Balu Venkataraman

Diffusion controlled reactions: Fluorescence quenching of cationic dyes by charged quenchers

Rate coefficients of diffusion‐limited fast reactions of charged reactants are predicted to be time dependent or time independent in different theoretical approaches to solve the Debye–Smoluchowski equation. Different rate coefficient expressions are also predicted in different time domains. Fluorescence quenching experiments using two cationic dyes (rhodamine B and cresyl violet) and four efficient ionic quenchers (iron complexes) in aqueous solutions were carried out to verify the theoretical prediction. A detailed data analysis of the quenched fluorescence decay over a range of concentration of the quenchers supports the prediction that the ‘‘long’’ time rate coefficient is k(t)∼a+bt−1/2. Exact agreement with the theoretical predictions is not obtained in any of the fluorophore–quencher systems described in this work. Cresyl violet–ferrocyanide and cresyl violet–ferricyanide are two systems for which the experimental values of diffusion coefficient D and distance parameter RHN are justifiably close to ...

Rotational reorientation dynamics of polar dye molecular probes by picosecond laser spectroscopic technique

Fluorescence lifetimes and rotational reorientation times for four structurally similar dye molecules—three monocations: cresyl violet, nile blue, and oxazine 720 and one neutral but polar: nile red—have been measured by picosecond time‐resolved fluorescence depolarization spectroscopy using the single‐photon counting technique, in a number of solvents, which included a wide range of alcohols, other hydrogen‐bonding liquids, and a few aprotic liquids. The rotational reorientation of the dye probes (assumed to be oblate ellipsoids) are sought to be explained in terms of the Stokes–Einstein–Debye theory and dielectric friction. The individual contributions to the rotational friction due to the above two factors were calculated using reasonable values for the molecular volume and dipole moment of the solute. The rotational behavior of all the four dyes in amides and aprotic solvents is reasonably well explained in terms of the simple stick hydrodynamic model with the ‘‘molecular volume’’ obtained by using th...

Diffusion controlled reactions: Experimental verification of the time‐dependent rate equation

The transient effect in a diffusion limited reaction is described by a time‐dependent rate coefficient: k(t)=a+b exp(c2t)erfc(ct1/2), where a, b, and c are expressed in terms of the diffusion coefficient (D), the encounter distance (R), and the absolute rate coefficient (ka); and for ionic reactions, the Onsager length (rc). Time resolved fluorescence quenching studies on cresyl violet–potassium iodide system in water confirm the validity of the above equation. The values of D and R obtained from the fluorescence quenching studies are in good agreement with the values inferred from other sources, and the value of ka is reasonable.

Electron spin‐lattice relaxation times of transient free radicals

The saturation recovery method has been used to measure the electron spin relaxation times of short‐lived free radicals in solution. The radicals were produced by continuous in situ radiolysis with a 2.8 MeV electron beam (except for two cases where stable radicals were formed by air oxidation). The ESR spectrometer used broad‐band amplification of the detected ESR signal (time constant of 100 ns) and signal averaging by means of a transient recorder and minicomputer. The magnetic field was stepped off the ESR line for alternate saturating pulses; the phase of the microwave saturating pulse could be changed by 180 ° to allow separation of the saturation recovery and free induction decay. The sensitivity was such that T1 values greater than 1 ms could be determined for samples of 1014 spins (i.e., 3×10−6 M radical in 50 ml) if the ESR linewidth was 100 mG and transient curves from 40 000 saturating pulses were averaged. Somewhat smaller values of T1 could be determined with higher radical concentrations. T...

Electron spin resonance of Eu2+ in KCl

Abstract Electron spin resonance of Eu 2+ in KCl has been observed at X -band using a Superheterodyne spectrometer. The spectrum was examined along [001] and [110] directions by rotating the magnetic field in (100) plane of the crystal. In a general orientation there are six non-equivalent sites with six sets of principal axes. The spectrum of each site is described by the following rhombic spin-Hamiltonian H = gβ H.S + B 2 0 O 2 0 + B 4 0 O 4 0 + B 6 0 O 6 0 + B 2 2 O 2 2 + B 4 2 O 4 2 + B 6 + B 6 6 O 6 6 + A h S.I where O m n are the spin operators and B m n are empirical coefficients determined by experiment. The sign of B 0 2 was determined from observations at liquid helium temperatures. The hyperfine structure arising from Eu 151 and Eu 153 isotopes have also been observed. The results can be understood on the basis of a vacancy-Eu 2+ complex in the crystal and the present results have been compared with those of Mn 2+ in KCl crystals.

Study of molecular motions in liquids by electron spin-lattice relaxation measurements, I: Semiquinone ions in hydrogen bonding solvents

