• Ranjit Biswas

Articles written in Journal of Chemical Sciences

• Solvation dynamics of a charge bubble in water

A microscopic theory is used to calculate the solvation-time correlation function, (S(t)), of a light, non-stationary charge bubble in water. The calculated correlation function is found to be similar to the energy-time correlation function of a solvated electron. The ionic mobility of a charge bubble of the size of the hydrated electron is also calculated. It is found that the mobility of the charge plays a very important role in its own solvation.

• Anomalous solubility of organic solutes in supercritical water: A molecular explanation

In its supercritical state water exhibits anomalous solvent properties, the most important being its ability to solubilize organic solutes of various sizes which are sparingly soluble under ambient conditions. This phenomenon occurs at high pressure where the density is rather large (0.6–0.9 gm/cm3). In this work, a microscopic explanation for the anomalous solubility of organic substances in supercritical water is presented by using the quasi-chemical approximation of Bethe and Guggenheim. The theory suggests the enhanced anomalous solubility arises because the critical temperature of the binary mixture (waterplus organic solute) could be slightly lower than the gas-liquid critical temperature of pure water. Several exotic solvent properties may arise due to the subtle interplay between these two critical temperatures.

• Non-ideality in Born-free energy of solvation in alcohol-water and dimethylsulfoxide-acetonitrile mixtures: Solvent size ratio and ion size dependence

Recent extension of mean spherical approximation (MSA) for electrolyte solution has been employed to investigate the non-ideality in Born-free energy of solvation of a rigid, mono-positive ion in binary dipolar mixtures of associating (ethanol-water) and non-associating (dimethylsulfoxide-acetonitrile) solvents. In addition to the dipole moments, the solvent size ratio and ion size have been treated in a consistent manner in this extended MSA theory for the first time. The solvent-solvent size ratio is found to play an important role in determining the non-ideality in these binary mixtures. Smaller ions such as Li+ and Na+ show stronger non-ideality in such mixtures compared to bigger ions (for example, Cs+ and Bu4N+). The partial solvent polarization densities around smaller ions in tertiary butanol (TBA)-water mixture is found to be very different from that in other alcohol-water mixtures as well as to that for larger ions in aqueous solutions of TBA. Non-ideality is weaker in mixtures consisting of solvent species possessing nearly equal diameters and dipole moments and is reflected in the mole fraction dependent partial solvent polarization densities.

• Structural transition in alcohol-water binary mixtures: A spectroscopic study

The strengthening of the hydrogen bonding (H-bond) network as well as transition from the tetrahedral-like water network to the zigzag chain structure of alcohol upon increasing the alcohol concentration in ethanol-water and tertiary butanol (TBA) - water mixtures have been studied by using both steady state and time resolved spectroscopy. Absorption and emission characteristics of coumarin 153 (C153), a widely used non-reactive solvation probe, have been monitored to investigate the structural transition in these binary mixtures. The effects of the hydrogen bond (H-bond) network with alcohol concentration are revealed by a minimum in the peak frequency of the absorption spectrum of C153 which occur at alcohol mole fraction ∼ 0.10 for water-ethanol and at ∼ 0.04 for water-TBA mixtures. These are the mole fractions around which several thermodynamic properties of these mixtures show anomalous change due to the enhancement of H-bonding network. While the strengthening of H-bond network is revealed by the absorption spectra, the emission characteristics show the typical non-ideal alcohol mole fraction dependence at all concentrations. The time resolved anisotropy decay of C153 has been found to be bi-exponential at all alcohol mole fractions. The sharp change in slopes of average rotational correlation time with alcohol mole fraction indicates the structural transition in the environment around the rotating solute. The changes in slopes occur at mole fraction ∼ 0.10 for TBA-water and at ∼ 0.2 for ethanol-water mixtures, which are believed to reflect alcohol mole fraction induced structural changes in these alcohol-water binary mixtures.

