Density functional study of the conformations and intramolecular proton transfer in thiohydroxamic acids

The conformational preferences of thiohydroxamic acids (N-hydroxythioamides) are investigated by the density functional B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) method in this work. Unlike hydroxamic acids, the thione and thiol forms are found to be equally stable in the gas phase, and the reaction pathways for the interconversion between the thione and thiol forms have been deduced to involve rotation about the C[double bond, length as m-dash]N bond of the thiol tautomer in the rate-determining step. The effect of aqueous solvation on the reactions has also been investigated. It is found that inclusion of a few explicit water molecules in an implicit solvent calculation is necessary in order to accurately account for hydrogen bonding effects. Thiohydroxamic acids, like their hydroxamic acid analogues, are found to be N-acids, both in the gas phase and in aqueous solution.     

Role of surface modification of colloidal CdSe quantum dots on the properties of hybrid organic–inorganic nanocomposites

In this work, tri-octyl phosphine/tri-octyl phosphine oxide (TOPO)-capped cadmium selenide (CdSe) quantum dots (QDs) of varied sizes (5–9 nm), prepared by varying the input Cd:Se precursor ratio using chemical route, were dispersed in conducting polymer matrices viz. poly[2-methoxy, 5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly(3-hexylthiophene) (P3HT). By using a binary solvent mixture (pyridine–chloroform), homogeneous dispersion of CdSe nanocrystals in polymers (MEH-PPV, P3HT) could be realized. The properties of the resulting dispersions could be tailored by the composition and concentration of QDs in polymer. The emission and structural properties of polymer–CdSe nanocomposites are found to be dependent on the crystallite size and morphology of CdSe nanocrystallites. An effective quenching of photoluminescence emission in the polymer nanocomposite was observed for smaller CdSe quantum dots (size ∼6 nm) as compared to larger CdSe quantum dots (size ∼9 nm), thus ensuring efficient charge transfer process across the polymer–CdSe interface in the former case. The incomplete quenching, particularly for MEH-PPV:CdSe nanocomposites, could be as a result of insufficient coverage of polymers on the surface of CdSe nanocrystallites, mainly due to phase segregation for TOPO-stripped CdSe nanocrystallites. The superior morphology and optical properties of polymer nanocomposite (P3HT:CdSe QDs) could play a pivotal role for the realization of effective charge separation and transport in hybrid solar cells.     

Advances in the Elegance of Chemistry in Designing Dendrimers

Highly functional and monodisperse macromolecules with tailored architecture constitute the key to designing efficient and smart nanomaterials. Dendrimers offer real potential to achieve this goal, and one of the earlier challenges faced by this novel class of polymers has been addressed by the evolution of synthetic methodologies. This review provides an evaluation of the role played by chemistry in taking these macromolecules of academic relevance to practical industrial and biological applications, in a relatively short period. One can now construct dendrimers in a ‘made‐to‐order’ fashion, for numerous applications in a variety of disciplines.

     

Single-crystal magnetism of sodium magnesium chromium(III) oxalate,NaMgCr(C2O4)3·9H2O

Magnetic anisotropies of a trigonal crystal of NaMgCr(C₂O₄)₃·9H₂O measured between 300 and 77 K are approximately inversely proportional to the square of the absolute temperature; moreover K₆ > K_. although g_. > g₆ from EPR measurements. The sign of D, the zero-field splitting, which was not determined from EPR, is shown to be negative.     

Determination of molybdenum by extraction of its thiocyanate into ethyl methyl ketone

A simple, sensitive and selective spectrophotometric method for determination of molybdenum is described. A solution containing 100 mug of Mo in 2.5M hydrochloric acid is treated with ascorbic acid and ammonium thiocyanate and after standing for 8 min is shaken with an equal volume of ethyl methyl ketone for 30 sec. The absorbance of the complex is measured at 465 nm against a reagent blank. The complex is stable for 1 hour. There is no interference from Re(VII), SO(2-)(4), Cl(-), CH(3)COO(-), PO(3-)(4), NO(-)(3), C(2)O(2-)(4), citrate or tartrate, and at least 5 mg of U(VI), 10 mg of Cr(III, VI), Th, or Ni, and 20 mg of W(VI) Can be tolerated. Vanadium(V) interferes at the 500 mug level, and fluoride slightly decreases the absorbance.     

