Conformations, energies, and intramolecular hydrogen bonds in dicarboxylic acids: implications for the design of synthetic dicarboxylic acid receptors

The various conformers of the dicarboxylic acids HO2C--(CH2)n--CO2H, n = 1-4, were obtained using density functional methods (DFT), both in the gas phase and in the aqueous phase using a polarized continuum model (PCM). Several new conformers were identified, particularly for the two larger molecules glutaric (n = 3) and adipic acid (n =4). The PCM results show that the stability of most conformers were affected, many becoming unstable in the aqueous phase; and the energy ordering of conformers is also different. The results suggest that conformational preferences could be important in determining the design and stability of appropriate synthetic receptors for glutaric and adipic acid. Geometry changes between gas and aqueous phases were most marked in those conformers containing an intramolecular hydrogen bond. Additional calculations have probed the strength of intramolecular hydrogen bonds in these dicarboxylic acids. In the cases of glutaric and adipic acid, the strength of the intramolecular hydrogen bond were estimated to be around 28-29 kJ/mol, without any vibrational energy correction. The intramolecular hydrogen bond energies in malonic and succinic acid were also estimated from the calculated H-bond distances using an empirical relationship. Intramolecular H-bond redshifts of 170-250 cm(-1) have been estimated from the results of the harmonic frequency analyses.     

A new detection method for purines in thin-layer chromatography

Summary The separation of some purines by thin-layer chromatography, utilizing precoated plates with Silicagel R as stationary phase and the following mobile phases: i-butanol: methylethylketone: ammonium hydroxide 25%: water (40252015) and n-propanol: ammonium hydroxide 25%: water (80155) is reported.Visualisation of the spots has been achieved by spraying with aqueous 1% uranyl acetate and irradiating with UV both at 254 and 366 nm. The visual detection of a spot was 0.01 g.     

Atmospheric chemistry of propionaldehyde: kinetics and mechanisms of reactions with OH radicals and Cl atoms, UV spectrum, and self-reaction kinetics of CH3CH2C(O)O2 radicals at 298 K

The kinetics and mechanism of the reactions of Cl atoms and OH radicals with CH3CH2CHO were investigated at room temperature using two complementary techniques: flash photolysis/UV absorption and continuous photolysis/FTIR smog chamber. Reaction with Cl atoms proceeds predominantly by abstraction of the aldehydic hydrogen atom to form acyl radicals. FTIR measurements indicated that the acyl forming channel accounts for (88 +/- 5)%, while UV measurements indicated that the acyl forming channel accounts for (88 +/- 3)%. Relative rate methods were used to measure: k(Cl + CH3CH2CHO) = (1.20 +/- 0.23) x 10(-10); k(OH + CH3CH2CHO) = (1.82 +/- 0.23) x 10(-11); and k(Cl + CH3CH2C(O)Cl) = (1.64 +/- 0.22) x 10(-12) cm3 molecule(-1) s(-1). The UV spectrum of CH3CH2C(O)O2, rate constant for self-reaction, and rate constant for cross-reaction with CH3CH2O2 were determined: sigma(207 nm) = (6.71 +/- 0.19) x 10(-18) cm2 molecule(-1), k(CH3CH2C(O)O2 + CH3CH2C(O)O2) = (1.68 +/- 0.08) x 10(-11), and k(CH3CH2C(O)O2 + CH3CH2O2) = (1.20 +/- 0.06) x 10(-11) cm3 molecule(-1) s(-1), where quoted uncertainties only represent 2sigma statistical errors. The infrared spectrum of C2H5C(O)O2NO2 was recorded, and products of the Cl-initiated oxidation of CH3CH2CHO in the presence of O2 with, and without, NO(x) were identified. Results are discussed with respect to the atmospheric chemistry of propionaldehyde.     

Lower semicontinuous solutions for a class of Hamilton-Jacobi-Bellman equations

The present paper is concerned with the study of a Hamilton-Jacobi-Bellman equation in finite-dimensional spaces, from both the point of view of l.s.c. viscosity solutions and the point of view of l.s.c. contingent solutions. The results have been used in the study of the uniqueness problem for the Bellman equation associated to a time-optimal control problem (Ref. 1).This paper was completed while the author was visiting the University of California at Los Angeles as a Fulbright Scholar.     

Selective Synthesis of 2-, 4-, and 5-Cyano Substituted Imidazoles from Imidazole N-Oxides and Trimethylsilyl Cyanide

1-Substituted 2-, 4-, and 5-cyanoimidazoles were produced by reaction of 3-cyclohexyl or 3-p-tolylimidazole 1-oxide and trimethylsilyl cyanide in the presence of triethylamine. The product composition depended on reaction temperature and solvent polarity. By proper choice of these parameters each isomer could be obtained selectively in reasonable yield.     

Photovoltaic performance of nanostructured zinc oxide sensitised with xanthene dyes

The photovoltaic properties of nanostructured ZnO electrodes prepared from commercially available ZnO nanoparticles (Degussa) and sensitised with xanthene dyes are explored. We have used Eosin Y, Eosin B and Mercurochrome and compared their performance to that of the more common N719 dye. We observe that these dyes efficiently sensitise commercial ZnO nanopowder and yield efficiencies that are very competitive with respect to those provided by N719. Local photocurrent and transmittance measurements as a function of the wavelength confirm the good performance of the xanthene dyes in the absorption maxima. We have prepared polymer-sealed cells and measured the short-circuit current of the devices under continuous illumination at 200 mW/cm² corresponding to twice the standard solar light intensity. The organic dyes show very good stability properties under these conditions. The combination of a versatile metal oxide such as ZnO and inexpensive organic dyes should be considered as a promising alternative in the field of dye-sensitised solar cells.     

The liquid crystalline properties of some ferrocene‐containing Schiff bases

The synthesis and characterization of new ferrocene‐containing mesomorphous compounds is described. In order to induce mesogenic behavior, the structure of the substituent was systematically modified. Derivatives with no more than two rings in the core did not exhibit liquid crystalline properties. The synthesized products were characterized using NMR, IR and mass spectroscopy, DSC and polarized optical microscopy. Copyright © 2007 John Wiley & Sons, Ltd.     

On the index of fractions with square-free denominators in arithmetic progressions

We prove asymptotic formulas for the first and second moments of the index of fractions with square-free denominators of order Q streaming in a given arithmetic progression as Q→∞. A. Zaharescu was supported by NSF grant number DMS-0456615. This research was also partially supported by the CERES Program 4-147/2004 of the Romanian Ministry of Education and Research.     

Control and characterization of the nucleation process during co-expansion of ternary mixtures

In this paper, we show how the use of a third non-condensable species gives both new insight and control on the binary nucleation process during supersonic expansion of gas mixtures. We present the case of an oxygen-nitrogen mixture diluted in various proportions of helium. Using beam diagnostics, we determine the mean cluster composition and size as well as the percentage of uncondensed matter present in the beam. The presence of helium permits us to understand the cooling and clustering role played by each species during the expansion process. We discuss, in particular, its influence on the dramatic composition change observed at the nucleation onset. Received 18 April 2001