Diels-Alder approach to biaryls (DAB): importance of the ortho-nitro moiety in the [4 + 2] cycloaddition

A series of nitrophenyl acetylenes were evaluated for their utility in a cycloaddition-cycloreversion approach to polysubstituted biaryls. ortho-Nitrophenyl acetylene consistently provided the target biaryls in superior yields as compared to the meta and para nitro substituted variants.     

Salt effects on poly(N-isopropylacrylamide) phase transition thermodynamics from NMR spectroscopy

NMR spectra were collected for cross-linked poly(N-isopropylacrylamide), poly(NIPAM), hydrogels in the presence of NaCl and CaCl2 aqueous solutions. Intensity variations in the 1H NMR signals of the polymer provide insight into the phase transition process. These data were used to observe a two-stage phase transition process. Thermodynamic quantities were obtained from a van't Hoff analysis of the temperature-dependent equilibrium constants, which were derived from the NMR data. The Delta H degrees and Delta S degrees values for the hydrogel in D2O are 3.4 kJ/mol and 11.2 J/mol.K for stage I, which is attributed to the formation of hydrophobic bonds between neighboring isopropyl groups. The formation of hydrogen bonds during stage II yielded Delta H degrees and Delta S degrees values of 14.8 kJ/mol and 48.4 J/mol.K in D2O. However, the corresponding Delta H degrees values in 150 mM NaCl and 150 mM CaCl2 are reduced to 1.5 and 1.8 kJ/mol for stage I of the dehydration process. This corresponds to the known effect of salts on hydrophobic bond energetics. The value of Delta S degrees also decreased to 4.9 and 5.9 J/mol.K in NaCl and CaCl2 solutions, respectively. However, the thermodynamic values during stage II were only slightly affected by the salts. The lower temperatures required to induce spontaneous precipitation implies that Delta G degrees of precipitation is reduced. With our measurement of equilibrium thermodynamics, we see that 150 mM NaCl and CaCl2 solutions have a greater effect on hydrophobic bond formation associated with the phase transition process. In this manner, these salts aid in solvent reorganization necessary to form the hydrophobic bond, and this suggests that the formation of hydrophobic bonds is a strong determining factor in the stability of poly(NIPAM) hydrogels in water.     

N-(p-dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide: a practical proline mimetic for facilitating enantioselective aldol reactions

A highly practical and readily available proline surrogate has been developed with improved solubility properties in common, nonpolar organic solvents. This sulfonamide-based catalyst has proven highly effective at facilitating enantioselective and diastereoselective aldol reactions with a range of substrates in nonpolar organic solvents in the presence of a single equivalent of water. Additionally, catalyst loading as low as 2 mol % can be employed in the absence of any organic solvent with continued high levels of selectivity.     

Vibrato Rate Adjustment

The goal of the present study was to document the acoustic changes that occur as singers attempt to increase or decrease their vibrato rate to match target stimuli. Eight advanced singing students produced vowels with vibrato in three registers, both naturally and while attempting to match faster or slower rate stimuli. Slower rates were associated with lower intensity and less steady vibrato. Faster rates involved increased vibrato extent in the chest register and increased intensity in the head register. Singers whose spontaneous vibrato rates were naturally either slower or faster tended to also be relatively slower or faster when matching target rates. This ability to modify rate may have beneficial effects on the artistic quality of the voice for performance.     

Non-innocent Dissociation of H(2)O on GaP(110): Implications for Electrochemical Reduction of CO(2)

The structural and electronic properties of the GaP(110)/H(2)O interface have been investigated by first-principles density functional theory calculations. Our results suggest that hydride-like H atoms are present on the surface as a consequence of the dissociation of water in contact with the GaP surface. This feature opens up a new feasible reduction pathway for CO(2) where the GaP(110) surface is the electrochemically active entity.     

Hierarchically structured porous cadmium selenide polycrystals using polystyrene bilayer templates

In this study, a novel approach is demonstrated to fabricate hierarchically structured cadmium selenide (CdSe) layers with size-tunable nano/microporous morphologies achieved using polystyrene (PS) bilayered templates (top layer: colloidal template) via potentiostatic electrochemical deposition. The PS bilayer template is made in two steps. First, various PS patterns (stripes, ellipsoids, and circles) are prepared as the bottom layers through imprint lithography. In a second step, a top template is deposited that consists of a self-assembled layer of colloidal 2D packed PS particles. Electrochemical growth of CdSe crystals in the voids and selective removal of the PS bilayered templates give rise to hierarchically patterned 2D hexagonal porous CdSe structures. This simple and facile technique provides various unconventional porous CdSe films, arising from the effect of the PS bottom templates.     

Silica nanocoils

We report here the results of nanoparticle-catalyzed synthesis of a variety of silica nanocoils (SiNCs) under similar growth conditions to those of making regular straight silica/silicon nanowires (SiNWs). Synthesis of individual SiNCs with alternating coiled and straight segments was achieved with cobalt nanoparticle catalysts at alternating temperatures. In addition, a variety of SiNCs, including multiple SiNCs sharing the same nanoparticle catalyst, SiNC double helices, and individual SiNCs with varying pitches and diameters were made.     

Structure characterization of lipocyclopeptide antibiotics,aspartocins A,B & C,by ESI‐MSMS and ESI‐nozzle‐skimmer‐MSMS

Three lipocyclopeptide antibiotics, aspartocins A (1), B (2), and C (3), were obtained from the aspartocin complex by HPLC separation methodology. Their structures were elucidated using previously published chemical degradation results coupled with spectroscopic studies including ESI‐MS, ESI‐Nozzle Skimmer‐MSMS and NMR. All three aspartocin compounds share the same cyclic decapeptide core of cyclo [Dab2 (Asp1‐FA)‐Pip3‐MeAsp4‐Asp5‐Gly6‐Asp7‐Gly8‐Dab9‐Val10‐Pro11]. They differ only in the fatty acid side chain moiety (FA) corresponding to (Z)‐13‐methyltetradec‐3‐ene‐carbonyl, (+,Z)‐12‐methyltetradec‐3‐ene‐carbonyl and (Z)‐12‐methyltridec‐3‐ene‐carbonyl for aspartocins A (1), B (2), and C (3), respectively. All of the sequence ions were observed by ESI‐MSMS of the doubly charged parent ions. However, a number of the sequence ions observed were of low abundance. To fully sequence the lipocyclopeptide antibiotic structures, these low abundance sequence ions together with complementary sequence ions were confirmed by ESI‐Nozzle‐Skimmer‐MSMS of the singly charged linear peptide parent fragment ions H‐Asp5‐Gly6‐Asp7‐Gly8‐Dab9‐Val10‐Pro11‐Dab2¹⁺‐Asp1‐FA. Cyclization of the aspartocins was demonstrated to occur via the β‐amino group of Dab2 from ions of moderate intensity in the ESI‐MSMS spectra. As the fatty acid moieties do not undergo internal fragmentations under the experimental ESI mass spectral conditions used, the 14 Da mass difference between the fatty acid moieties of aspartocins A (1) and B (2) versus aspartocin C (3) was used as an internal mass tag to differentiate fragment ions containing fatty acid moieties and those not containing the fatty acid moieties. The most numerous and abundant fragment ions observed in the tandem mass spectra are due to the cleavage of the tertiary nitrogen amide of the pipecolic acid residue‐3 (16 fragment ions) and the proline residue‐11 (7 fragment ions). In addition, the neutral loss of ethanimine from α,β‐diaminobutyric acid residue 9 was observed for the parent molecular ion and for 7 fragment ions. Copyright © 2009 John Wiley & Sons, Ltd.