Broad area of research:


Theoretical and computational studies of structure, dynamics and nonlinear spectroscopy of complex chemical systems, from bulk solutions to clusters, nano-confined fluids, interfaces, reactive systems, and biological systems using classical and quantum methods.

Some specific research interests:

• Structure and dynamics of molecular solutions in bulk and confined systems
• Hydration structure and dynamics of charge defects in clusters and solutions
• Chemical reaction dynamics in condensed phases and in clusters
• Molecular properties of solid-liquid, liquid-liquid and liquid-vapour interfaces
• Structure and dynamics of supercooled and supercritical fluids
• Structure, dynamics and energetics of biological systems: Protein aggregation
• Reaction pathways and free energy landscapes of enzymatic reactions
• Theoretical vibrational spectroscopy: THz, 2D-IR and Sum Frequency Generation (VSFG) Spectroscopy
• Computer simulations of clusters, surfaces, interfaces and bulk condensed phases using classical, quantum and hybrid quantum-classical (QM/MM) methods.

We work in the broad area of structure, dynamics and theoretical spectroscopy of complex chemical systems of aqueous and non-aqueous solutions, surfaces, interfaces and confined systems, clusters, and biomolecular systems. We use both classical and quantum methods for our studies. To be more specific, some of our research interests include (i) Structure and dynamics of hydrogen bonds and their relations to vibrational spectral diffusion in associated liquids including theoretical studies of nonlinear vibrational photon echo and two-dimensional infrared spectroscopy, (ii) Dielectric and dynamical properties of electrolyte solutions at high concentrations, (iii) Structure, dynamics and polarity of molecular liquids at solid-liquid, liquid-liquid, and liquid-vapour interfaces and in confined environment including theoretical studies of surface-specific vibrational sum frequency generation spectroscopy, (iv) Solvation of hydrophobic solutes, (v) Chemical reaction dynamics in condensed phases including proton transfer processes in aqueous solutions, clusters and low-dimensional systems, (vi) Behavior of supercooled and supercritical solutions and molecular relaxation at high pressures, (vii) Ion solvation and electron localization in molecular liquids and clusters, (viii) Complex enzymatic reactions in aqueous biomolecular solutions and, also on (ix) Protein aggregation and its possible inhibition through nanoparticles. Our work includes both development of theories as well as applications of state-of-the-art simulation techniques based on modern statistical mechanical concepts.

Hydrogen bond dynamics and vibrational spectral diffusion in associated liquids
Studies of hydrogen bond dynamics in associated liquids in bulk phases and at interfaces constitute a major area of our research over past two decades. We have shown how the presence of ions affects the structure and dynamics of hydrogen bonds in aqueous ionic solutions and how such ion induced effects show up in time dependent vibrational spectroscopy. We have also done important work on the dynamics of hydrogen bonds at liquid-vapour interfaces and also on the role that hydrogen bonds play in the adsorption of molecular solutes at interfaces. We presented first principles theoretical studies of vibrational spectral diffusion, photon echo and two-dimensional infrared spectroscopy in aqueous systems and their relations to hydrogen bond fluctuations from ab initio simulations without using any empirical potentials. By combining ab initio simulations with time series analysis, we also unearthed the relations between hydrogen bond distance and stretch frequency of water molecules which is so crucial in the interpretation of time dependent spectroscopic results. We have also established the critical role that hydrogen bond dynamics plays in chemical dynamics in aqueous systems such as in proton transfer processes in liquid water, clusters, and in confined aqueous systems.

Dielectric and dynamical properties of electrolyte solutions
A major part of our work over past many years has been on the structural, dielectric and dynamical properties of electrolyte solutions at high ion concentrations. We developed molecular theories of ion atmosphere relaxation and its effects on dynamic response functions of ions and solvent molecules, frequency dependent ion conductivity, dielectric relaxation and solvation dynamics in concentrated electrolyte solutions and also on the dynamics of chemical reactions such as charge transfer and isomerization reactions occurring in solutions at finite ion concentrations. He have also done significant work on the effects of ions on single-particle, pair and collective dynamics of solvent molecules in electrolyte solutions at high ion concentrations, on the tracer diffusion of ionic and hydrophobic solutes in highly nonideal solvent mixtures and also first principles studies of the behavior of hydration shell water around complex hydrophobic solutes and ions.

Molecular liquids at interfaces and in cavity, and vibrational sum frequency generation spectroscopy
We have done extensive work on the behaviour of molecular liquids at solid-liquid interfaces and in confined environment, including calculations of nonlinear response of vibrational sum frequency generation spectroscopy. We have succeeded in developing microscopic theories and performing classical and ab initio simulations to investigate interfacial and confined dipolar liquids and electrolyte solutions. We developed a nonlinear theory of the structure of dipolar liquids near solid surfaces by using a nonlocal density functional theory. Subsequently, by means of molecular dynamics simulations and analytical theories, we showed how different the dynamics of molecular relaxation at solid-liquid interfaces and in cavity can be than that in the bulk phases and to what extent the modifications of the dynamics depend on the nature of the surfaces and on the degree of confinement. We have also shown how the dielectric constant of a solvent is reduced when it is confined in cavities of nano-dimensions. This is a very important piece of work which has huge implications in studies of many biological systems. We have also been able to unearth several molecular aspects of liquid-liquid and liquid-vapor interfaces such as the changes of polarity and hydrogen bonds and possible complex formation at interfaces that are relevant to atmospheric chemistry.

