Effect of the excitation wavelength on the ultrafast charge recombination dynamics of donor-acceptor complexes in polar solvents

The effect of the excitation wavelength on the charge recombination (CR) dynamics of several donor-acceptor complexes (DACs) composed of benzene derivatives as donors and of tetracyanoethylene or pyromellitic dianhydride as acceptors has been investigated in polar solvents using ultrafast time-resolved spectroscopy. Three different wavelength effects have been observed. (1) With complexes exhibiting two well-separated charge-transfer bands, the CR dynamics was found to be slower by a factor of about 1.5 upon excitation in the high-energy band. This effect was measured in both fast and slow relaxing solvents and was discussed in terms of different DAC geometries. (2) When the CR is faster than diffusive solvation, a slowing down of the CR with increasing excitation wavelength accompanied by an increase of the nonexponential character of the dynamics was measured. This effect appears only when exciting on the red edge of the charge-transfer absorption band. (3) When the driving force for CR is small, both nonequilibrium (hot) and thermally activated CR pathways can be operative. The results obtained with such a complex indicate that the relative contribution of these two paths depends on the excitation wavelength.     

Physical significance of contact angles

Summary The physical significance of contact angles has been interpreted on the basis of a model derived from known surface energy relationships. The degrees of non-spreading and spreading have been expressed in terms of the magnitude of contact angles. On the basis of the physical picture, hysteresis of contact angle has been calculated from the experimental values of equilibrium contact angle and surface tension of the liquid. It has been suggested that it is not necessary to assume that hysteresis of contact angles is due to surface roughness of solids. The picture also explains why apparent contact angle on a non-flat solid surface is more than that on a flat solid.With 2 figures and 1 table     

The spectrophotometric determination of molybdenum and uranium with quinalizarin

Quinalizarin reacts very sensitively with molybdenum and uranium to form colored chelates having λ_max at 520 and 630 nm, respectively. The molar ratio for both the chelates is 1:1 (metal:reagent). Optimum conditions including the range for adherence to Beer's law, effect of pH on color intensity, effect of excess of the reagent, sensitvity, and interference of the foreign ions has been reported for the photometric determination of these metal ions using quinalizarin, in 50% ethanolic medium and at 30 °C.     

Complex Formation of Lead (II) with Bromopyrogallol Red

The complex formation of lead with Bromopyragallol Red has been studied spectrophotometrically in an attempt to establish composition, stability, thermodynamic parameters and optimum condition for determining small amounts of lead. The blue complex of lead had λ_max at 630 nm (against a reagent blank). The composition as determined by different methods is 1:1 at pH 5.5±0.1. The mean value of log K, free energy (ΔG), the heat content (ΔH) and entropy change (ΔS) of the complex are found to be 6.28, ?10.46 kcal/mole and 6.31 e. u. respectively at 25°C. The molecular extinction coefficient and Sendall's sensitivity is 5000 and 0.0140 μg of lead/cm². The effect of diverse ions was examined with eighteen cations and ten anions, in the determination of lead.     

A new conformer of 20-hydroxyecdysone from Sesuvium portulacastrum: an x-ray crystallographic study

X-ray single crystal analysis of 20-hydroxyecdysone (20E, 2), an important insect moulting hormone, isolated from Sesuvium portulacastrum Linn. was found to exhibit a unique stereochemical configuration revealing it to be a different conformer (polymorph) possessing the aliphatic chain atoms in a trans configuration. Moreover, the analysis also revealed the presence of three molecules of water of crystallisation thereby restricting the freedom of the aliphatic side chain.     