AbstractThe electron spin-lattice relaxation times (T1) of a variety of semiquinone ions in hydrogen bonding solvents have been measured by the pulsed saturation recovery technique as a function of temperature (T) and viscosity (η) of the solvent. Also linewidths (ΔH) have been measured in suitable cases in such solvents at low radical concentrations (∼10−4M). It is observed that (i) the temperature and viscosity dependence ofT1 can be fitted to an equation of the form 1/T1=A(T/η)+Bexp(-ΔE/RT) whereA andB are constants and ΔE is an activation energy of the order of 1 kcal mole−1 for these systems; (ii)T1 is essentially independent of the radical concentration within the range 10−3 to 5×10−2M; (iii) the concentration independent part of the linewidth (ΔH) increases linearly with (η/T) at sufficiently low temperatures, and (iv) the (η/T) dependent part ofT1 is sensitive to the size of the semiquinone as well as that of the solvent molecule, whereas the linewidth which is proportional to (η/T) at high viscosity, low temperature region is not sensitive to the size of the semiquinone and that of the solvent. Based on these observations, it is postulated that in hydrogen bonding solvents, three types of motion contribute significantly to electron spin relaxation:(a)A restricted small step diffusional motion, not involving large changes in the orientation of the molecule, leading to the dominant viscosity dependent contributions toT1 and ΔH, due to spin rotation interaction;(b)a large amplitude reorientation of the semiquinone, coupled to translational diffusion, resulting in viscosity dependent contributions toT1 and ΔH, throughg-modulation;(c)a hindred rotation of the semiquinone within the solvent cage, contributing toT1 due to spin rotation interaction. The fact thatT1 is not sensitive to the concentration of the radicals, is ascribed to the formation of the solvent cage that prevents the close approach of radicals, thereby rendering radical-radical interactions to be weak mechanisms for relaxation, even at relatively high radical concentrations.

Magnetic field-pulsed ELDOR spectroscopy

Abstract A preliminary account is given of attempts to perform ELDOR spectroscopy utilizing a pulsed EPR spectrometer that operates in the saturation-recovery mode for measurements of electron spin-lattice relaxation times. The procedure is to saturate one of the hyperfine lines with a pulse of high microwave power, shift the magnetic field at the end of the microwave pulse within a time shorter than the electronic T 1 to the resonant value of another hyperfine component, and measure the saturation transfer that has taken place by monitoring the EPR signal intensity as a function of time as in a conventional saturation-recovery experiment. The saturation transfer has been measured as a function of the microwave pulse width and the peak power of the saturating microwave pulse in order to establish that the extent of saturation transfer is related to the extent of saturation of the transition which was directly saturated in the first instance. The studies are carried out on 2,5-ditertiary butyl p -benzosemiquinone ion in ethanol. The lack of signal-averaging techniques in these preliminary experiments forces the use of a high free radical concentration (10 −2 M ) leading to considerable overlap between hyperfine components-the spectral overlap aiding saturation transfer. However, these preliminary experiments have demonstrated the feasibility of the field-pulsed ELDOR method in the investigation of saturation transfer. By a minor modification of the pulsing circuit, it has been possible to obtain linear magnetic field scans of the order of 6 G at the maximum rate of 1 G/μsec. Since the spectrometer has a response time of the order of 3 μsec, it should be possible to obtain EPR spectra of transient species with lifetimes of the order of 10 μsec or longer by such rapid scan techniques.

Study of molecular motions in liquids by electron spin-lattice relaxation measurements. II. 2,5-Di-tert-butylsemiquinone ion in acetonitrile and tetrahydrofuran

Abstract Electron spin-lattice relaxation times ( T 1 ) have been measured by the saturation recovery technique for 2,5-di- tert -butylsemiquinone ion (2,5-DTBSQ) in acetonitrile and tetrahydrofuran in the radical concentration range 2 × 10 −3 to 10 −1 M as a function of temperature T from 300 K down to the freezing points of the solvents. The dependence of T 1 on the radical concentration C and η / T is given by 1/ T 1 = ( A 0 + AC ) T / η , where A 0 and A are constants and η is the viscosity of the solvent. The linewidths of the individual hyperfine lines decrease with decreasing temperature, suggesting the presence of exchange interaction. The observed concentration dependence of T 1 is explained by taking into account the relaxation caused by electron-electron dipolar interaction at higher radical concentrations. The fact that T 1 is proportional to η / T even at low concentrations in these solvents suggests that the semiquinone molecule undergoes only viscosity-dependent rotational motion, in contrast to the behavior in hydrogen-bonding solvents, where the temperature dependence of T 1 suggests that in addition to the viscosity-dependent motion, the molecule undergoes hindered rotation in a cage of solvent molecules.

An X-band time domain electron paramagnetic resonance spectrometer

A computer-controlled X-band time domain electron paramagnetic resonance (EPR) spectrometer, with a time resolution of the order of 0.5μsec, has been constructed with many of the crucial microwave components designed and fabricated by the Microwave Engineering Group of TIFR. The spectrometer operates either in a microwave power pulsed mode for determination of spin-lattice relaxation times by the saturation recovery technique or in the kinetic mode for determination of the time dependence of EPR signal after laser excitation. It has an automatic frequency control, an automatic phase control and, most importantly, a field-frequency lock which ensures good stability of the EPR line positions enabling signal averaging for extended periods. The constructional details of the spectrometer and its performance in both the modes are described here by reporting results on certain typical systems.

Hydrogen abstraction from solvents by the triplet state of p-benzoquinone: a time-resolved electron paramagnetic resonance and laser flash photolysis study

The reaction of triplet p-benzoquinone in several solvents, such as ethanol, 2-propanol, ethylene glycol, t-butanol, 1,4-dioxane, tetrahydrofuran and cyclohexane, has been studied. The primary photochemical event was shown to be a hydrogen atom abstraction from the solvent and not an electron transfer. In the time-resolved EPR experiments, in all cases, except t-butanol and cyclohexane, the radicals from the solvent were recorded along with p-benzosemiquinone radical and the spectra were dominated by triplet mechanism of spin polarization, giving totally emissive EPR signals. The sites of abstracted hydrogen atom have been identified. In particular, whereas the methine hydrogen atom is abstracted from 2-propanol, both the methylene and hydroxyl hydrogen atoms are abstracted from ethanol. From laser flash photolysis experiments, the rate constants of the hydrogen abstraction in all solvents, except t-butanol and cyclohexane, were found to be (1–5) × 108 M−1 s−1.