• Volume 121, Issue 5

September 2009,   pages  559-950

• Foreword

• Test of theoretical models for ultrafast heterogeneous electron transfer with femtosecond two-photon photoemission data

The energy distribution of electrons injected into acceptor states on the surface of TiO2 was measured with femtosecond two-photon photoemission. Shape and relative energetic position of these distribution curves with respect to the corresponding donor states, i.e. of perylene chromophores in the first excited singlet state attached via different bridge-anchor groups to the TiO2 surface, were compared with the predictions of different theoretical models for light-induced ultrafast heterogeneous electron transfer (HET). Gerischer’s early scenario for light-induced HET was considered and two recent explicit calculations, i.e. a fully quantum mechanical analytical model and a time-dependent density functional theory model based on molecular dynamics simulations for the vibrational modes were also considered. Based on the known vibrational structure in the photoionization spectrum of perylene in the gas phase and that measured in the linear absorption spectra of the perylene chromophores anchored on the TiO2 surface the energy distribution curves for the injected electrons were fitted assuming the excitation of the dominant 0.17 eV vibrational mode in the ionized perylene chromophore leading to a corresponding Franck-Condon dictated progression in the energy distribution curves. Each individual peak was fitted with a Voigt profile where the Lorentzian contribution was taken from the time-resolved HET data and the Gaussian contribution attributed to inhomogeneous broadening. The measured room temperature energy distribution curves for the injected electrons are explained with the fully quantum mechanical model for light-induced HET with the high energy, 0.17 eV, skeletal stretching mode excited in the ionized perylene chromophore. The corresponding energy distribution of the injected electrons is fully accommodated in acceptor states on the TiO2 surface fulfilling the wide band limit.

• Some properties of electrochemical nanostructures

The physical and electronic properties of several platinum nanostructures have been investigated by density functional calculations. Particular attention has been paid to the structure of the 𝑑-band. Our results predict, that nanowires and small platinum clusters supported on Au(111) should be excellent catalysts for the hydrogen evolution reaction; a monolayer of platinum on Au(111) should also be better than pure platinum.

• Generalized Warburg impedance on realistic self-affine fractals: Comparative study of statistically corrugated and isotropic roughness

We analyse the problem of impedance for a diffusion controlled charge transfer process across an irregular interface. These interfacial irregularities are characterized as two class of random fractals: (i) a statistically isotropic self-affine fractals and (ii) a statistically corrugated self-affine fractals. The information about the realistic fractal surface roughness has been introduced through the bandlimited power-law power spectrum over limited wave numbers. The details of power spectrum of such roughness can be characterized in term of four fractal morphological parameters, viz. fractal dimension ($D_H$), lower ($\ell$), and upper (𝐿) cut-off length scales of fractality, and the proportionality factor (𝜇) of power spectrum. Theoretical results are analysed for the impedance of such rough electrode as well as the effect of statistical symmetries of roughness. Impedance response for irregular interface is simplified through expansion over intermediate frequencies. This intermediate frequency expansion with sufficient number of terms offers a good approximation over all frequency regimes. The Nyquist plots of impedance show the strong dependency mainly on three surface morphological parameters i.e. $D_H$, $\ell$ and 𝜇. We can say that our theoretical results also provide an alternative explanation for the exponent in intermediate frequency power-law form.

• Theory of coherent molecule to surface electron injection: An analytical model

Electron transfer from a molecular level to empty continuum levels of a substrate is described theoretically. Using a quasicontinuum approach to model the substrate, analytical expressions pertaining to the time-dependent probability among the various levels of the substrate is presented along with its extension to coherently excited molecular vibrational modes. Hidden time scales and dynamics are revealed in the analysis and possible experiments to observe the new results are suggested. We note the applicability of the model to the description of a variety of other phenomena that are formally similar to the electron injection problem, although pertaining to different physics.

• Partition function of nearest neighbour Ising models: Some new insights

The partition function for one-dimensional nearest neighbour Ising models is estimated by summing all the energy terms in the Hamiltonian for N sites. The algebraic expression for the partition function is then employed to deduce the eigenvalues of the basic $2 \times 2$ matrix and the corresponding Hermitian Toeplitz matrix is derived using the Discrete Fourier Transform. A new recurrence relation pertaining to the partition function for two-dimensional Ising models in zero magnetic field is also proposed.

• Simulation for memory effect of Fick’s first law

The memory effect of the Fick’s first law, expressed by $\tau(\partial J/\partial t$) = $-J$ - Dgradc, was confirmed by means of the 3D Monte Carlo simulation, where 𝜏 is the relaxation time, 𝐽 is the flux of the diffusing particles, 𝐷 is the diffusion coefficient, and 𝑐 is the concentration of the particles. The delay has been observed by chronoamperometry at a pair electrode. It behaves as if it were due to a slow electron transfer reaction. A diffusion model was composed of two cubic cells with different volumes in contact with each other by their faces, which worked as the boundary for the flux. Each cell contained one diffusing particle and solvent molecules for a given concentration. The particle moved randomly in the 3D lattice until it traversed the boundary. The number of the random steps before the traverse was equivalent to the relaxation time. It was proportional to ca 2/3 powers of the number of solvent molecules or was inversely proportional to 0.63 powers of the concentration. The relaxation time was roughly equivalent to the lapse of taking for the particle to visit every lattice site impartially.

