NOPR Collection:

Triplet state and semiquinone free radical of 5-methoxyquinizarin : A laser flash photolysis and pulse radiolysis study

1992-11-01T00:00:00Z

Title: Triplet state and semiquinone free radical of 5-methoxyquinizarin : A laser flash photolysis and pulse radiolysis study Authors: Pal, H; Palit, D K; Mukherjee, T; Mittal, J P Abstract: The triplet(T) state properties like T-T absorption spectra, quantum yield, energy level and decay kinetics of 5-methoxyquinizarin (5-methoxy-1,4-dihydroxy-9,10-anthraquinone; MQZ) have been investigated in cyclohexane, acetonitrile and isopropyl alcohol using nanosecond laser flash photolysis technique. In isopropyl alcohol, a neutral semiquinone radical is also formed which has been characterised by comparing the long lived transient spectra with the MQZ-semiquinone spectra obtained by pulse radiolysis of MQZ in the same solvent. A relatively small amount of a long lived transient formed in cyclohexane and acetonitrile, along with the triplet state of MQZ, could not be characterized unambiguously, but has been attributed to the semiquinone radical of MQZ, produced by the reaction of the excited states of the quinone with the solvent. The quantum yield of the semiquinone radical in isopropyl alcohol is considerably higher than the triplet quantum yield, showing that both the excited singlet and the triplet states of the quinone probably react with the solvent molecules to form the semiquinone radical. The photophysical properties of the triplet and the semiquinone radical of MQZ have been compared with those of simple 1,4-disubstituted anthraquinones. Page(s): 811-816

Thermal and spectral studies on cellulose arylphosphinites in air

1992-11-01T00:00:00Z

Title: Thermal and spectral studies on cellulose arylphosphinites in air Authors: Bhagwan, Jai; Lal, Krishan Abstract: The thermal behaviour of cellulose and cellulose arylphosphinite samples have been studied using DTA, TG and DTG techniques from ambient temperature to 700°C in dynamic air atmosphere. First exotherms in DTA curves of cellulose arylphosphinite samples attributable to the decomposition process have been found to be lower than that of cellulose. Various kinetic and thermodynamic parameters have been obtained from TG curves using Broido method and transition state theory. For the decomposition stage of cellulose arylphosphinite samples the values of the activation energies are found to be in the range 83-138 kJ mol^-1 and are lower than that of cellulose (187 kJ mol^-1). The EPR signals have given indication of the formation of trapped and stable free radicals in the thermal degradation of cellulose and cellulose arylphosphinite samples. Higher char yields of cellulose arylphosphinite samples as compared to cellulose lead to the conclusion that cellulose arylphosphinites are good flame retardants. A flame-retardancy mechanism for the thermal degradation of cellulose arylphosphinites has been proposed. Page(s): 817-823

Medium effects on the electron-transfer processes: Reduction of aquapentaamminecobalt(III) by hexacyanoferrate(II) in ethyleneglycol-water, ethylene carbonate-water and urea-water media

1992-11-01T00:00:00Z

Title: Medium effects on the electron-transfer processes: Reduction of aquapentaamminecobalt(III) by hexacyanoferrate(II) in ethyleneglycol-water, ethylene carbonate-water and urea-water media Authors: Dash, Anadi C; Dash, Neelamadhab; Pradhan, Jyotsnamayee Abstract: The outer-sphere electron transfer between Co(NH₃)₅OH and Fe(CN) - which proceeds via precursor ion-pair formation has been investigated in ethylene glycol-water (0-47.6 wt% of EG), ethylene carbonate-water (0-40 wt% of EC) at 20-35ᵒC and in urea-water (0-19.4 wt% of EG) at 25ᵒC (I = 0.50 mol dm^-3 NaNO₃)· The electron transfer rate constant for the ion pair (k, s^-1) follows the order H₂O ~ U-H₂O < EG-H₂O << EC-H₂O. The specific solvent effect on the rate constant for EGH₂O and EC-H₂O media can be account d for in the frame work of the Marcus theory by considering the ion solvent dipole interaction in the solvent cospheres of the precursor and the successor ionpairs. Both H^≠ and S^≠ decrease linearly with increase in mole fraction of EC(X_EC) while for the EG-H₂O media a broad maximum s discernible in the plot of H^≠ (or S^≠)versus X_EG indicating thereby that the solvent structural effects might mediate the activation process. Page(s): 824-831

Kinetics and mechanism of acid and base hydrolysis of cis-(bioxalato) (imidazole)bis(ethylenediamine)cobalt (III) ion: A study of the role of the coordinated imidazole and its anion

1992-11-01T00:00:00Z

Title: Kinetics and mechanism of acid and base hydrolysis of cis-(bioxalato) (imidazole)bis(ethylenediamine)cobalt (III) ion: A study of the role of the coordinated imidazole and its anion Authors: Dash, Anadi C; Nanda, Rabindra K; Acharya, Achyuta N Abstract: The aquation and base hydrolysis of cis-[(en)₂(imH)CoC₂O₄H]² + (imH = imidazole) have been investigated in 1.0 mol dm^-3 (CIO), ionic strength. The aquation of the complex at [H⁺] = 0.50-1.0 mol dm^-3 (60°C) proceeds via spontaneous and H⁺ catalysed paths, k_obs = k + K_H[H⁺], with k = (1.2± 0.2) x 10^-6 s^-1 and k_H= (3.2± 0.4)10^-6 dm³ mol^-1s^-1. In the range [OH^-]_T= 0.010-1.0 mol dm^-3 and 15-35°C, the complex exists in doubly deprotonated form, cis-[(en)₂Co(im)C₂O₄] and the pseudo-first order rate constant fits the relationship, k_obs, = k₀ + k₁[OH^-] + k₂[OH^-]². The rate constants (k₀, k₁, and k and the associated activation parameters (H^≠ and S^≠) arc reported. Comparison of k₀ and k show that imidazolate anion (im^-) has stronger σ-bonded labilisation of Co - O bond relative to imidazole. Base hydrolysis in 50% medium results in rate acceleration relative to aqueous medium, and k_obs = k₁[OH^-l]_T is obeyed at [OH^-]_T = 0.050-0.70 mol dm^-3 (30°C). Page(s): 832-837