S.K. Sarkar

How ions affect the structure of water: a combined Raman spectroscopy and multivariate curve resolution study.

Raman spectroscopy in combination with multivariate curve resolution (Raman-MCR) is used to explore the interaction between water and various kosmotropic and chaotropic anions. Raman-MCR of aqueous Na-salt (NaI, NaBr, NaNO3, Na2SO4, and Na3PO4) solutions provides solute-correlated Raman spectra (SC-spectra) of water. The SC-spectra predominantly bear the vibrational characteristics of water in the hydration shell of anions, because Na(+)-cation has negligible effect on the OH stretch band of water. The SC-spectra for the chaotropic I(-), Br(-), and NO3(-) anions and even for the kosmotropic SO4(2-) anion resemble the Raman spectrum of isotopically diluted water (H2O/D2O = 1/19; v/v) whose OH stretch band is largely comprised by the response of vibrationally decoupled OH oscillators. On the other hand, the SC-spectrum for the kosmotropic PO4(3-) anion is quite similar to the Raman spectrum of H2O (bulk). Comparison of the peak positions of SC-spectra and the Raman spectrum of isotopically diluted water suggests that the hydrogen bond strength of water in the hydration shell of SO4(2-) is comparable to that of the isotopically diluted water, but that in the hydration shell of I(-), Br(-), and NO3(-) anions is weaker than that of the latter. Analysis of integrated area of component bands of the SC-spectra reveals ∼80% reduction of the delocalization of vibrational modes (intermolecular coupling and Fermi resonance) of water in the hydration shell of I(-), Br(-), NO3(-), and SO4(2-) anions. In the case of trivalent PO4(3-), the vibrational delocalization is presumably reduced and the corresponding decrease in spectral response at ∼3250 cm(-1) is compensated by the increased signal of strongly hydrogen bonded (but decoupled) water species in the hydration shell. The peak area-averaged wavenumber of the SC-spectrum increases as PO4(3-) < SO4(2-) < NO3(-) < Br(-) < I(-) and indeed suggests strong hydrogen bonding of water in the hydration shell of PO4(3-) anion.

Change of boron substitution improves the lasing performance of Bodipy dyes: a mechanistic rationalisation.

Bodipy laser dyes are highly efficient but degrade rapidly in solution by reacting with in situ generated singlet oxygen ((1)O(2)). To increase the lasing lifetimes of these dyes, we have designed and synthesised two different congeners of the widely studied Pyrromethene 567 (PM567) by substitution at the boron centre and/or at both the boron centre and the meso position. The two new dyes showed high lasing efficiencies with increased photostability. The results of theoretical and pulse radiolysis studies revealed that the substitution at the boron centre reduced the (1)O(2) generation capacity of these dyes as well as their rate of reaction with (1)O(2), thereby enhancing their lifetimes even under lasing conditions.

Water in the hydration shell of halide ions has significantly reduced Fermi resonance and moderately enhanced Raman cross section in the OH stretch regions.

Water in the presence of electrolytes plays an important role in biological and industrial processes. The properties of water, such as the intermolecular coupling, Fermi resonance (FR), hydrogen-bonding, and Raman cross section were investigated by measuring the Raman spectra in the OD and OH stretch regions in presence of alkali halides (NaX; X = F, Cl, Br, I). It is observed that the changes in spectral characteristics by the addition of NaX in D2O are similar to those obtained by the addition of H2O in D2O. The spectral width decreases significantly by the addition of NaX in D2O (H2O) than that in the isotopically diluted water. Quantitative estimation, on the basis of integrated Raman intensity, revealed that the relative Raman cross section, σ(H)/σ(b) (σ(H) and σ(b) are the average Raman cross section of water in the first hydration shell of X(-) and in bulk, respectively), in D2O and H2O is higher than those in the respective isotopically diluted water. These results suggest that water in the hydration shell has reduced FR and intermolecular coupling compared to those in bulk. In the isotopically diluted water, the relative Raman cross section increases with increase in size of the halide ions (σ(H)/σ(b) = 0.6, 1.1, 1.5, and 1.9 for F(-), Cl(-), Br(-), and I(-), respectively), which is assignable to the enhancement of Raman cross section by charge transfer from halide ions to the hydrating water. Nevertheless, the experimentally determined σ(H)/σ(b) is lower than the calculated values obtained on the basis of the energy of the charge transfer state of water. The weak enhancement of σ(H)/σ(b) signifies that the charge transfer transition in the hydration shell of halide ions causes little change in the OD (OH) bond lengths of hydrating water.