• Graham’s law of diffusion: Quantum analogy and non-ideality

We focus attention on two equivalent forms of Graham’s law of diffusion that is valid for an ideal gas mixture. This equivalence is shown to be lost by the empirical equations of state in presence of an attractive nonideality. The modified forms are noted. We then construct a simple quantum mechanical model to simulate these results and obtain a one-to-one correspondence. We see how these equations of interest may be extended to 𝐷-dimensions. By employing the quantum model, we next observe the equivalence of the results found above with those obtained via statistical mechanics. As an added advantage, we demonstrate that the virial theorem for confined quantum stationary states retains its validity in the statistical domain too, though here the averaging scheme is correspondingly different.

• Spectrometric mixture analysis: An unexpected wrinkle

The spectrometric analysis of a mixture of two chemically and spectroscopically similar compounds is illustrated for the simultaneous spectrometric determination of caffeine and theobromine, the primary stimulants in coffee and tea, based on their ultraviolet absorbances. Their analysis indicates that such measurements may need an unexpectedly high precision to yield accurate answers, because of an artifact of inverse cancellation, in which a small noise or drift signal is misinterpreted in terms of a concentration difference. The computed sum of the concentrations is not affected.

• Electrochemical studies of redox probes in self-organized lyotropic liquid crystalline systems

Lyotropic liquid crystalline phases formed by surfactants are of special importance due to their close resemblance to biological systems. The redox reactions in such ordered media are of fundamental interest in understanding several complex processes occurring in the biological media, where the former can act as model systems. In this work, we have carried out the redox reactions of benzoquinone| hydroquinone, methyl viologen and ferrocenemethanol probes in a lyotropic hexagonal columnar phase (H1 phase) using cyclic voltammetry and electrochemical impedance spectroscopic studies. The liquid crystalline phase we have studied is made up of the non-ionic surfactant, Triton X-100 and water. Polarizing optical microscopic examination confirmed that the columnar hexagonal phase is retained even after the addition of redox probe as well as the supporting electrolyte. Our studies show a significant shift in the half-peak potentials of the redox probes in the H1 phase as compared to the solvent phase. The diffusion coefficient values for different redox probes in the H1 phase were also found to be significantly reduced when compared to the corresponding solvent media.

• Surface modification of RuO2 electrodes by laser irradiation and ion implantation: Evidence of electrocatalytic effects

RuO2 thin layers were deposited on Ti supports by thermal decomposition of RuCl3 at 400°C. Some of the samples were subjected to laser irradiation between 0.5 and 1.5 J cm-2. Some others to Kr bombardment with doses between 1015 and 1016 cm-2. Modifications introduced by the surface treatments were monitored by cyclic voltammetry and O2 evolution in H2SO4 solution. The voltammetric charge increased with surface treatment almost to the same extent for irradiation and bombardment. The electrocatalytic activity turned out much higher for Kr bombarded samples. Raw experimental data were scrutinized in an attempt to separate geometric from electronic factors. True electrocatalytic effects are clearly seen to prevail over purely surface area effects.

• Poly (vinyl alcohol) hydrogel membrane as electrolyte for direct borohydride fuel cells

A direct borohydride fuel cell (DBFC) employing a poly (vinyl alcohol) hydrogel membrane electrolyte (PHME) is reported. The DBFC employs an AB5 Misch metal alloy as anode and a goldplated stainless steel mesh as cathode in conjunction with aqueous alkaline solution of sodium borohydride as fuel and aqueous acidified solution of hydrogen peroxide as oxidant. Room temperature performances of the PHME-based DBFC in respect of peak power outputs; ex-situ cross-over of oxidant, fuel, anolyte and catholyte across the membrane electrolytes; utilization efficiencies of fuel and oxidant, as also cell performance durability are compared with a similar DBFC employing a Nafion®-117 membrane electrolyte (NME). Peak power densities of ∼30 and ∼40 mW cm-2 are observed for the DBFCs with PHME and NME, respectively. The crossover of NaBH4 across both the membranes has been found to be very low. The utilization efficiencies of NaBH4 and H2O2 are found to be ∼24 and ∼59%, respectively for the PHME-based DBFC; ∼18 and ∼62%, respectively for the NME-based DBFC. The PHME and NME-based DBFCs exhibit operational cell potentials of ∼ 1.2 and ∼ 1.4 V, respectively at a load current density of 10 mA cm-2 for ∼100 h.