• Excited state intramolecular charge transfer reaction in 4-(1-azetidinyl)benzonitrile: Solvent isotope effects

Excited state intramolecular charge transfer reaction of 4-(1-azetidinyl) benzonitrile (P4C) in deuterated and normal methanol, ethanol and acetonitrile has been studied in order to investigate the solvent isotope effects on reaction rates and yields. These quantities (reaction rates and yields) along with several other properties such as quantum yield and radiative rates have been found to be insensitive to the solvent isotope substitution in all these solvents. The origin of the solvent isotope insensitivity of the reaction is discussed and correlated with the observed slowing down of the solvation dynamics upon isotope substitution.

• Excited state intramolecular charge transfer reaction of 4-(morpholenyl) benzonitrile in solution: Effects of hetero atom in the donor moiety

An intramolecular charge transfer (ICT) molecule with an extra hetero atom in its donor moiety has been synthesized in order to investigate how ICT reaction is affected by hetero atom replacement. Photo-physical and photo-dynamical properties of this molecule, 4-(morpholenyl)benzonitrile (M6C), have been studied in 20 different solvents. The correlation between the reaction driving force ($-\Delta G_r$) and activation barrier ($\Delta G$#) has been explored in order to understand the solvent effects (static and dynamic) on the photo-excited ICT reaction in this molecule. A Kramer’s model analysis of the experimentally observed reaction rate constants indicates a solvent-averaged activation barrier of $\sim 4 k_BT$ in the absence of solvent dynamical control. The reaction in M6C is therefore not a barrier-less reaction but close to the limit where conventional kinetics might break down.

• Excited state charge transfer reaction in (mixed solvent + electrolyte) systems: Role of reactant-solvent and reactant-ion interactions

Fluorescence spectroscopic techniques have been used to study the excited state intramolecular charge transfer reaction of 4-(1-azetidinyl)benzonitrile (P4C) in two sets of mixed solvents, (1-propanol + ethyl acetate) and (propylene carbonate + acetonitrile), in the absence and presence of a strong electrolyte, lithium perchlorate. These two sets of mixed solvent systems represent binary solvent mixtures of low and high polarities, respectively. Density, sound velocity and viscosity measurements indicate that these two mixed solvent systems are structurally different. Stronger ion-reactant interaction is evidenced in the mole fraction independence of emission frequencies in electrolyte solutions of low polar binary solvent mixtures. For both these mixtures, the reaction driving force ($-\Delta G_r$) decreases with increase in mole fraction of the relatively less polar solvent component of the mixture. Interestingly, $-\Delta G_r$ increases significantly on addition of electrolyte in low polar mixtures and exhibits mixture composition dependence but, in contrast, $-\Delta G_r$ in high polar mixtures does not sense variation in mixture composition in presence of electrolyte. This insensitivity to mixture composition for high polar mixtures is also observed for the measured reaction time constant. In addition, the reaction time constant does not sense the presence of electrolyte in the high polar solvent mixtures. The reaction time constant in low polar mixtures, which becomes faster on addition of electrolyte, lengthens on increasing the mole fraction of the relatively less polar solvent component of the mixture. These observations have been qualitatively explained in terms of the measured solvent reorganization energy and reaction driving force by using expressions from the classical theory of electron transfer reaction.

• Excited state intramolecular charge transfer reaction in non-aqueous reverse micelles: Effects of solvent confinement and electrolyte concentration

Steady state and time resolved fluorescence emission spectroscopy have been employed to investigate the effects of solvent confinement and electrolyte concentration on excited state intramolecular charge transfer (ICT) reaction in 4-(1-pyrrolidinyl) benzonitrile (P5C), 4-(1-piperidinyl) benzonitrile (P6C), and 4-(1-morpholenyl) benzonitrile (M6C) in AOT/n-heptane/acetonitrile and AOT/n-heptane/methanol reverse micelles. Dramatic confinement effects have been revealed via a huge reduction (factor ranging between 100 and 20) over bulk values of both equilibrium and reaction rate constants. A strong dependence on the size of the confinement ($W_s$) of these quantities has also been observed. $W_s$ dependent average static dielectric constant, viscosity and solvation time-scale have been determined. Estimated dielectric constants for confined methanol and acetonitrile show a decrease from the respective bulk values by a factor of 3-5 and viscosities increased by a factor of 2 at the highest $W_s$ considered. Addition of electrolyte at $W_s = 5$ for acetonitrile is found to produce a linear increase of confined solvent viscosity but leads to a non-monotonic electrolyte concentration dependence of average solvation time. Reaction rate constant is found to decrease linearly with electrolyte concentration for P5C and P6C but non-monotonically for M6C, the highest decrease for all the molecules being ∼ 20% over the value in the absence of added electrolyte in the solvent pool. The observed huge reduction in reaction rate constant is attributed to the effects of decreased solution polarity, enhanced viscosity and slowed-down solvent reorganization of the solvent under confinement in these non-aqueous reverse micelles.

