Absolute configuration and conformational stability of (+)-2,5-dimethylthiolane and (-)-2,5-dimethylsulfolane

Enantiopure (+)-2,5-dimethylthiolane and (-)-2,5-dimethylsulfolane were prepared using literature procedures and investigated using vibrational circular dichroism (VCD). Experimental absorption and VCD spectra of (+)-2,5-dimethylthiolane and (-)-2,5-dimethylsulfolane in CCl(4) solution in the 2000-900 cm(-)(1) region were compared with the ab initio predictions of absorption and VCD spectra obtained with density functional theory using the B3LYP/6-311G(2d, 2p) basis set for different conformers of (2R,5R)-2,5-dimethylthiolane and (2R,5R)-2,5-dimethylsulfolane. This comparison indicates that (+)-2,5-dimethylthiolane is of the (2R,5R)-configuration and has two predominant conformations in CCl(4) solution. In addition, (-)-2,5-dimethylsulfolane is of (2R,5R)-configuration and has only one predominant conformation. The stereochemical assignment is in agreement with literature.     

The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsg _xx andg _xxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of theh _xx andh _xxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects. Preliminary data were presented at the International Conferences on Chemical Thermodynamics at Baden (1973) and Montpellier (1975). The experimental part was carried out at the Istituto Chimico of the University of Trieste. To whom correspondence should be addressed.     

Regioselective synthesis of beta-N1- and beta-N3-alloxazine nucleosides

[structures: see text] A regio- and stereoselective glycosylation of ribose tetraester with persilylated alloxazine to give either beta-N1 or beta-N3 nucleosides is described. The N3 product is potentially of interest as a fluorescent nucleoside and is predicted to have the hydrogen-bonding characteristics of thymidine.     

210Pb fluxes in sediment layers sampled from Northern Patagonia lakes

Unsupported ²¹⁰Pb fluxes were determined from sediment core inventories in lakes located in Northern Patagonia, Argentina. Total ²¹⁰Pb, ²²⁶Ra, associated with supported ²¹⁰Pb, and ¹³⁷Cs specific activity profiles were measured by gamma-ray spectrometry. Unsupported ²¹⁰Pb fluxes showed very low values when compared to other regions, with a 12 fold variation, ranging from 4 to 48 Bq m^–2.y^–1. The linear correlation observed between the ²¹⁰Pb fluxes and¹³⁷Cs cumulative fluxes in sediment cores sampled from water bodies within a zone with similar precipitation demonstrated that both radioisotopes behave in the same manner in these systems concerning the processes occurred from fallout to sediment deposition, and that there are no appreciable local or regional sources of unsupported ²¹⁰Pb. Positive correlation of ²¹⁰Pb fluxes with organic matter contents of the uppermost sediment core layers was also observed.