• Platinum-carbon black-titanium dioxide nanocomposite electrocatalysts for fuel cell applications

New-generation Pt/C-TiO2 nanocomposite electrocatalysts for fuel cells, prepared by a heterogeneous photocatalytic method, have been characterized using techniques such as cyclic voltammetry, rotating disk electrode (RDE) voltammetry, and electrochemical impedance spectroscopy (EIS). Importantly, galvanostatic data confirm the superior stability of these materials against corrosion under anodic polarization conditions relative to commercial benchmark fuel cell electrocatalysts. EIS spectra from ETEK 5, SIDCAT 405 and SIDCAT 410 membrane electrode assemblies (MEAs) were fit to a Randles equivalent circuit with a Warburg element to show the presence of O2 transport limitation arising from the use of thicker electrodes (lower Pt loading on carbon). The use of a constant phase element (CPE) instead of pure capacitor was justified from the fit procedure as CPE represents the porous electrode system more precisely with its distributive elements. EIS spectra from Tanaka, SIDCAT 451 and SIDCAT 452 MEAs (thinner electrodes) were fit to a Randles circuit with a pure capacitor and no Warburg element. The use of a transmission line model for fitting these data independently provided information about the catalyst layer resistance while all other parameters matched well with that of the Randles circuit. The effective proton transport in cathodes was quantified using polarization data for both classes of MEAs. Trends in the previously reported performance of MEAs prepared using these electrocatalysts were justified based on the relative contributions of kinetic, Ohmic and mass transfer losses to the overall overpotential, which in turn were estimated from impedance and polarization data analyses.

• Tailoring self-assembled monolayers at the electrochemical interface

The main focus of this review is to illustrate the amenability of self-assembled monolayers (SAMs) for functionalisation with different receptors, catalytic materials, biomolecules, enzymes, antigen-antibody, etc for various applications. The review discusses initially about the preparation and characterization of SAMs and tailoring of SAMs by incorporation of suitable recognition elements. A description of how the molecular recognition is achieved through forces like electrostatic, covalent and host-guest interactions is included in the review.

• Electrochemical reduction of hydrogen peroxide on stainless steel

Electrochemical reduction of hydrogen peroxide is studied on a sand-blasted stainless steel (SSS) electrode in an aqueous solution of NaClO4. The cyclic voltammetric reduction of H2O2 at low concentrations is characterized by a cathodic peak at $-0.40$ V versus standard calomel electrode (SCE). Cyclic voltammetry is studied by varying the concentration of H2O2 in the range from 0.2 mM to 20 mM and the sweep rate in the range from 2 to 100 mV s-1. Voltammograms at concentrations of H2O2 higher than 2 mM or at high sweep rates consist of an additional current peak, which may be due to the reduction of adsorbed species formed during the reduction of H2O2. Amperometric determination of H2O2 at $-0.50$ V vs SCE provides the detection limit of 5 𝜇M H2O2. A plot of current density versus concentration has two segments suggesting a change in the mechanism of H2O2 reduction at concentrations of H2O2 ≥ 2 mM. From the rotating disc electrode study, diffusion co-efficient of H2O2 and rate constant for reduction of H2O2 are evaluated.

• Electrodeposition of BaCO3 coatings on stainless steel substrates: Oriented growth in the presence of complexing agents

Electrodeposition of BaCO3 from aminecarboxylate stabilized-Ba(HCO3)2 baths, results in oriented crystallization when the bath conditions promote the decomposition of the Ba complex. Crystal growth is predominant along the 𝑐-crystallographic axis. The crystallites orient themselves with their 𝑐-axis normal to the substrate. The crystallites exhibit three-fold twinning (trilling) consequent to the evolution of the {110} planes as planes of reflection. Pairs of trillings are seen to grow about a four-sided polygon formed by the {010} crystal faces whose centre is a point of inversion.

• A comparative electrochemical study of electrosorbed 2- and 4-mercaptopyridines and their application as corrosion inhibitors at C60 steel

The electrochemistry of self-assembled monolayers (SAMs) prepared from 2-mercaptopyridine (2 Mpy) and 4-mercaptopyridine (4 Mpy) dissolved in either water or an aqueous solution of 0.1 M H2SO4 on a polycrystalline gold electrode has been investigated in an aqueous electrolyte solution (0.5 M H2SO4) using cyclic voltammetry. Results suggest that 2 Mpy is adsorbed more strongly than 4 Mpy due to the formation of a S-Au-N chelate. The under- and over-potential deposition of copper from an aqueous solution of 0.1 M sulphuric acid is inhibited in the presence of these SAMs suggesting strong interactions between these adsorbates and the gold surface. A copper adlayer was partially displaced by adsorbing 2 Mpy and 4 Mpy. The inhibition effect of these SAMs on the corrosion of C60 steel has been investigated using electrochemical impedance measurements (EIM) in an aqueous 3.5 wt% NaCl solution. The polarization resistance and the inhibition efficiency were calculated. 2 Mpy has higher inhibition efficiency than 4 Mpy.