• Does polar interaction influence medium viscosity? A computer simulation investigation using model liquids

Molecular dynamics simulations of model liquids interacting via Lennard-Jones (L-J) and Stockmayer (SM) interactions have been carried out to explore the effects of the longer-ranged dipole-dipole interaction on solvent viscosity and diffusion. Switching on of the dipolar interaction at a fixed density and temperature has been found to increase the viscosity over that of the LJ liquid, the extent of increase being a few percent to as large as ∼60% depending on the magnitude of the solvent dipole moment used in the SM potential. The simulated translational and rotational diffusion coefficients show strong dipole moment and temperature dependences, eventhough effects of these parameters on solvent-solvent radial distribution function are moderate. Interestingly, a partial solute-solvent decoupling is observed when the simulated translational and rotational diffusion coefficients are connected to the simulated viscosity coefficients via the Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations. In the limit of large dipole moment, simulated self-part of the van Hove correlation function at intermediate times reveals a departure from the Gaussian distribution with particle displacement. This suggests that dynamic heterogeneity is one of the reasons for the departure of centre-of-mass diffusion from the SE relation in these model systems.

• Effects of acid concentration on intramolecular charge transfer reaction of 4-(azetidinyl) benzonitrile in solution

Effects of acid concentration on excited state intramolecular charge transfer reaction of 4-(azetidinyl) benzonitrile (P4C) in aprotic (acetonitrile and ethyl acetate) and protic (ethanol) solvents have been studied by means of steady state absorption and fluorescence, and time resolved fluorescence spectroscopic techniques. While absorption and fluorescence bands of P4C have been found to be shifted towards higher energy with increasing acid concentration in acetonitrile and ethyl acetate, no significant dependence has been observed in ethanolic solutions. Reaction rate becomes increasingly slower with acid concentration in acetonitrile and ethyl acetate. In contrast, acid in ethanolic solutions does not produce such an effect on reaction rate. Time-dependent density functional theory calculations have been performed to understand the observed spectroscopic results.

• Solvent sorting in (mixed solvent + electrolyte) systems: Time-resolved fluorescence measurements and theory

In this manuscriptwe explore electrolyte-induced modification of preferential solvation of a dipolar solute dissolved in a binary mixture of polar solvents. Composition dependence of solvation characteristics at a fixed electrolyte concentration has been followed. Binary mixtures of two different polarities have been employed to understand the competition between solute-ion and solute-solvent interactions. Time-resolved fluorescence Stokes shift and anisotropy have been measured for coumarin 153 (C153) in moderately polar (ethyl acetate + 1-propanol) and strongly polar (acetonitrile + propylene carbonate) binary mixtures at various mixture compositions, and in the corresponding 1.0M solutions of LiClO4. Both the mixtures show red shifts in C153 absorption and fluorescence emission upon increase of mole fraction of the less polar solvent component in presence of the electrolyte. In addition, measured average solvation times become slower and rotation times faster for the above change in the mixture composition. A semi-molecular theory based on solution density fluctuations has been developed and found to successfully capture the essential features of the measured Stokes shift dynamics of these complex multi-component mixtures. Dynamic anisotropy results have been analyzed by using both Stokes-Einstein-Debye (SED) and Dote-Kivelson-Schwartz (DKS) theories. The importance of local solvent structure around the dissolved solute has been stressed.

