Small heterocyclics as hydrogen bond acceptors and donors: the case of the C2H3XS···NH3 complexes (X = H, F and CH3)

B3LYP/6-311++G(d,p) calculations were employed in order to examine the molecular parameters of the C₂H₃XS···NH₃ heterocyclic hydrogen-bonded complexes with X = H, F and CH₃. Intermolecular criteria were taken into account when studying the formation of these hydrogen-bonded complexes, such as geometry analysis, charge density quantification and interpretation of the harmonic vibrational spectrum, in which case the appearance of red-shift and blue-shift effects was discussed. It was assumed from the outset that many hydrogen bond types may exist in these systems, and these were investigated using the results of topological integrations from the quantum theory of atoms in molecules (QTAIM) and intermolecular charge transfer calculations using the ChelpG scheme. The proton donor/acceptor behavior of C₂H₃XS was interpreted in terms of hydrogen bond energies, whose values were corrected using the basis sets superposition error (BSSE) and zero point energy (ZPE). Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Density functional study for the polymerization of ethylene monomer using a new nickel catalyst

The present computational study was designed to study the polymerization of ethylene catalyzed by a new Ni‐based PymNox organometallic compound. Recently, we have synthesized and tested the behavior of this type of catalyst in olefin polymerization. It has been experimentally observed that the unsubstituted catalyst Ni2 (aldimino PymNox catalyst ) is less active than the methyl substituted Ni1 (acetaldimino PymNox catalyst ) analogue. The reactivity of both catalysts was examined using density functional theory (DFT) models. Our results indicate that the methyl substituted Ni1 introduces some additional steric hindrance that probably renders a more suitable catalyst conformation for the monomer incorporation. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1160–1165, 2010     

Chromium(VI) oxide oxidation of non‐ethoxylated and ethoxylated alcohols for determination by electrospray ionization mass spectrometry

A new derivatization procedure to increase the sensitivity of electrospray ionization mass spectrometry (ESI‐MS) to non‐ethoxylated and ethoxylated alcohols was investigated. The analytes were oxidized with chromium(VI) oxide and the resulting carboxylic and ethoxy‐carboxylic acids were isolated by extraction with ethyl acetate; the extracts were alkalinized and infused into the ESI‐MS system working in the negative‐ion mode. The yields of the combined oxidation‐extraction were ca. 100% for non‐ethoxylated fatty alcohols dissolved in acetone and they decreased moderately in samples containing increasing amounts of water (e.g., a 75% yield was obtained with 50% water). Ethoxylated alcohols with more than two ethylene oxide units resulted in yields of ca. 60%. Low limits of detection (LODs) were obtained when the procedure was applied to the analysis of body‐care products and cosmetics containing fatty alcohols, e.g., in a varicose‐vein cream, the LODs were 25 µg cetyl alcohol and 7.5 µg stearyl alcohol (detected as palmitic acid and stearic acid, respectively) per gram of sample. High molecular mass alcohols were also detected in seawater after pre‐concentration by solid‐phase extraction. Thus, the proposed method is particularly valuable for use in industrial samples having complex matrices and in environmental samples and it is competitive with other methods for the analysis of trace amounts of fatty alcohols. Copyright © 2010 John Wiley & Sons, Ltd.     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

Separation and quantitation of colour pigments of chili powder (Capsicum frutescens) by high-performance liquid chromatography-diode array detection

The performance of reversed-phase thin-layer (RP-TLC) and reversed-phase high-performance liquid chromatography (RP-HPLC) was compared for the separation and determination of the colour pigments of chili (Capsicum frutescens) powder using a wide variety of eluent systems. No separation of pigments was achieved in RP-TLC, however, it was established that tetrahydrofuran shows an unusually high solvent strength. RP-HPLC using water-methanol-acetonitrile gradient elution separated the chili pigments in many fractions. Diode array detection (DAD) indicated that yellow pigments are eluted earlier than the red ones and chili powder contains more yellow pigments than common paprika powders. It was established that the very different absorption spectra of pigments make the use of DAD necessary.     

Novel bis-betaines and betaines within [1(4)]meta-heterophane frameworks

After prior selection of betaine building blocks for the construction of quadrupolar heterophane frameworks, a convergent "3+1" synthetic strategy is reported for the synthesis of the title macrocycles composed of heterocyclic betaine subunit(s). These typify the first example of simple cyclophanes constructed out of both highly pi-excessive and highly pi-deficient heteroaromatic moieties linked in a 1,3-alternating fashion. The chemical reactivity of the quadrupolar heterophanes 1a and 1c toward electrophiles under neutral conditions corroborated their bis-betaine structure. The structural features of the bis-betaines 1, betaines 2 x PF6 and 5 x X, and the corresponding dicationic [1(4)]heterophanes 3 x 2X and 4 x 2Cl were studied by 1H and 13C NMR spectroscopy and electrospray ionization mass spectrometry, and confirmed by single-crystal X-ray diffraction analysis of macrocycles 1a and 2a x PF6.     

Generation and reactivity of 3‐carbethoxy‐5‐phenyl‐5H,7H‐thiazolo[3,4‐c]oxazol‐4‐ium‐1‐olate

3‐Carbethoxy‐5‐phenyl‐5H,7H‐thiazolo[3,4‐c]oxazol‐4‐ium‐1‐olate was generated from (2R,4R)‐N‐ethoxyoxalyl‐2‐phenylthiazolidine‐4‐carboxylic acid and its reactivity studied. This münchnone showed low reactivity as dipole although from the reaction with dimethyl acetylenedicarboxylate the corresponding (3R)‐3‐phenyl‐17H,3H‐pyrrolo[1,2‐c]thiazole‐5,6,7‐tricarboxylate could be isolated. The thermolysis of (2R,4R)‐N‐ethoxyoxalyl‐2‐phenylthiazolidine‐4‐carboxylic acid in refluxing acetic anhydride led to the synthesis of N‐(1‐ethoxycarbonyl‐2‐phenylvinyl)‐2‐phenyl‐4‐thioxo‐1,3‐thiazolidine. The structure of methyl (2R,4R)‐N‐ethoxyoxalyl‐2‐phenylthiazoliddine‐4‐carboxylate was determined by X‐ray crystallography.     

Theoretical study of ribonucleotide reductase mechanism-based inhibition by 2'-azido-2'-deoxyribonucleoside 5'-diphosphates

2'-azido-2'-deoxyribonucleoside 5'-diphosphates are mechanism-based inhibitors of Ribonucleotide Reductase. Considerable effort has been made to elucidate their mechanism of inhibition, which is still controversial and not fully understood. Previous studies have detected the formation of a radical intermediate when the inhibitors interact with the enzyme, and several authors have proposed possible structures for this radical. We have conducted a theoretical study of the possible reactions involved, which allowed us to identify the structure of the new radical among the several proposals. A new reactional path is also proposed that is the most kinetically favored to yield this radical and ultimately inactivate the enzyme. The energetic involved in this mechanism, both for radical formation and radical decay, as well as the calculated Hyperfine Coupling Constants for the radical intermediate, are in agreement with the correspondent experimental values. This mechanistic alternative is fully coherent with remaining experimental data.