• A Nafion®-based co-planar electrode amperometric sensor for methanol determination in the gas phase

A co-planar electrode device, fabricated with all electrodes (working, counter and reference) on the same face of a Nafion® polymer electrolyte membrane, is proposed for the amperometric detection of gaseous methanol using Pt as the working electrode. Clear voltammetry is obtained for methanol oxidation from its vapours in equilibrium with methanol aqueous solutions, both in the absence and presence of oxygen in the gas stream. Using an appropriate pulse sequence to keep the indicator electrode active, methanol vapours in the 1-13 Torr partial pressure range (in equilibrium with methanol aqueous solutions in the 1-10% w/w concentration range) could be determined, in a constant potential, amperometric mode. The methanol detector could be operated both in a nitrogen stream and (in what is essential for practical applications) in an air atmosphere too, with estimated detection limits of 1.2 and 1.4 Torr respectively.

• X-Ray photoelectron spectroscopic investigation of phenosafranine adsorbed onto micro and mesoporous materials

The phenosafranine adsorbed onto the micro and mesoporous materials prepared by ion exchange method and interaction of the dye with host materials were studied by X-ray photoelectron spectroscopy to elucidate the influence of the host matrix on the binding energy of N 1s orbital. Core level N 1s X-ray photoelectron spectroscopy reveals the interaction between the dye and the solid surface through the hydrogen bonding between the hydrogen atoms of primary amino groups in dye molecule and the oxygen atom of surface hydroxyl groups. The strength of the hydrogen bonding depends on the nature of the solid surface. In the dye adsorbed onto the micro and mesoporous materials the interaction between adsorbed phenosafranine and the surfaces of the porous materials are found to modify the optical spectra and the excited state dynamics of the confined phenosafranine molecules. The change in photophysical properties of phenosafranine adsorbed on to the host materials on dehydration at elevated temperatures is attributed to the modification of host surface during dehydration process.

• Imaging hydrogen oxidation activity of catalyst-coated perfluoro sulfonic acid-polymer electrolyte membranes using Scanning Electrochemical Microscopy

Scanning Electrochemical Microscopy (SECM) is a unique technique for studying fast heterogeneous kinetics and to map reactivity gradients along the surface of an electrocatalyst, especially when it involves multiple surface sites of varying reactivity. It combines the dual advantages offered by ultramicroelectrode (UME) voltammetry in terms of reduced ohmic drop and insignificant double layer charging contribution with the advantages of imaging by rastering the UME across an electro-active surface. In this work, we demonstrate these distinctive features of SECM in evaluating reactivity gradients on catalyst (Pt/C) coated Nafion® films towards hydrogen oxidation activity, a reaction of immense technological relevance. Imaging has been performed in the feedback mode by allowing H2 evolution at the tip (25 𝜇m Pt UME), which is reoxidized at the substrate electrode containing Pt/C-Nafion film. Interesting distribution in H2 oxidation activity has been observed as a function of potential applied to the Pt/CNafion film. In addition, a plot of normalized tip current versus the substrate electrode potential indicates the effect of potential-induced reactivity change in the catalyst-coated membranes. The results of the present investigation are believed to be useful to H2/O2 PEM fuel cells with respect to evaluating reactivity gradients of catalyst-coated polymer electrolyte membranes, which is important to rectify problems related to catalyst utilization.

• Hydrogel membrane electrolyte for electrochemical capacitors

Polymer electrolytes are known to possess excellent physicochemical properties that are very useful for electrochemical energy systems. The mobility in polymer electrolytes is understood to be mainly due to the segmental motion of polymer chains and the ion transport is generally restricted to the amorphous phase of the polymer. Gel polymer electrolytes (GPE) that are formed using plastizicers and polymers along with ionic salts are known to exhibit liquid-like ionic conductivity while maintaining the dimensional stability of a solid matrix. In the present study, the preparation and characterization of poly(vinyl alcohol)-based hydrogel membranes (PHMEs) as electrolytes for electrochemical capacitors have been reported. Varying HClO4 dopant concentration leads to different characteristics of the capacitors. The EC comprising PHME doped with 2 M HClO4 and black pearl carbon (BPC) electrodes has been found to exhibit a maximum specific capacitance value of 97 F g-1, a phase angle value of 78°, and a maximum charge-discharge coulombic efficiency of 88%.

