Pulsed double resonance in the pure quadrupole regime: off-resonant DRRF technique for detection of rare quadrupolar nuclei

A scheme for the detection of rare quadrupolar nuclei based on off-resonant pulse excitation of the abundant quadrupolar nuclei coupled to the rare ones in the sample has been outlined and demonstrated. Pure quadrupole resonance spectrum of ²³Na in poly-crystalline samples of sodium chlorate has been thus recorded at room temperature. The present double resonance scheme in the pure quadrupole regime is marked by a substantial reduction in spectrometer transmitter power and hardware requirements.     

Reduced fluidity of dipalmitoyl phosphatidic acid membranes by salicylic acid

This paper presents DSC and NMR study of how the kerotolytic drug, salicylic acid (SA), affects the thermotropic and morphological behavior of a model membrane, dipalmitoyl phosphatidic acid (DPPA). The membrane-drug system has been studied in the multilamellar vesicular (MLV) and in the unilamellar vesicular (ULV) forms, for SA/DPPA molar ratios from 0 to 0.5. The mode of interaction of SA molecules with DPPA is similar in MLV and ULV. Chain-melting transition becomes sharper and shifts to higher temperatures in the presence of the drug, implying an enhanced co-operativity of the acyl chains. NMR and DSC data indicate that the drug molecules are located in the aqueous interfacial region neighboring the lipid headgroups. The membrane becomes more rigid in the presence of the drug molecules, due to a stronger interaction between the lipid headgroups leading to reduced permeability. ULVs are destroyed by even a short equilibration at room temperature, whereas prolonged equilibration of the MLV only leads to a slightly reduced interaction between the lipid headgroups due to sequestering of the drug molecules in the interfacial aqueous region.     

Polarographic reduction behaviour of copper in the presence of 1-amino-3-methoxypropane and 2-dimethylaminoethanol

1-Amino-3-methoxypropane (3MPA) and 2-dimethylaminoethanol (DMAE) are potential volatile amines for boiler water treatment and were investigated for their complexation behaviour with copper. Their pK_b values were 3.67 and 4.52 at 25°C and they formed coloured complexes with absorption in the region of 644 and 510 nm, respectively. In the pH range 10–11, the Cu-3MPA complex exhibited stepwise reduction and [Cu(3MPA)₂]⁺ was identified, with a stability constant of 10^9.52. In the pH range 8–10.5, the Cu-DMAE complex exhibited 2e^? reduction and the species [Cu(DMAE)₂(OH)₂] was identified, with a stability constant of 10^20.39. A correlation between visible and ESR spectra and reduction behaviour was established.     

Thermische Reaktionen mit 3-Phenyl-2H-azirinen; 1, 3-dipolare Cycloadditionen und En-Reaktionen

Benzonitrile p-nitrobenzylide ( 5 ) undergoes 1,3-dipolar cyclo-additions in the presence of 3-phenyl-2H-azirines ( 1 ), yielding in benzene at 0° 2-(p-nitrophenyl)-4,5-diphenyl-1,3-diazabicyclo[3.1.0]hex-3-enes ( 7 , scheme 2). Under the basic conditions of the reaction mixture, 7 a and 7 b are partially converted to 2-(p-nitrophenyl)-4,5-diphenyl-1,6-dihydropyrimidines ( 8a, b ) which are dehydrogenated by oxygen to the corresponding pyrimidines 9a and 9b , respectively. 3-Phenyl-2H-azirines ( 1 ) form, on heating at 145° in xylene in the presence of the azalactone 32 (2,4-diphenyl-Δ²-oxazolin-5-one), 4-(aziridin-2′-yl)-2,4-diphenyl-Δ²-oxazolin-5-ones ( 33 , scheme 11). 33 arises from an ene reaction of the enol form of 32 with 1 . Similar ene reactions are observed with the azirines 1 and dimedone ( 37 , scheme 12). Under the ene reaction conditions (xylene, 145°), the non-isolated intermediate primary adducts ( 38a and 38b ) undergo rearrangements of the vinylcyclopane-cyclopentene type to give 6,6-dimethyl-4-oxo-1,3-diphenyl-4, 5, 6, 7-tetrahydroisoindole ( 40 ) and 6, 6-dimethyl-4-oxo-3-phenyl-4, 5, 6, 7-tetrahedroindole ( 42 ), respectively.     

Valence bond calculations of vicinal proton coupling constants in cyclopropane

Vicinal proton couplings in cyclopropane have been calculated using Dirac-Van Vleck and variational methods in valence bond formalism. The couplings may be considered as taking place along a path comprising either three or four intervening bonds. Inclusion of all exchange integrals in the appropriate fragment models leads to a satisfactory ratio of J_cis/J_trans . In this respect valence bond results appear to be superior to those obtained using a molecular orbital approach. The present valence bond results on couplings in cyclopropane are not affected by the choice of models for bonding in cyclopropane, namely the Coulson-Moffitt model or the Walsh model.     

Reduction of Carboxylic Acids by Tetraalkyl Ammonium Borohydride

Tetraalkylammonium borohydride reduces carboxylic acids to the corresponding alcohols in good yields utilizing only stoichiometric quantities of hydride and also in the absence of any Lewis acids.     

Zirconium Borohydride - a Versatile Reducing Agent for the Reduction of Electrophilic and Nucleophilic Substrates

Zirconium borohydride, a potential reducing agent, reduces acids, esters, imines to the corresponding alcohols and secondary amines in good yield at room temperature within two hours. This facile reducing property was taken advantage off in the synthesis of pheromones and some novel chiral precursors for asymmetric synthesis.     

Reaction of Dichlorocarbene with Substituted Cyclopentadienones Under Phase Transfer Conditions

The cycloaddition reactions involving dichlorocarbene generated under phase transfer conditions have been studied extensively in the past few years due to the simplicity of the procedure involved.¹ The facile formation of dichlorocyclopropanes provides an easy entry to functionalised aromatics and expanded carbocyclic compounds.² The dichlorocarbene generated under phase transfer conditions is capable of adding even to electron deficient olefins. We describe here the cyclo-addition reaction of dichlorocarbene to tetrasubstituted cyclopentadienones.     

Calculations of 1H-19F nuclear spin couplings by the finite perturbation method

The direct correlation function for water near its triple point is obtained from published X-ray data. The core part of the function can be approximated by a hard-sphere function. The part beyond the core is discussed rather tentatively in the light of the Percus-Yevick relation.