Our recent theoretical work on vibrational sum frequency generation spectroscopy (VSFG) of aqueous surfaces covered with alcohol molecules showed that the apparent red-shift of the hydrogen bonded part of the VSFG spectrum does not arise from any strengthening of hydrogen bonds or ice-like structures, rather the red shift occurs due to partial cancellation of the responses from up and down-oriented OH bonds at the interfaces. This is an important result which helped in better understanding of the structure of aqueous surfaces in presence of surfactants and links of interfacial structure to observables of nonlinear spectroscopy.

Molecular solutions under extreme conditions: Supercooled, supercritical and high pressure systems
The behavior of molecular liquids under extreme conditions has been another major area of our research. Over the past years, we have made a detailed molecular-level investigation of the effects of pressure on anomalous diffusion in supercooled water and correlated the observed diffusion anomalies with changes of hydrogen bond properties. We have also done important work on the anomalous size dependence of ion diffusion in aqueous and non-aqueous solutions and showed that the anomaly is enhanced under supercooled or very cold conditions. More recently, we have extended their research to the other side of the phase diagram and investigated at great details the hydrogen bond and residence dynamics and vibrational spectral diffusion in aqueous solutions under supercritical conditions. We showed that, unlike ambient water, for supercritical water the dynamics of vibrational spectral diffusion does not necessarily capture the hydrogen bond dynamics. Rather, an interplay between the dynamics of hydrogen bonds, dangling OH and the inertial rotation of water molecules determines the time scales of spectral diffusion in a rather subtle manner. Our group has also looked at the behavior of water molecules confined in nano-bubbles at the hybrid interfaces of graphene and diamond surfaces at varying temperatures including supercritical temperatures. The calculated vibrational spectra were found to be in good agreement with available experiments which helped to clarify the origin of various spectral features arising from confined water at varying density and temperature. Further calculations of dynamical properties of water molecules in the graphene nano-bubbles revealed the presence of strong heterogeneity in nano-environments.

Solvation and migration of protonic defects and excess electron in molecular liquids and clusters
We, along with our coworkers, developed a statistical mechanical theory combined with ab initio simulations to establish the critical role that hydrogen bond dynamics plays in proton transfer processes in aqueous solutions. The work showed that the hydroxide ion features a nonclassical hypercoordinated solvation structure and the resulting mechanism deviates substantially from the traditional mirror image picture. For one dimensional chain systems in confinement, we showed that the hydroxide ion could migrate at a faster rate than an excess proton unlike their relative mobilities in bulk water. We have also made important contributions to answer some of the key questions on electron and metal solvation in liquids such as ammonia and clusters of water and ammonia molecules. Our group has also addressed the key issue of surface versus interior states of electron and ion solvation in molecular clusters.

More recently, we have been working on extending our earlier studies of acidic and basic solutions to calculations of two-dimensional infrared (2DIR) spectra of these solutions at high concentrations with an aim to connect the dynamics of proton transfer events in these solutions to observed nonlinear vibrational spectral features, such as the dynamics of spectral diffusion and appearance of cross peaks.

Hybrid quantum-classical studies of enzymatic reactions in biological systems
We expanded his research activities to biological systems using multi-scale models. We have done significant work in this area where we have looked at the mechanism and energetics of transimination and transamination reactions in aqueous medium. We have succeeded in resolving many of the existing ambiguities associated with such reactions through calculations of the relevant free energy surfaces and exploring the mechanism using the method of metadynamics coupled with hybrid quantum-classical simulations. Their recent work, along with his coworkers, on free energy landscape and mechanistic pathways of transimination process at the active site of aspartate aminotransferase by means of hybrid quantum-classical molecular dynamics simulations combined with various enhanced sampling techniques revealed a novel proton hopping mechanism through the coenzyme pyridoxal 5’-phosphate (PLP) rather than a direct proton transfer from the enzyme to the substrate.

Protein-nanoparticle interactions and protein aggregation
The aggregation of proteins triggered by their conformational changes leads to various neurodegenerative diseases. For example, it is believed that formation of -sheets of the amyloid  (A) peptide plays a key role in its aggregation and subsequent fibrilization that leads to Alzheimer disease. We have looked at protein-nanoparticle interactions as possible means to prevent protein aggregation. In a recent work, our group looked at the interactions of A(1-42) peptide with boron nitride nanotube (BNNT) and investigated how the secondary structure of the A peptide gets affected by the nanoparticle and which, in turn, influences the amyloid oligomerization process. It was shown that the nanoparticle provides structural stability to the peptide by keeping the monomeric units separated and preserving the initial helical conformations, and thus tends to inhibit aggregation of the monomeric peptides.

Terahertz spectroscopy of water and electrolytes: Probing intermolecular interactions in bulk and at interfaces
The infrared spectroscopy in the far infrared, also known as the terahertz (THz) spectroscopy, has proved to be a viable technique to study such inter-molecular modes directly. Indeed, in recent years, the THz spectroscopic technique has been employed not only to study the hydrogen bonded intermolecular interactions in liquid water, but also in a variety of aqueous solutions containing solutes of different degrees of complexity, including battery electrolytes, ionic liquids and deep eutectic solvents. We have been working on theoretical THz spectroscopy for water and electrolyte solutions, and also of liquid-vapor interfaces of selected systems, using explicit polarizable models for the solvent and the solutes. Theoretical calculations have shown that induced dipoles play important roles in producing some of the essential features of THz spectrum of aqueous systems. The analysis of observed spectroscopic features in the THz region can be a rather complex task as the spectrum is generally broad with many complex features arising from solvent and solutes, and theoretical calculations can dissect the total spectrum into various components and help in analyzing the origins of different parts of the spectrum.

Full list of publications

 

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