• Core-shell Au/Ag nanoparticles embedded in silicate sol-gel network for sensor application towards hydrogen peroxide

The electrocatalytic activity of core-shell Au$_{100-x}$Ag$_x$ ($x = 15$, 27, 46, and 60) bimetallic nanoparticles embedded in methyl functionalized silicate MTMOS network towards the reduction of hydrogen peroxide was investigated by using cyclic voltammetry and chronoamperometric techniques. Core-shell Au/Ag bimetallic nanoparticles were characterized by absorption spectra and HRTEM. The MTMOS silicate sol-gel embedded Au73Ag27 core-shell nanoparticles modified electrode showed better synergistic electrocatalytic effect towards the reduction of hydrogen peroxide when compared to monometal MTMOS-Aunps and MTMOS-Agnps modified electrodes. These modified electrodes were studied without immobilizing any enzyme in the MTMOS sol-gel matrix. The present study highlights the influence of molar composition of Ag nanoparticles in the Au/Ag bimetallic composition towards the electrocatalytic reduction and sensing of hydrogen peroxide in comparison to monometal Au and Ag nanoparticles.

• Structure transitions between copper-sulphate and copper-chloride UPD phases on Au(111)

Structure transitions between copper UPD adlayers on Au(111)-($1 \times 1$) in sulfuric acid and chloride containing electrolyte were investigated by in situ scanning tunnelling microscopy. We demonstrate that co-adsorbed sulphate ions in the ($\sqrt{3} \times \sqrt{3}$)R30° UPD adlayer are replaced by chloride ions and, depending on the halide coverage, a commensurate ($2 \times 2$) or a slightly distorted ($5 \times 5$)-like Cu-Cl UPD adlayer are formed. The stability ranges of these phases are controlled both by the electrode potential and the Cl- concentration. Phase transitions between the three UPD phases were monitored by time-resolved in situ STM. The observed structure details were attributed to mechanisms based on two-dimensional nucleation and growth processes.

• On optimal designing of low frequency polychromatic fields for facile photo-dissociation of model diatomic molecules

The dissociation of a diatomic molecule in low frequency polychromatic fields of moderate intensities is studied. Genetic Algorithm is invoked to search out a set of four optimal non-resonant frequencies ($\omega_1$ - $\omega_4$), intensities ($\epsilon_1$ - $\epsilon_4$) the and phase angles ($\delta_1 - \delta_4$), for achieving a facile photo dissociation. Time-dependent Hellmann-Feynman theorem is used to gain insight into the frequency resolved energy absorption pattern. The ‘quantum phase space’ structures indicate occurrence of bond breaking dynamics akin to the classical one.

• Quantum entanglement and teleportation using statistical correlations

A study of quantum teleportation using two and three-particle correlated density matrix is presented. A criterion based on standard quantum statistical correlations employed in the many-body virial expansion is used to determine the extent of entanglement for a $2N$-particle system. A relation between the probability and statistical parameters is established using the correlated density matrices for the particles.

• Distance dependence of fluorescence resonance energy transfer

Deviations from the usual $R^{-6}$ dependence of the rate of fluorescence resonance energy transfer (FRET) on the distance between the donor and the acceptor have been a common scenario in the recent times. In this paper, we present a critical analysis of the distance dependence of FRET, and try to illustrate the non-$R^{-6}$ type behaviour of the rate for the case of transfer from a localized electronic excitation on the donor, a dye molecule to three different energy acceptors with delocalized electronic excitations namely, graphene, a two-dimensional semiconducting sheet and the case of such a semiconducting sheet rolled to obtain a nanotube. We use simple analytic models to understand the distance dependence in each case.

• Nonadiabatic quantum wave packet dynamics of the H + H2 reaction including the coriolis coupling

The effect of coriolis coupling on the dynamics of H + H2 reaction is examined by calculating the initial state-selected and energy resolved reaction probabilities on the coupled manifold of its degenerate $2p$ ($E'$) ground electronic state. H3 in this state is prone to the Jahn-Teller (JT) instability and consequently the degeneracy is split upon distortion from its $D_{3h}$ equilibrium geometry. The orbital degeneracy is, however, restored along the $D_{3h}$ symmetry configuration and it results into conical intersections of the two JT split component states. The energetically lower adiabatic component of latter is repulsive, and mainly (rather solely’) drive the H + H2 reaction dynamics. On the otherhand, the upper adiabatic component is of bound type and can only impart non-adiabaticity on the dynamics of lower state. Comparison calculations are therefore also carried out on the uncoupled lower adiabatic sheet to assess the nonadiabatic effect. Exact quantum scattering calculations are performed by a chebyshev polynomial propagator and employing the double many body expansion potential energy surface of the electronic ground state of H3. Reaction probabilities are reported up to a total energy of ∼ 3.0 eV, slightly above the energetic minimum of the seam of conical intersections at ∼ 2.74 eV. Reaction probabilities are calculated up to the total angular momentum, $J = 20$ and for each value of 𝐽, the projection quantum number 𝐾 is varied from 0 to min ($J, K_{\text{max}}$), with $K_{\text{max}} = 4$. Probability results are compared and discussed with those obtained without the coriolis coupling.

