NOPR Collection:

Ultrafast solvation dynamics of an ion in a restricted environment

Wed, 01 Nov 1995 00:00:00 GMT

Title: Ultrafast solvation dynamics of an ion in a restricted environment Authors: Nandi, Nilashis; Bagchi, Biman Abstract: The solvation dynamics of an excited coumarin dye molecule (C-480) enclosed within a restricted space have been studied using molecular hydrodynamic theory (MHT) and compared with the recent experimental findings. The solvation dynamics of the dye molecule within the cavity of a toroidal γ-cyclodextrin molecule have been shown to be explained only in terms of the freezing of the solvent translational modes using MHT. The results of the theoretical calculation are in good agreement with the experimental results. The inertial components of the solvation time correlation function remain the same in both the restricted environment and in the free space. These results are interesting in the light of the simulation studies of Maroncelli and Fleming [J chem Phys, 89 (1988) 5044] which concludes that the participation of the different solvation shells in controlling the dynamics are much different. The earlier studies have been reviewed and the recent findings are discussed. Page(s): 845-849

Ground and excited state proton transfer of 4-methyl- 2,6-diacetylphenol in some highly polar aprotic solvents: Interaction with base

Wed, 01 Nov 1995 00:00:00 GMT

Title: Ground and excited state proton transfer of 4-methyl- 2,6-diacetylphenol in some highly polar aprotic solvents: Interaction with base Authors: Das, Ranjan; Mitra, Sivaprasad; Nath, Debnarayan; Mukherjee, Samaresh Abstract: The proton transfer processes of the ground and excited states of 4-methyl-2, 6-diacetylphenol (MAOH) have been investigated by means of absorption, emission, nanosecond and picosecond transient emission spectroscopy at different temperatures. The fluorescence quantum yields are found to be dependent on temperature, excitation energy and added base. Unlike 4-methyl-2, 6-diformylphenol (MFOH), MAOH shows a single fluorescence band at room temperature in dimethyl sulphoxide and N,N-dimethylformamide and at 77K it does not exhibit any phosphorescence emission. The fluorescence lifetime and decay. rates of MAOH are temperature dependent and are relatively faster than those of MFOH. The solute-solvent interaction appears to play an important role in the proton transfer reaction of MAOH. Page(s): 850-856

Transfer energetics of uracil, thymine, cytosine and adenine in aqueous mixtures of lithium chloride, sodium chloride and potassium chloride

Wed, 01 Nov 1995 00:00:00 GMT

Title: Transfer energetics of uracil, thymine, cytosine and adenine in aqueous mixtures of lithium chloride, sodium chloride and potassium chloride Authors: Ganguly, Sonali; Kundu, Kiron K Abstract: Standard free energies ( G) and entropies ( S) of transfer of nucleic acid bases viz. uracil (URA), thymine (THY), cytosine (CYT) and adenine (ADE) from water to aqueous mixtures of lithium chloride, sodium chloride and potassium chloride have been evaluated from solubility measurements at different temperatures. The interaction effect of G_int, as guided by the combined effects of hydrophilic hydration (H₁H) of hydrophilic parts and hydrophobic hydration (H_bH) of hydrophobic parts of these bases in these ionic cosolvents relative to that in water, have been obtained by subtraction of the cavity effect G, _cav (i) as computed by SPT formulations from the corresponding G values. The composition profiles of G and G_int indicate that each of the bases is found to be increasingly desolvated in these aqueous electrolytes save URA and THY at higher compositions of aqueous KCl, their order being KCI < NaCI < LiCI at any of the compositions, as expected from the primary solvation effect of the cations of the ionic cosolvents. The observed T Sand TS, _int (i)-composition profiles are, however, found to be dictated by the relative 3D structure breaking effect as well as the relative H₁H_bH effect, induced by these ionic cosolvents. The observed increased desolvation of the bases reflect that the intramolecular H-bonds between the bases of the double strands of DNA/RNA helices are likely to get strengthened in these ionic cosolvent systems. Page(s): 857-865

Kinetics and mechanism of iridium (III) catalysed oxidation of formic acid by Cerium(IV) in aqueous sulphuric acid media

Wed, 01 Nov 1995 00:00:00 GMT

Title: Kinetics and mechanism of iridium (III) catalysed oxidation of formic acid by Cerium(IV) in aqueous sulphuric acid media Authors: Das, Asim K; Das, Mahua Abstract: The kinetics of oxidation of formic acid by cerium(IV) in the presence of iridium(lll) (ca. 10^-6 mol dm^{- 3}) in aqueous sulphuric acid media have been followed at different temperatures (30-50°C). At fixed [H⁺], under the conditions [HCO₂H]_T.»[Ce^IV]_T»[lr]_T, the rate of disapperance of cerium(IV) in the reaction has been found to be first order with respect to cerium(IV) and the observed first order rate constant conforms to: where [HCO₂H]_T and [Ir]_T represent the total concentrations of formic acid and iridium(III) respectively; A, B, C, D and E are constants at a particular temperature and at a fixed [H ⁺] The values of the constants (at 30°C, 1.0 mol dm^-3 sulphuric acid, l= 3.0 mol dm^-3) are: A/EE=6X10^-3 s^-1; B/D= 1.0 mol dm^{- 3}; C= 6.0, and D/E= 2.3 x 10^{- 6} mol dm^-3. Formation of a dinuclear complex involving the oxidant, catalyst and the substrate before the electron transfer step has been proposed in the reaction mechanism. The electron transfer (innersphere mechanism) occurs within the dinuclear complex to give the partially oxidised product (i.e. formate radical) which is rapidly oxidised by Ce(IV) at a fast step. Page(s): 866-870