Confined systems and the modified virial theorem from semiclassical considerations

We show that two simple semiclassical strategies, one based on the Wilson–Sommerfeld rule and the other on the uncertainty principle, yield the exact modified form of the virial theorem for confined systems. An alternative, easier quantum mechanical route to arrive at this result is also sketched. Pilot calculations on confined oscillators reveal decisive trends. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

p-Toluenesulfonic acid catalyzed regiospecific nitration of phenols with metal nitrates

Highly regiospecific mononitration of phenols and substituted phenols is accomplished employing a metal nitrate and a catalytic amount of p-toluenesulfonic acid in acetone. An exclusive ortho-selectivity was observed with excellent yields. A variety of metal nitrates were used to obtain o-nitrophenols exclusively in good to excellent yields. The use of p-toluenesulfonic acid is key for the selectivity observed.     

Oligosaccharides through reactivity tuning: convergent synthesis of the trisaccharides of the steroid glycoside Sokodoside B isolated from marine sponge Erylus placenta

Chemical synthesis of the trisaccharide of the steroid glycoside Sokodoside B isolated from Erylus placenta is reported. Stereoselective, high-yielding glycosylation strategies through thioglycoside activation using H₂SO₄ immobilized on silica in conjunction with N-iodosuccinimide are used for better results. A late stage TEMPO-mediated oxidation was performed for the formation of required uronic acid moiety. An analog of the target trisaccharide is also prepared by using a bis-glycosylation approach.     

Oxovanadium(IV) and -(V) complexes of dithiocarbazate-based tridentate Schiff base ligands: syntheses,structure, and photochemical reactivity of compounds involving imidazole derivatives as coligands

The tridentate dithiocarbazate-based Schiff base ligands H(2)L (S-methyl-3-((5-R-2-hydroxyphenyl)methyl)dithiocarbazate, R = NO(2), L = L(2); R = Br, L = L(3)) react with [VO(acac)(2)] in the presence of imidazole derivatives as coligands to form oxovanadium(IV) and cis-dioxovanadium(V) complexes. With benzimidazole and N-methylimidazole, the products are oxovanadium(IV) complexes, viz. [VOL(3)(BzIm)].0.5CH(3)CN (1a) and [VOL(N-MeIm)(2)] (L = L(3), 1b; L = L(2), 1c), respectively. In both 1a,b, the O and S donor atoms of the tridentate ligand are cis to the terminal oxo group (in the "equatorial" plane) and mutually trans, but the N donor atom is respectively cis and trans to the oxo atom, as revealed from X-ray crystallography. When imidazole or 4-methylimidazole is used as the ancillary ligand, the products obtained are water-soluble cis-dioxovanadium(V) complexes [VO(2)L(R'-ImH)] (L = L(3) and L(2), R' = H and Me, 2a-d). These compounds have zigzag chain structures in the solid state as confirmed by X-ray crystallographic investigations of 2a,d, involving an alternating array of LVO(2)(-) species and the imidazolium counterions held together by Coulombic interactions and strong hydrogen bonding. Complexes 2a-d are stable in water or methanol. In aprotic solvents, viz. CH(3)CN, DMF, or DMSO, however, they undergo photochemical transformation when exposed to visible light. The putative product is a mixed-oxidation divanadium(IV/V) species obtained by photoinduced reduction as established by EPR, electronic spectroscopy, and dynamic (1)H NMR experiments.     

High resolution spectral analysis of 13CH3OH in the excited torsional states

In this work, we have extended our previous analysis of the Hamiltonian of 13C substituted methanol to include a large number of spectral lines involving the second excited torsional state using an improved model. The data set consisted of 2529 Fourier transform and microwave transitions with the rotational angular momentum J < or = 10, K < or = 6 and n < or = 2 (with 336 MW lines). The data set was fitted with the new Hamiltonian model to derive the molecular parameters. The results indicate that the model developed for the other methanol species (CH3OH, CH3(18)OH and CH3OD) is also valid for the C-13 substituted species. The results will allow the energy levels of the molecule to be calculated for higher torsional levels above the internal rotational barrier with improved precision and allow the analysis to be carried out for more excited torsional states.     

Tamarindus indica mucilage and its acrylamide-grafted copolymer as flocculants for removal of dyes

The application of Tamarindus indica seed mucilage (Tam), a food grade polysaccharide, and its acrylamide grafted copolymer as flocculants was assessed for the first time for removal of various types of dyes from model textile wastewater containing azo, basic, and reactive dyes. Acrylamide grafted T. indica mucilage (Tam-g-PAM) was obtained by ceric ion initiated polymerization technique. A series of contact time experiments were conducted to assess the system variables such as concentrations of mucilage and dyes and pH. These flocculants reduce the dye concentration by flocculation and settling. The grafted copolymer, Tam-g-PAM showed better results for dye removal. The optimal flocculant concentration, which was required to effect flocculation, was independent of dye concentration within the range examined. Both the flocculants performed better for removal of azo dyes than for reactive and basic dyes.     