• Quantum dynamics of vibrational excitations and vibrational charge transfer processes in H+ + O2 collisions at collision energy 23 eV

Quantum mechanical study of vibrational state-resolved differential cross sections and transition probabilities for both the elastic/inelastic and the charge transfer processes have been carried out in the H+ + O2 collisions at the experimental collision energy of 23 eV. The quantum dynamics has been performed within the vibrational close-coupling rotational infinite-order sudden approximation framework employing our newly obtained quasi-diabatic potential energy surfaces corresponding to the ground and the first excited electronic states which have been computed using ab initio procedures and Dunning’s correlation consistent-polarized valence triple zeta basis set at the multireference configuration interaction level of accuracy. The present theoretical results for elastic/inelastic processes provide an overall agreement with the available state-selected experimental data, whereas the results for the charge transfer channel show some variance in comparison with those of experiments and are similar to the earlier theoretical results obtained using model effective potential based on projected valence bond method and using semi-empirical diatomics-in-molecules potential. The possible reason for discrepancies and the likely ways to improve the results are discussed in terms of the inclusion of higher excited electronic states into the dynamics calculation.

• Effective harmonic oscillator description of anharmonic molecular vibrations

The validity of an effective harmonic oscillator approximation for anharmonic molecular vibrations is tested and compared with vibrational self consistent field and vibrational configurational interaction results. The effective harmonic oscillator is constructed variationally, by taking the trial wave function as a harmonic oscillator eigenfunction with the centroid and width parameter as variational paraeters. It is found that the effective harmonic oscillator approximation provides a description of the anharmonic eigenstates very similar to the vibrational self consistent field results. Coriolis coupling is also included in these studies.

• A new scaling algorithm for predicting vibrational spectra of polyatomic molecules

We propose here a relationship between the off-diagonal and diagonal force constants which is valid theoretically under harmonic approximation provided the 𝐿 matrix is available from some theoretical calculation using $L^TFL = \Lambda$.

• Signatures of molecular recognition from the topography of electrostatic potential

The recognition of interaction between two molecules is analysed via the topography of their molecular electrostatic potentials (MESP). The point of recognition between two species is proposed to be the geometry at which there is a change in the nature of the set of MESP critical points of one of the molecules vis-a-vis with its MESP topography at infinite separation. These results are presented for certain model systems such as pyridine and benzene dimers, cytosine-guanine and adenine-thymine base pairs in various orientations of approach of the two species.

• Computing magnetic anisotropy constants of single molecule magnets

We present here a theoretical approach to compute the molecular magnetic anisotropy parameters, $D_M$ and $E_M$ for single molecule magnets in any given spin eigenstate of exchange spin Hamiltonian. We first describe a hybrid constant $M_S$-valence bond (VB) technique of solving spin Hamiltonians employing full spatial and spin symmetry adaptation and we illustrate this technique by solving the exchange Hamiltonian of the Cu6Fe8 system. Treating the anisotropy Hamiltonian as perturbation, we compute the D$_M$ and E$_M$ values for various eigenstates of the exchange Hamiltonian. Since, the dipolar contribution to the magnetic anisotropy is negligibly small, we calculate the molecular anisotropy from the single-ion anisotropies of the metal centers. We have studied the variation of D$_M$ and E$_M$ by rotating the single-ion anisotropies in the case of Mn12Ac and Fe8 SMMs in ground and few low-lying excited states of the exchange Hamiltonian. In both the systems, we find that the molecular anisotropy changes drastically when the single-ion anisotropies are rotated. While in Mn12Ac SMM $D_M$ values depend strongly on the spin of the eigenstate, it is almost independent of the spin of the eigenstate in Fe8 SMM. We also find that the $D_M$ value is almost insensitive to the orientation of the anisotropy of the core Mn(IV) ions. The dependence of $D_M$ on the energy gap between the ground and the excited states in both the systems has also been studied by using different sets of exchange constants.

• Structure and stability of spiro-cyclic water clusters

The structure and stability of spiro-cyclic water clusters containing up to 32 water molecules have been investigated at different levels of theory. Although there exist minima lower in energy than these spiro-cyclic clusters, calculations at the Hartree-Fock level, density functional theory using B3LYP parametrization and second order Møller-Plesset perturbation theory using 6-31G and 6-311++G∗∗ basis sets show that they are stable in their own right. Vibrational frequency calculations and atoms-inmolecules analysis of the electron density map confirm the robustness of these hydrogen bonded clusters.

• Bonding, aromaticity and reactivity patterns in some all-metal and non-metal clusters

Several sandwich-like metal clusters have been studied at the B3LYP/6-311 + G level of theory. Bonding and reactivity have been analysed through various geometrical parameters and conceptual density functional theory based global reactivity descriptors. Aromaticity patterns have been understood in terms of the associated nucleus independent chemical shift values. Possibility of bond-stretch isomerism in some doped clusters is explored. Preferable sites for electrophilic and nucleophilic attacks have been identified using different local reactivity descriptors.