Extraction and extraction chromatographic separation of minor actinides from sulphate bearing high level waste solutions using CMPO

Bench-Scale studies on the partitioning and recovery of minoractinides from the actual and synthetic sulphate-bearing high level waste (SBHLW) solutions have been carried out by giving two contacts with 30% TBP to deplete uranium content followed by four contacts with 0.2M CMPO+1.2M TBP in dodecane. The acidity of the SBHLW solutions was about 0.3M. In the case of actual SBHLW, the final raffinate contained about 0.4% -activity originally present in the HLW, whereas with synthetic SBHLW the -activity was reduced to the background level.¹⁴⁴Ce is extracted almost quantitative in the CMPO phase,¹⁰⁶Ru about 12% and¹³⁷Cs is practically not extracted at all. The extraction chromatographic column studies with synthetic SBHLW (aftertwo TBP contacts) has shown that large volume of waste solutions could be passed through the column without break-through of actinide metal ions. Using 0.04M HNO₃>99% Am(III) and rare earths could be eluted/stripped. Similarly >99% Pu(IV) and U(VI) could be eluted.stripped using 0.01M oxalic acid and 0.25M sodium carbonate, respectively. In the presence of 0.16M SO ₄ ^2– (in the SBHLW) the complex ions AmSO ₄ ⁺ , UO₂SO₄, PuSO ₄ ²⁺ and Pu(SO₄)₂ were formed in the aqueous phase but the species extracted into the organic phase (CMPO+TBP) were only the nitrato complexes Am(NO₃)₃·3CMPO, UO₂(NO₃)₂·2CMPO and Pu(NO₃)₄·2CMPO. A scheme for the recovery of minor actinides from SBHLW solution with two contacts of 30% TBP followed by either solvent extraction or extraction chromatographic techniques has been proposed.     

Thermal behaviour of copper oxalate and its mixed ligand complexes with amines

The thermal decomposition behaviour of copper oxalate hemihydrate and its complexes with different amines has been studied by TG and DTA methods in nitrogen and air atmospheres. The complexes CuC₂O4 · 0.5 H₂O, CuC₂O₄(NH₃)₂ and CuC₂O₄(py) produced Cu₂O as final residue after decomposition, whereas, another group of complexes, CuC₂O₄(EtNH₂)₂, CuC₂O₄(MeNH₂)₂, CuC₂O₄(en)₂ and CuC₂O₄(An)₂, gave a final residue of CuO. All the complexes decomposed without forming any isolable stable intermediate; an exception was CuC₂O₄(en)₂, which formed a binuclear intermediate complex, [CuC₂O₄en]₂.     

Structure elucidation and X-ray crystallographic study of 3-(naphth-1-ylmethyl)indole

Synthesis and structure determination of 3-(naphth-1-ylmethyl)indole are reported. The molecular and crystal structures together with the molecular formula were determined by spectral and single crystal X-ray studies. X-ray crystallography revealed the presence of two conformers arising from the flipping of the naphthalene unit. The crystal of the compound belongs to the triclinic crystal system and space group \(P\bar 1\). Crystal data are as follows: a = 10.302(5) Å, b = 12.522(4) Å, c = 13.383(4) Å, α = 111.9(1)°, β = 116.86(6)°, γ = 71.65(5)°; V = 1726.429 ų; Z = 4. The final R and Rw are R = 0.0744 (on observed F′s); R = 0.0924 (all F data), R _w = 0.1757 (observed F ²) and R _w = 0.1834 (all F ² data).     

Kinetic study of the oxidation of thioacids by pyridinium fluorochromate

Summary Kinetics of oxidation of thioglycollic, thiolactic and thiolmalic acids by pyridinium fluorochromate (PFC) to the corresponding disulphides have been studied using a stopped-flow technique. The reaction is first order with respect to PFC; Michaelis-Menten type kinetics were observed with respect to the thioacid. The formation constants for the thioacid-PFC complexes and the rates of their decomposition at different temperatures have been evaluated. Thermodynamic parameters for complex formation and the activation parameters for their decomposition have also been calculated. The oxidation was studied in nineteen different organic solvents; analysis of the solvent effect indicated that both cation-and anion-solvating powers of the solvents play important roles. A suitable mechanism has been proposed.