• Are N-methyl groups of Tetramethylurea (TMU) Hydrophobic? A composition and temperature-dependent fluorescence spectroscopic investigation of TMU/water binary mixtures

This paper reports results from temperature and composition dependent steady state UV-Visibleabsorption and fluorescence measurements and time-resolved fluorescence experiments, using a dipolar probecoumarin 153 (C153), in TMU/water binary mixtures.Q1Both steady state and time-resolved spectroscopic dataindicate, much like in water/alcohol mixtures, TMU-induced structural stiffening and transition of the tetrahedralH-bond network of water. The structural transition referred to above means cosolvent-induced transitionof three dimensional tetrahedral H-bond network to two dimensional zig-zag chain-like structure often foundin alcohols. A comparison to the results obtained for aqueous alcohol solutions suggests that the cosolvent concentrationat which the structural transition occurs depends both on the polarity of the cosolvent and the size ofthe cosolvent molecules. UV-Visible absorption measurements reveal aggregation among TMU molecules atlower TMU concentration which shows a temperature maximum. In addition, red edge excitation effects havebeen observed at very dilute TMU concentration suggesting distribution of C153 among heterogeneous environments.All these results indicate hydrophobic interaction-induced aggregation of TMU in dilute aqueoussolutions which corroborate well with the existing simulation observation.

• Is dynamic heterogeneity of water in presence of a protein denaturing agent different from that in presence of a protein stabilizer? A molecular dynamics simulation study

Rotational and translational dynamic heterogeneities (DHs) of ambient aqueous solutions of trimethylamine-N-oxide (TMAO) and tetramethylurea (TMU) at several solute concentrations have been investigated and compared. Motional characteristics of water molecules at solute interfaces and in bulk solutionshave been thoroughly examined for the search of slow dynamics. Note, TMAO possesses zwitterionic structure and is a protein stabilizer whereas TMU is a neutral dipolar molecule and a strong denaturant. Results suggest that water-TMAO solutions possess stronger DH than water-TMU solutions with the solute concentration dependence being stronger for TMAO than for TMU. Diffusive dynamics slows down near the solute surface for both the solutes. Solvation structure shows TMAO-water interaction is stronger than TMU-waterinteraction, producing longer H-bond fluctuation timescale in TMAO solutions. In short, this paper presents, for the first time, a systematic and comparative study of motional features and inter-species interactions between aqueous solutions containing solutes that differ in their individual impacts on protein stability.

• Microstructures and their lifetimes in acetamide/electrolyte deep eutectics: anion dependence

Extensive computer simulations with deep eutectics made of acetamide (CH₃CONH₂) and lithium salts (LiX) have been performed at 303 K and 350 K to identify the solution-phase microstructures in these media and investigate the anion dependence of the size and lifetime distributions of these microstructures. In addition, we explore how the added electrolyte interferes with the natural hydrogen bonded (H-bonded) network structure of liquid acetamide. For this purpose several radial distribution functions have been analysedand visualised. The results reveal that amide–amide H-bond interaction decreases significantly upon the addition of electrolyte, and the interactions of Li⁺ and X⁻ (X⁻ being NO⁻3 , Br⁻ and ClO⁻4 ) with CH₃CONH₂ lead to heterogeneous solution structures. Furthermore, we have obtained the cluster size and lifetime distributions in order to estimate the size of local microstructures and their stability. Both these distributions are analysed by separating the contributions arising from (a)CH₃CONH₂−CH₃CONH₂, (b)Li⁺−CH₃CONH₂ and (c) Li⁺−X⁻ interactions. The size distribution of Li⁺−X⁻ clusters is found to be different from those for the other two. Also, the lifetime distributions show a pronounced anion dependence and suggest cluster stability time up to a fewnanoseconds.