• Fluorine effect on pericyclic and pseudopericyclic processes: Evidences and ab initio theory

Electrocyclic ring opening (ERO) reactions of 2-pyrone, 2-pyranol and pyran and their fluoro compounds (1-6) have been studied at MP2/6-31G(𝑑) level with special emphasis on the influence of fluorine on these pericyclic/pseudopericyclic processes. Calculations clearly predict that substitution of fluorine at C6 favour the reaction both kinetically and thermodynamically. Magnetic susceptibility anisotropy ($\Delta_{\chi\text{aniso}}$), NICS(0), NBO and bond critical property (BCP) analyses clearly illustrate the following; 2-pyrone (1) and 6-fluoro-2-pyrone (2) reactions are pseudopericyclic; 6-fluoro-2-pyranol (reaction 4) corresponds to a borderline case; 2-pyranol (3) and pyran (5) and 6-fluoro pyran (6) reactions are clearly pericyclic in character. Correspondingly pseudeopericyclic reactions show up orbital disconnections and fluorine delays the occurrence of orbital disconnections on the reaction trajectory.

• Electronic structure analysis and vertical ionization energies of thiophene and ethynylthiophenes

Results from different decouplings of the electron propagator theory using MP2/6-311$g$ ($2df$, $2p$) and MP2/6-311$++g$ ($2df$, $2p$) optimized geometries have been performed to investigate first eight vertical ionization energies and the corresponding Dyson orbitals. The results computed are in good agreement with experimental ionization energies and help clear the ambiguities of experimental photoelectron spectrum (PES) assignments. Detailed examination of the 𝜋-orbital density distribution of Dyson orbitals provides clarity in PES assignments and new insights about the topology of ring 𝜋 and ethynyl $\pi_{c-c}$ electron density distribution which may be tapped for improved nonlinear optical/electrochemical response from the thiophenic conjugated polymers.

• Electronic absorption spectra and nonlinear optical properties of CO2 molecular aggregates: A quantum chemical study

We have investigated the structural aspects of several carbon dioxide molecular aggregates and their spectroscopic and nonlinear optical properties within the quantum chemical theory framework. We find that, although the single carbon dioxide molecule prefers to be in a linear geometry, the puckering of angles occur in oligomers because of the intermolecular interactions. The resulting dipole moments reflect in the electronic excitation spectra of the molecular assemblies. The observation of significant nonlinear optical properties suggests the potential application of the dense carbon dioxide phases in opto-electronic devices.

• Comparison between implicit and hybrid solvation methods for the determination of $pK_a$ of mono-protonated form of $13^2$-(demethoxycarbonyl) pheophytin 𝑎 in methanol

Both implicit solvation method (dielectric polarizable continuum model, DPCM) and hybrid solvation method (cluster-continuum model) were adopted to calculate the $pK_a$ of mono-protonated form of $13^2$-(demethoxycarbonyl) pheophytin 𝑎 (Pheo) in methanol. In the cluster-continuum model calculations, we considered only 1 solvent molecule attached explicitly and others treated implicitly whereas in the DPCM calculations all the solvent molecules were treated implicitly. DPCM calculations were carried out on Pheo, PheoH+, Pheo-CH3OH and PheoH+-CH3OH in methanol solution. The aim of these calculations was to determine the free energy changes involved in the deprotonation of PheoH+ ($\Delta G_{\text{sol}}$) and finally to obtain the corresponding $pK_a$ value. DPCM calculations were carried out employing the restricted open-shell density functional treatment (ROB3LYP) using the 6-31G(𝑑) basis set to determine the free energy of solvation of bare Pheo and PheoH+ and of the clusters, Pheo-CH3OH and PheoH+-CH3OH in methanol. In-vacuo geometries of all the species were obtained by performing optimizations at ROB3LYP level using the 6-31G(𝑑) basis. Electronic energies of all the species were then obtained by carrying out single point DFT calculations using 6-311+G(2𝑑, 2𝑝) basis set on the respective optimized geometries. Differences in thermal energy and molecular entropy were calculated by carrying out frequency calculations at ROB3LYP/STO-3G level on the optimized geometries of the truncated models. The optimized geometries of the clusters display intermolecular hydrogen bonding interactions. The $pK_a$ values of PheoH+ calculated by DFT-DPCM and cluster-continuum methods are 6.12 and 4.70 respectively while the observed value is 4.14. The hydrogen bonding interaction between the solute and the solvent can be attributed for the good performance of the cluster-continuum model over pure continuum model.