Laser isotope separation of 13C: A comparative study

IR laser chemistry of (CF3Br/Cl2) mixture and neat CF2HCl are examined in the context of 13C enrichment. Decomposition extent, enrichment factor and energy absorbed are measured for both systems at their respective optimum conditions. A direct comparison is obtained by keeping extraneous factors such as laser, its pulse duration, cell, irradiation geometry etc. the same. The halogen scavenged CF3Br MPD requires lower fluence compared to neat CF2HCl irradiation. Overall throughput for a product with 60–65% 13C content in a single stage is the same for both systems requiring a similar amount of energy. However, at lower enrichment levels, CF2HCl MPD is better than (CF3Br/Cl2) photolysis in terms of both product yield and energy absorption.

Effect of donor orientation on ultrafast intermolecular electron transfer in coumarin-amine systems

Effect of donor amine orientation on nondiffusive ultrafast intermolecular electron transfer (ET) reactions in coumarin-amine systems has been investigated using femtosecond fluorescence upconversion measurements. Intermolecular ET from different aromatic and aliphatic amines used as donor solvents to the excited coumarin-151 (C151) acceptor occurs with ultrafast rates such that the shortest fluorescence lifetime component (tau(1)) is the measure of the fastest ET rate (tau(1)=tau(ET) (fast)=(k(ET) (fast))(-1)), assigned to the C151-amine contact pairs in which amine donors are properly oriented with respect to C151 to maximize the acceptor-donor electronic coupling (V(el)). It is interestingly observed that as the amine solvents are diluted by suitable diluents (either keeping solvent dielectric constant similar or with increasing dielectric constant), the tau(1) remains almost in the similar range as long as the amine dilution does not cross a certain critical limit, which in terms of the amine mole fraction (x(A)) is found to be approximately 0.4 for aromatic amines and approximately 0.8 for aliphatic amines. Beyond these dilutions in the two respective cases of the amine systems, the tau(1) values are seen to increase very sharply. The large difference in the critical x(A) values involving aromatic and aliphatic amine donors has been rationalized in terms of the largely different orientational restrictions for the ET reactions as imposed by the aliphatic (n-type) and aromatic (pi-type) nature of the amine donors [A. K. Satpati et al., J. Mol. Struct. 878, 84 (2008)]. Since the highest occupied molecular orbital (HOMO) of the n-type aliphatic amines is mostly centralized at the amino nitrogen, only some specific orientations of these amines with respect to the close-contact acceptor dye [also of pi-character; A. K. Satpati et al., J. Mol. Struct. 878, 84 (2008) and E. W. Castner et al., J. Phys. Chem. A 104, 2869 (2000)] can give suitable V(el) and thus ultrafast ET reaction. In contrary, the HOMO of the pi-type aromatic amines is largely distributed throughout the whole molecule and thus most of the orientations of these amines can give significant V(el) for ultrafast ET reactions with close-contact C151 dyes.

Oxidation Reactions of Thymol: A Pulse Radiolysis and Theoretical Study

The reactions of (•)OH and O(•-), with thymol, a monoterpene phenol and an antioxidant, were studied by pulse radiolysis technique and DFT calculations at B3LYP/6-31+G(d,p) level of theory. Thymol was found to efficiently scavenge OH radicals (k = 8.1 × 10(9) dm(3) mol(-1) s(-1)) to produce reducing adduct radicals, with an absorption maximum at 330 nm and oxidizing phenoxyl radicals, with absorption maxima at 390 and 410 nm. A major part of these adduct radicals was found to undergo water elimination, leading to phenoxyl radicals, and the process was catalyzed by OH(-) (or Na(2)HPO(4)). The rate of reaction of O(•-) with thymol was found to be comparatively low (k = 1.1 × 10(9) dm(3) mol(-1) s(-1)), producing H abstracted species of thymol as well as phenoxyl radicals. Further, these phenoxyl radicals of thymol were found to be repaired by ascorbate (k = 2.1 × 10(8) dm(3) mol(-1) s(-1)). To support the interpretation of the experimental results, DFT calculations were carried out. The transients (both adducts and H abstracted species) have been optimized in gas phase at B3LYP/6-31+G(d,p) level of calculation. The relative energy values and thermodynamic stability suggests that the ortho adduct (C6_OH adduct) to be most stable in the reaction of thymol with OH radicals, which favors the water elimination. However, theoretical calculations showed that C4 atom in thymol (para position) can also be the reaction center as it is the main contributor of HOMO. The absorption maxima (λ(max)) calculated from time-dependent density functional theory (TDDFT) for these transient species were close to those obtained experimentally. Finally, the redox potential value of thymol(•)/thymol couple (0.98 V vs NHE) obtained by cyclic voltammetry is less than those of physiologically important oxidants, which reveals the antioxidant capacity of thymol, by scavenging these oxidants. The repair of the phenoxyl radicals of thymol with ascorbate together with the redox potential value makes it a potent antioxidant with minimum pro-oxidant effects.