• Are water-xylitol mixtures heterogeneous? An investigation employing composition and temperature dependent dielectric relaxation and time-resolved fluorescence measurements

Aqueous xylitol solutions at six different concentrations were studied employing dielectric relaxation (DR) and time-resolved fluorescence (TRF) measurements in the temperature range 295–323 K. The focus was to explore the solution heterogeneity aspect via monitoring the viscosity coupling of the average relaxation rates at various temperatures. TRF measurements were done using both hydrophobic and hydrophilic probes to explore the preferences, if any, for solute locations in these binary mixtures. Energy-selective population excitations and the corresponding fluorescence emissions did not suggest any significant spatial heterogeneity in solution structure within the lifetimes of these probes. DRmeasurements and TRF experiments indicated mild deviations from the hydrodynamic viscosity dependence of themeasured relaxation rates. All these suggestmild spatiotemporal heterogeneity for these water-xylitol mixtures in the temperature range considered. In addition, DRtimescales appear to originate from reorientational and H-bond relaxation dynamics, excluding the possibilityof full molecular rotations

• Dielectric relaxations of molten acetamide: dependence on the model interaction potentials and the effects of system size

Molecular dynamics simulations of dielectric relaxations (DRs) in neat molten acetamide(CH3CONH2) at ~358 K have been carried out by employing two different versions of the OPLS force fieldparameters, namely, the OPLS-UA (united-atom) and the OPLS-AA (all-atom) model interactions. Threesystems consisting of 250, 500, and 1000 molecules have been studied to examine the impact of system sizeon the simulated dielectric properties. A comparison between our simulation predictions and the experimentalDR data in the MHz-GHz frequency regime reveals that the OPLS-UA interaction parameters betterreproduce the experimental static dielectric constant, whereas the OPLS-AA interaction describes well themeasured DR time constants. Moreover, a weak system size dependence has been observed. A Cole-Cole plotof the simulated and experimental dielectric spectra reveal non-Debye nature of liquid acetamide and corroborateswell with the earlier observation on the collective single-particle reorientational relaxation of liquidacetamide. The simulated single dipole reorientation dynamics also reflects this weak non-Debye nature andreveals its contribution to the collective polarization relaxation. Simulation results obtained here set the rightground for investigating the colossal dielectric constant (~106) of ionic acetamide deep eutectics reportedearlier via DR measurements in the KHz-MHz regime.

• Why do some reactions possess similar reaction rate in wildly different viscous media? A possible explanation via frequency-dependent friction

We explore herein the interconnection between the collective intermolecular solvent modes(CIM) and ultrafast reaction rate, assuming that frequency-dependent solvent friction controls the rate of suchreactions. We attempt to find a possible explanation for the observed near-insensitivity of ultrafast reactionrates (for example, charge transfer reaction) to the medium viscosity. Results are presented here by employingan analytical scheme that estimates the high-frequency solvent frictional response. Representative roomtemperature reaction media considered here are an ionic liquid (BMIMPF6, η~310cP), a dipolar solvent(ethanol, η~1:09cP) and a deep eutectic solvent (Acetamide+ LiBr, η~1950cP). It is found that the wavenumber and frequency-dependent rotational friction, ГR(ƙ,ᴢ), estimated by using the available experimental dielectric relaxation (DR) data for the ionic liquid and the deep eutectic solvent (DES), cannot predictthe viscosity independence of ГR(ƙ,ᴢ) at high frequency. Missing dispersion in the DR data of the DES appears to be critical and incorporation of this missing amplitude via collective solvent intermolecular modescentered around 100 cm-1 markedly improves the high-frequency behaviour of ГR(ƙ,ᴢ): Subsequently, thecalculated ГR(ƙ,ᴢ) for these solvents at high frequency exhibits near-insensitivity to medium viscosity andexplain the viscosity independence of ultrafast reaction rates.

Comparison of the frequency dependent rotational friction among three different solvent systems considered – normal liquid (ethanol), deep eutectic solvent (acetamide/LiBr) and ionic liquid ([BMIM][BF6]). Note the high frequency friction values for these solvents are very similar, although their zero frequency values differ quite considerably because of widely different viscosities.

• # Journal of Chemical Sciences

Volume 134, 2022
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• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019