]-omega-(4-decylanilinebenzylidene-4'-oxy)nonane can be fitted precisely to the theory, which provides new insight into the orientational and conformational dynamics of molecules in ordered liquid crystalline phases.     

Synthesis of 1,8-naphthyridine derivatives. Potential antihypertensive agents. 1

Reaction of substituted 7-chloro-1,8-naphthyridines with N-carbethoxypiperazine gave in good yields the corresponding 7-(4-carbethoxypiperazin-1-yl)-1,8-naphthyridines V as potential antihypertensive agents.     

From the double-stranded helix to the chiral nematic phase of B-DNA: a molecular model

B-DNA solutions of suitable concentration form left-handed chiral nematic phases (cholesterics). Such phases have also been observed in solutions of other stiff or semiflexible chiral polymers; magnitude and handedness of the cholesteric pitch are uniquely related to the molecular features. In this work we present a theoretical method and a numerical procedure which, starting from the structure of polyelectrolytes, lead to the prediction of the cholesteric pitch. Molecular expressions for the free energy of the system are obtained on the basis of steric and electrostatic interactions between polymers; the former are described in terms of excluded volume, while a mean field approximation is used for the latter. Calculations have been performed for 130 base pair fragments of B-DNA. The theoretical predictions provide an explanation for the experimental behavior, by showing the counteracting role played by shape and charge chirality of the molecule.     

One step synthesis of pyrimido[1,2-a][1,8]naphthyridinones,pyrido[1,2-a]pyrimidinones and 1,8-naphthyridinones. Antihypertensive agents. V

The condensation of 2,6-diaminopyridine and 2-acetamido-6-aminopyridine with β-keto esters in polyphosphophoric acid was studied. In this reaction some 1,8-naphthyridinones, pyrido[1,2-a]pyrimidinones and pyrimido[1,2-a][1,8]naphthyridinones variously substituted were obtained.