Ab initio rotational constants of interstellar species: Cyanoacetylene hydrogenated derivatives

By means of ab initio calculations, the rotational constants and dipole moments of H_nC₃N (n = 1, 3, 5, 7, and 9) species have been calculated at the HF / 6-31G * level of theory. Selected cases have been also calculated at the MP 2/6-31G * level and the influence of calculation level on rotational constant values is briefly discussed. Some of these species were discovered in the interstellar medium, while others have still not been detected there, although their existence is very probable. The results given here could help in their detection. © 1993 John Wiley & Sons, Inc.     

On projective tensor products with complemented subspaces isomorphic to ℓ1(*)

We give new sufficient conditions in order that a projective tensor product of Banach spaces contains complemented copies of ?¹, which provides us new examples of this situation.     

Recognition of guests bearing donor and acceptor hydrogen bonding groups by heteroditopic calix[4]arene receptors

The synthesis of new hosts specifically designed for the recognition of neutral guests bearing donor-acceptor hydrogen bonding groups is described. These hosts are characterized by the presence of two distinct binding region in close proximity: the rigid π-donor cavity and the H-bond donor N-methylene-N′-phenylureido group inserted onto the upper rim of the calix[4]arene skeleton. The binding abilities of these receptors were investigated toward a series of neutral ditopic organic molecules in CDCl₃ solution by ¹H NMR spectroscopy. The results obtained show that rigidity of the calix[4]arene apolar cavity is the control element in determining efficiency. In fact, compared with the more rigid 2, host 10, where the rigidity of the cone structure is maintained by hydrogen bonding of the OH of the lower rim, a decrease of efficiency of almost one order of magnitude was observed. The cooperative effect of the two binding region of host 2 was verified with different classes of ditopic guests. Good efficiency in the recognition of urea derivatives and dimethylsulfoxide was achieved.     

New Hg2+ and Cu2+ selective chromo- and fluoroionophore based on a bichromophoric azine

[graph: see text] A new probe, 1,4-bis(1-pyrenyl)-2,3-diaza-1,3-butadiene, selectively senses Hg2+ and Cu2+ through two different channels: the yellow-deep-pink color change and the enhancement of the fluorescence with the red shift of the excimer emission, which can visually be discernible by a green fluorescence in the presence of Hg2+ and an orange fluorescence in the presence of Cu2+.     

Electroreductive polymerization of phenylmethyldichlorosilane in a divided cell using a neutral membrane

Electrochemical formation of poly(phenylmethylsilane) in a divided cell containing a Teflon® neutral membrane was studied. The electrolysis of dichlorosilanes was carried out in a solution containing tetrahydrofuran + hexamethylphosphoramide as the solvent, tetrabutylammonium perchlorate as the support electrolyte and stainless steel as the cathode, with Pt and graphite as the resistant anodes or stainless steel as the sacrificial anode. Polysilanes with a number-average molecular weight in the range from 2,600 to 130,000 g/mol were obtained, depending on the conditions used.     

Synthesis of bridgehead nitrogen heterocycles from pyrylium salts and β-functionalized amines

β-Functionalized primary amines are converted by the 2-ethoxycarbonyl-4,6-diphenylpyrylium cation into the corresponding bicyclic salts 2 . The N-substituted 4,6-diphenylpyridine-2-thiones 6 undergo cyclization to the byciclic compounds 8 by action of methyl iodide.     

Dynamic contact angle and spreading rate measurements for the characterization of the effect of dentin surface treatments

A new methodology capable of providing reliable and reproducible contact angle (theta) data has been employed to study the effect of clinical treatments grinding, acid etching, and deproteinization on medial dentin tissue. It is based on the application of the ADSA-CD algorithm to the determination of low-rate dynamic contact angles, obtained from slowly growing drops, and on contact angle measurement, as well as spreading behavior analysis, during the relaxation of the system (water on treated dentin) after initial drop growth. The theta data obtained were substantially more reproducible than those obtained with classical methods. A net effect of the treatment on theta was found, increasing dentin wettability: theta (polished) >theta (etched) >theta (deproteinized). The spreading rates correlate with the angles and are adequate for the dentin surface characterization. ANOVA and SNK tests show that for advancing contact angles the means corresponding to all treatments are significantly different. In the relaxing phase, mean angle and spreading rates on polished dentin differ significantly from those on etched and deproteinized dentin, but the latter do not differ significantly from each other.     

Anion effects on the recognition of ion pairs by calix[4]arene-based heteroditopic receptors

A novel heteroditopic receptor (5) based on a rigid calix[4]arene cavity bearing at the upper rim four arylsulfonamido binding sites has been synthesized. The binding abilities of this new host have been investigated in apolar solvents toward a series of tetramethylammonium salts (tosylate, chloride, acetate, trifluoroacetate, and picrate) and compared with those of monotopic and heteroditopic calix[4]arene-bis(crown-3)-based receptors 1 and 2 in order to evaluate the role of the anion on ion-pair recognition. While monotopic host 1 shows an efficiency order toward the different salts that increases when the anion is less interactive (Hofmeister trend), an opposite role of the counteranion on the recognition process was observed with host 5 (anti-Hofmeister trend). A more complex behavior is experienced by host 2, which shows a high and leveled efficiency for all the anions tested. The results obtained were explained on the basis of the different types of ion pairs present in the recognition process. Further information on the role of the anion were obtained by the "dual host" strategy utilizing the tri-n-butylthioureido derivative of tren 7, which forms a stable complex with chloride anion. The very high efficiency shown by these heteroditopic hosts opens new routes in supramolecular projects and is a very interesting tool in the molecular recognition of ion-pairs and its applications.     

Stability study of simvastatin under hydrolytic conditions assessed by liquid chromatography

In this work, a liquid chromatography stability-indicating method was developed and applied to study the hydrolytic behavior of simvastatin in different pH values and temperatures. The selected chromatographic conditions were a C18 column; acetonitrile-28 mM phosphate buffer solution, pH 4 (65 + 35) as the mobile phase; 251 degrees C column temperature; and flow rate 1 mL/min. The developed method exhibited an adequate repeatability and reproducibility (coefficient of variation 0.54 and 0.74%, respectively) and a recovery higher than 98%. Furthermore, the detection and quantification limits were 9.1 x 10(-7) and 2.8 x 10(-6) M, respectively. The degradation of simvastatin fitted to pseudo-first order kinetics. The degradation was pH dependent, being much higher at alkaline pH than at acid pH. Activation energy, kinetic rate constants (k) at different temperatures, the half life (t1/2) and the time for 10% degradation to occur (t90) values are also reported.