Real-time observation of :CF2 formation in the infrared multiple-photon dissociation of fluoroform-d

Abstract :CF 2 radicals are directly detected by transient absorption at 249 nm in the TEA CO 2 laser-induced decomposition of CDF 3 . Vibrationally hot :CF 2 radicals, T v ≈725 K, are produced in a 4–5 μs time scale. Second-order decay of :CF 2 radicals is also observed, yielding the dimerisation rate constant K 2 = 4.2 × 10 7 M −1 s −1 .

Electron beam irradiation-induced transformations in the electrical properties of 4'-octyl-4- cyanobiphenyl (8CB)

Changes in the dielectric and thermodynamical properties of electron beam-irradiated 4′-octyl-4-cyanobiphenyl (8CB) were studied. Irradiation-induced changes in the phase transition temperature, dielectric anisotropy, relaxation frequency and activation energy of an observed non-collective relaxation mode corresponding to molecular rotation about the short axis were determined in both nematic and smectic Ad phases. In the nematic phase, dielectric anisotropy increased for a small dose but decreased for a relatively high dose, whereas the relaxation frequency increased due to the irradiation. The pure and irradiated samples were characterised by UV–visible spectroscopy, Fourier transform infrared spectroscopy, gas chromatography, gas chromatography coupled with mass spectroscopy and pulse radiolysis. The observed changes in the dielectric parameters are related to the detachment of the CN group from some of the 8CB molecules due to the electron beam irradiation.

Investigation of Dynamics of Radiolytic Formation of CdSe Nanoparticles in Aqueous Solutions

The formation of cadmium selenide, CdSe, nanoparticles in aqueous solutions containing equimolar ammoniated cadmium sulfate, [Cd(NH(3))](4)SO(4) and sodium selenosulfate, Na(2)SeSO(3) as the starting materials, has been investigated by electron pulse radiolysis coupled with kinetic spectrometry. The formation of CdSe nanoparticles was found to proceed through the generation of short-lived transient intermediate species having an absorption peak at 520 nm, which is formed only upon the reaction of hydrated electrons, e(aq) with the precursor ions under deaerated conditions. The transient intermediate species decays with a weighted average rate constant, 1.2 × 10(7) s(-1). The transient intermediate species formed in the case of individual precursors did not match with the transients formed when both the precursors are taken together in the solutions under the present experimental conditions. The reaction rate constants between the precursor ions, [Cd(NH(3))(4)](2+) and the transient intermediate species formed from [SeSO(3)](2-) was 1.9 × 10(10) M(-1) s(-1). Similarly, the reaction rate constants between the precursor ions, [SeSO(3)](2-) and the transient intermediate species formed from [Cd(NH(3))(4)](2+) was 5.5 × 10(10) M(-1) s(-1). This clearly indicates that the formation of CdSe nanoparticles occurs through both reaction channels. However, the major reaction channel is through the reaction of e(aq) with the [Cd(NH(3))(4)](2+) ions (k = 3.1 × 10 (10) M(-1) s(-1)), as its rate constant is one order higher than that of the reaction of e(aq) with the [SeSO(3)](2-) ions (k = 2.3 × 10(9) M(-1) s(-1)).

Tuning of the electrical parameters of a twisted-nematic display material by using electron beam irradiation

Electron beam irradiation studies on 4′-pentyl-4-cyanobiphenyl (5CB) have been carried out. Dielectric measurements demonstrate that even a low level of irradiation (beyond a critical dose of ∼6kGy) results in a dramatic increase in the transverse component of the dielectric permittivity, and a corresponding decrease in the dielectric anisotropy. It is shown that the changes in dielectric parameters due to the irradiation can be fruitfully utilized to control the display performance of the device to a certain extent. The observed change in the dielectric parameters is attributed to the formation of a donor-acceptor type charge-transfer complex due to the irradiation.