• Statistical theory of neutral protein evolution by random site mutations

Understanding the features of the protein conformational space represents a key component to characterize protein structural evolution at the molecular level. This problem is approached in a twofold manner; simple lattice models are used to represent protein structures with the ability of a protein sequence to fold into the lowest energy native conformation, quantified as the foldability, which measures the fitness of the sequence. Alternatively, a self-consistent mean-field based theory is developed to evaluate the protein neutrality through random single-point and multiple-point mutations by calculating the pair-wise probability profile of the amino acid residues in a library of sequences, consistent with a particular foldability criterion. The theory predicts the change in sequence plasticity with the foldability criterion and also correlates the effect of hydrophobic residues on the variation of the free energy surface of the protein as a function of the number of cumulative mutations. The results obtained from the theory are compared with the exact enumeration results of $3 \times 3 \times 3$ lattice protein and also with some small real proteins chosen from the protein databank. An excellent match of the results obtained from theory and exact enumeration with those of real proteins validates the range of applicability of the theory. The theory may provide a new perspective in de novo protein design, in-vivo/in-vitro protein evolution and site-directed mutagenesis experiments.

• Work distribution for a particle moving in an optical trap and non-Markovian bath

We propose a simple approach to derive an exact analytical expression of work distribution for a system consisting of a colloidal particle trapped in an optical harmonic potential well, which is being pulled at a constant velocity through a solution represented by a non-Markovian bath. The thermal environment is represented by a bath composed of an infinite set of harmonic oscillators, and a model Hamiltonian for the trapped colloidal particle is constructed by representing the interaction with the bathvia linear dissipative mechanism. We have studied the effects of pulling time, pulling speed, and the adiabatic limit. It is also observed that only at long time the total work is completely converted into dissipative work.

• Growth and decay of large fluctuations far from equilibrium

We have explored the weak noise limit of stochastic processes in nonlinear dissipative systems which admit of stable dynamical attractors in absence of noise. An interesting detailed balance’ like condition in the steady state which is manifested in the time reversal symmetry between growth and decay of fluctuation far from equilibrium, similar to what is observed in thermally equilibrated systems, is demonstrated.

• Evaluation of collective transport properties of ionic melts from molecular dynamics simulations

Molecular dynamics simulations of beryllium fluoride (BeF2) have been carried out in the canonical (NVT) ensemble using a rigid-ion potential model. The Green-Kubo formalism has been applied to compute viscosities and ionic conductivities of BeF2 melt. The computational parameters critical for reliably estimating these collective transport properties are shown to differ significantly for viscosity and ionic conductivity. In addition to the equilibrium values of these transport properties, structural relaxation times as well as high-frequency IR-active modes are computed from the pressure and charge-flux auto correlation functions (ACFs) respectively. It is shown that a network-forming ionic melt, such as BeF2, will display persistent oscillatory behaviour of the integral of the charge-flux ACF. By suitable Fourier transformation, one can show that these persistent oscillations correspond to highfrequency, infra-red active vibrations associated with local modes of the network.

• A comparative molecular dynamics study of diffusion of 𝑛-decane and 3-methyl pentane in Y zeolite

Molecular dynamics simulations are reported on the structure and dynamics of 𝑛-decane and 3-methylpentane in zeolite NaY. We have calculated several properties such as the center of mass-center of mass rdf, the end-end distance distribution, bond angle distribution and dihedral angle distribution. We have also analysed trajectory to obtain diffusivity and velocity autocorrelation function (VACF). Surprisingly, the diffusivity of 3-methylpentane which is having larger cross-section perpendicular to the long molecular axis is higher than 𝑛-decane at 300 K. Activation energies have been obtained from simulations performed at 200 K, 300 K, 350 K, 400 K and 450 K in the NVE ensemble. These results can be understood in terms of the previously known levitation effect. Arrhenious plot has higher value of slope for 𝑛-decane (5.9 kJ/mol) than 3-methylpentane (3.7 kJ/mol) in agreement with the prediction of levitation effect.

• 𝑛th-Nearest neighbour distribution functions of a binary fluid mixture

For obtaining microscopic structural information in binary mixtures, often partial pair correlation functions are used. In the present study, a general approach is presented for obtaining the neighbourhood structural information for binary mixtures in terms of nth nearest neighbour distribution (NND) functions (for $n = 1, 2, 3,\ldots$). These functions are derived from the partial pair correlation functions in a hierarchical manner, based on the approach adopted earlier by us for single component fluids. Comparison of the results with MD simulation for Lennard-Jones binary mixtures is also presented. NND functions show reasonable matching for smaller n values particularly at higher density. The average 𝑛th nearest neighbour distance shows interesting feature.

• An improved numerical approximation for the first derivative

The traditional numerical computation of the first derivative $f'(x)$ of a given function $f(x)$ of a single argument 𝑥 by central differencing is known to involve aspects of both accuracy and precision. By analysing both we arrive at an algorithm that closely approximates the most accurate answer obtainable by this method, typically with at least 9 accurate decimals, while preserving a minimal footprint. The results apply to software based on the IEEE-754 specification, and are illustrated with Excel.

• # Journal of Chemical Sciences

Volume 132, 2019
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019