Kinetics and mechanism of the displacement of aqua ligands from cis-diaqua-bis(biguanide)chromium(III) by pyridine-2-aldoxime in EtOH-H2O mixtures

Kinetic and mechanistic studies of the displacement of aqua ligands from cis-[Cr(BigH)₂(H₂O)₂]³⁺ (BigH = H₂NC(NH)CHC(NH)NH₂) by pyridine-2-aldoxime (LH) in EtOH-H₂O mixtures, in the 30–45° C range, obey the following rate law over the 3.5–6.0 pH range,

Molecular clips form isostructural dimeric aggregates from benzene to water

We report the synthesis and characterization of eight C-shaped methylene-bridged glycoluril dimers (1-8) bearing hydrogen-bonding amide groups on their aromatic rings. Compounds 1-6 undergo tight dimerization in CDCl3 solution (Ks > 9 x 10(5) M(-1)); binary mixtures of 1-7 form mixtures of homodimers and heterodimers in moderately selective dimerization processes (0.23 < or = Keq < or = 768; 0.253 < or = chiAB < 0.933). The high affinity formation of 1.1-6.6 is due to the commensurate nature of the geometrical constraints imposed by the pi-pi interactions and only two hydrogen bonds. The differential response of the strengths of the pi-pi interactions and H-bonds of 2.2 to changes in solvent polarity--from C6D6 to D2O--results in the formation of a solvent-independent isostructural aggregate that exhibits high affinity dimerization across the full range of solvents.     

Vinyl polymerization: Kinetics of Ce(IV)–acetophenone-initiated polymerization of acrylonitrile in acetic–sulfuric acid mixtures

The polymerization of acrylonitrile (M) initiated by the Ce(IV)–acetophenone (AP) redox pair has been studied in acetic–sulfuric acid mixtures in a nitrogen atmosphere. The rate of polymerization is proportional to [M]^3/2, [AP]^1/2 and [Ce(IV)]^1/2. The rate of disappearance of ceric ion,–R_Ce, is proportional to [AP], [M], and [Ce(IV)]. The effect of certain salts, solvent, acid and temperature on both the rates have been investigated. A suitable kinetic scheme has been proposed, and the composite rate constants k_p ²(k/k/_t) and k₀/k_i are reported.     

Cleavage of the peptide bond of beta-alanyl-L-histidine (carnosine) induced by a Co(III)-amine complexes: reaction, structure and mechanism

Cleavage of the peptide bond occurs when beta]-alanyl-L-histidine (carnosine) reacts with [Co(tren)Cl2]+ (tren = tris(2-aminoethyl)amine) to give [Co(tren)(histidine)](2+) 1 and [Co(tren)(beta-alanine)](2+) 2. [Co(tren)(histidine)](2+) 1 crystallizes in the enantiomorphic space group P2(1)2(1)2(1) and 2 crystallizes in the P2(1)/c space group. The mechanism of the cleavage reactions were studied in detail for the precursor [Co(tren)Cl2]+ and [Co(trien)Cl2]+, which convert into [Co(tren)(OH)2]+/[Co(tren)(OH)(OH2)]2+ and [Co(trien)(OH)2]+/[Co(trien)(OH)(OH2)]2+ in water at basic pH (trien = 1,4,7,10-tetraazadecane). At a slightly basic pH, the initial coordination of the substrate (beta-alanyl-L-histidine) is by the carboxylate group for the reaction with [Co(tren)Cl2]+. This is followed by a rate-limiting nucleophilic attack of the hydroxide group at the beta-alanyl-L-histidine carbonyl group. In a strongly basic reaction medium substrate, binding of the metal was through carboxylate and amine terminals. On the other hand, for the reaction between [cis-beta-Co(trien)Cl2]+ and beta-alanyl-L-histidine, the initial coordination of the substrate takes place via an imidazole ring nitrogen, independently, and followed by a nucleophilic attack of the hydroxide group at the beta-alanyl-L-histidine carbonyl group. The circular dichroism spectrum for 1 suggests that a very small extent of racemization of the amino acid (L-histidine) takes place during the cleavage reaction between [Co(tren)Cl2]+ and beta-alanyl-L-histidine. Reaction between [cis-beta-Co(trien)Cl2]+ and beta-alanyl-L-histidine also causes cleavage of the peptide bond, producing a free beta-alanyl molecule and a cationic fragment [cis-alpha-Co(trien)(histidine)](2+) 3 that crystallizes in the optically active space group P2(1)2(1)2(1). Unlike the previous case an appreciable degree of racemization of the L-histidine takes place during the reaction between [cis-beta-Co(trien)Cl2]+ and beta-alanyl-L-histidine. Crystals containing L-histidine and D-histidine fragments in the [cis-alpha-Co(trien)(histidine)]2+ moiety were crystallographically documented by mounting a number of randomly selected crystals.