Unimolecular rearrangements of ethylnitrene: An exploratory theoretical study

The products of the rearrangement of ethylnitrene are ethylideneimine via a 1,2 hydrogen shift, N-methyl methyleneimine via a 1,2 methyl shift, and the corresponding aziridine by cyclization. The three reactions have been studied by the semiempirical MNDO method. The charge transfer effects have been investigated. Theoretically calculated vibrational frequencies and kinetic properties have been used in calculating macroscopic thermodynamic and kinetic properties. The entropies of activation, the high pressure rate constants, k_∞, and the high pressure Arrhenius parameters, A_∞ and E_∞, have been computed. The kinetic isotope effects have also been studied.     

C3H4: Density functional study of interconversion of isomers

The B3LYP density functional has been used to calculate the reaction paths for the interconversion of the three stable isomers of C₃H₄, namely, propyne, allene, and cyclopropene, and the results compared with experimental data. It is found that the trans nonplanar isomer of vinylmethylene plays a central role in the interconversions. Other thermochemical parameters, like the entropies of reaction and activation, have also been calculated. The rate constants of the various competing reactions have been calculated using the Rice–Ramsperger–Kassel–Marcus theory. Kinetic isotope effects for the reactions have also been investigated to gain an insight into their mechanisms. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Extractive spectrophotometric determination of palladium using 2-(2-hydroxyimino-1-oxoethyl)furan

A rapid, simple, selective and sensitive method for the determination of palladium in trace amounts has been developed, based on the reaction of 2-(2-hydroxyimino-1-oxoethyl)furan [-isonitroso-2-acetyl-furan] with the metal ion in acetic acid. The intense yellow complex is quantitatively extractable into chloroform, whose absorbance is measured at 425 nm. The method is free from the interference of a large number of other metal ions. It obeys Beer's law in the range 0.0–3.8 g Pd mL^–1 with Sandell's sensitivity of 0.003 g Pd cm^–2. The ratios of metal to ligand in the extracted complex are 1:1 and 1:2. The analysis of various samples has been carried out with satisfactory and reproducible results.     

Extractive spectrophotometric determination of niobium(V) with 3-hydroxy-2-(2-thienyl)-4H-chromen-4-one

A coloured complex of niobium (V) with 3-hydroxy-2-(2-thienyl)-4H-chromen-4-one (HTC) is produced in perchloric acid medium (4 mol/L); it is quite stable and extractable into dichloromethane. Beer's law is obeyed in the range 0.0–1.9 g Nb(V) mL^–1. The molar absorptivity at _max=420 nm is 5.0357×10⁴ L mol^–1 cm^–1. Using Job's method and the mole ratio method, the ratio of metal to ligand (Nb:HTC) in the extracted species has been found to be 1:2. The effect of anions and foreign metals is described.     

Metal ion complexes of thioformin: A density functional study

Some metal ion complexes of thioformin have been investigated by the density functional approach in order to understand its coordination chemistry. Due to the presence of the soft sulfur atom in addition to the oxide ion, thiohydroxamates exhibit a different coordination chemistry compared to their hydroxamate counterparts. It is found that the oxide ion forms bonds with metal ions that are predominantly ionic in nature while the sulfur atom is involved in weak covalent bond formation. Most of the complexes are found to be tetrahedrally distorted square planar structures, showing distinct cis and trans isomers. The exception is Zn(II), which forms tetrahedral complexes. The stabilities of the complexes have been linked with their antibiotic activity.