My research program can be divided into three main areas:
1. Mixed quantum-classical formulation of nonlinear spectroscopy
(a) Simulating the quantum dynamics of processes occurring in classical condensed phase hosts
Starting from the mixed quantum-classical Liouville (MQCL) equation of motion, it is possible to construct practical trajectory-based algorithms for simulating the dynamics of a quantum subsystem coupled to a classical environment. Over the past few years, several methods have been successfully applied in the treatment of simple model systems, but their implementation has been less straightforward in the case of more complex systems. It is our desire to develop and implement schemes based on the MQCL equation, which overcome the pitfalls of existing algorithms when dealing with complex, many-body systems.
(b) Modeling of multidimensional nonlinear spectra
Ultrafast nonlinear spectroscopy is capable of probing molecular dynamics on the femtosecond time scale. Often the resulting spectra are complex, thereby requiring a theoretical framework for their interpretation. Moreover, with comparisons between simulated and experimentally measured spectra becoming increasingly sophisticated, the development and implementation of accurate methods for modeling spectroscopic response is timely. The MQCL approach provides a convenient way for simulating laser-driven dynamics and will thus provide a suitable platform for the development of a general framework for calculating multidimensional nonlinear spectra.
2. Applications to the multidimensional infrared spectroscopy of chemical and biological systems in nanoconfined environments
We are interested in simulating one- and two-dimensional infrared spectra to study the structure and dynamics of a wide variety of nanoconfined systems of experimental interest, some examples of which include:
(a) Nanoconfined water
In many chemical and biological systems, water molecules can be confined on nanometer length scales. Under these conditions, the molecules are in contact with different types of interfaces. Near an interface, the hydrogen bonding network of water changes considerably because it must adjust to the shape of that interface. As a result, the properties and dynamics of nanoconfined water differ substantially from those of bulk water and must therefore be studied in the presence of the interface.
(b) Nanoconfined nonaqueous polar liquid clusters
Proton transfer in nanoconfined nonaqueous polar liquid clusters represents a class of reactions that are ubiquitous in chemistry. This charge transfer reaction is strongly coupled to the polar solvent and will therefore be greatly affected by solvent confinement. For example, several experiments have shown that the proton transfer rate constant can decrease significantly upon confinement. As a result of this sensitivity, one may design materials with specific chemical purposes by simply varying properties of the cluster such as its size and shape.
(c) Hydrogen transfer in enzymatic catalysis
Hydrogen transfer reactions are ubiquitous in enzymatic catalysis. The interior of an enzyme can form a nanoconfined environment around its active site and this confinement may play an important role in its function. Studying the effects of factors such as hydrogen tunneling and enzymatic motions is crucial for a detailed understanding of the transfer mechanism.
3. Ab-initio molecular dynamics of rare chemical events in condensed phase environments
We are interested in investigating the microscopic mechanisms, energetics, and kinetics of high-barrier chemical reactions and other rare events in both liquids and solids with the aid of ab-initio molecular dynamics and metadynamics. Examples of such processes under study in our group include:
(a) Dissociation and decomposition of carbonic acid in water
The reactions of carbonic acid (H2CO3) in water are important in many chemical and biological processes such as those involved in the pH regulation in blood, CO2 transport in the lungs, and the global carbon cycle. H2CO3 undergoes dissociation (H2CO3 --> HCO3- + H+) in water, which is followed by decomposition either via the hydroxide route (HCO3- --> CO2 + OH-) or the water route (HCO3- + H3O+ --> CO2 + 2H2O). Hydrogen bonding between H2CO3 and neighbouring water molecules plays an important role in determining the mechanisms, energetics, and kinetics of these reactions, and therefore an explicit and accurate treatment of the solvation is needed.
(b) Absorption of CO2 in aqueous solutions of NH3 and alkanolamines
Amine scrubbing is the leading technology currently used for capturing CO2 from flue gases in power plants. Typically, ammonia and alkanolamines (R-NH2) are used, which react with CO2 in aqueous solution according to the reaction CO2 + 2R-NH2 --> RNHCOO- + RNH3+. Several mechanisms for this reaction have been proposed, but they are still speculative. Therefore, further theoretical studies are required since a detailed understanding of the mechanisms of aqueous CO2/R-NH2 systems is important for the development and improvement of amine scrubbing technologies.
(c) Polymorphism in solid carbonic acid
Solid carbonic acid ((H2CO3)n) has been recently proposed as a potential candidate for CO2 capture. Experimental studies have shown that it can exist in two different polymorphs (i.e., α- and β-carbonic acid), but their exact crystal structures are still unknown. Theoretical studies can aid in the search for likely crystal structures. Moreover, in order to assess the feasibility of storing CO2 as solid H2CO3, the physical and chemical stability of the various polymorphs of solid H2CO3 must be investigated under different conditions.
Galib, M.; Hanna, G. “Mechanistic Insights into the Dissociation and Decomposition of Carbonic Acid in Water via the Hydroxide Route: An ab-initio Metadynamics Study ” Journal of Physical Chemistry B, 2011, 115: 15024–15035.
Hanna, G.; Geva, E. “Signature of Nonadiabatic Transitions on the Pump-Probe Infrared Spectra of a Hydrogen-Bonded Complex Dissolved in a Polar Solvent: A Computational Study” Journal of Physical Chemistry B, 2011, 115: 5191–5200.
Hanna, G.; Geva, E. “Computational study of the signature of hydrogen-bond strength on the infrared spectra of a hydrogen-bonded complex dissolved in a polar liquid” Chemical Physics, 2010, 370: 201-207.
Hanna, G.; Geva, E. “Multi-dimensional spectra via the mixed quantum-classical Liouville method: Signatures of nonequilibrium dynamics” Journal of Physical Chemistry B, 2009, 113: 9278–9288.
McRobbie, P.; Hanna, G.; Shi Q.; Geva, E. “Signatures of nonequilibrium solvation dynamics on multi-dimensional spectra” Accounts of Chemical Research, 2009, 42: 1299-1309.
Hanna, G.; Geva, E. “Isotope effects on the vibrational relaxation and multidimensional infrared spectra of the hydrogen-stretch in a hydrogen-bonded complex dissolved in a polar liquid” Journal of Physical Chemistry B, 2008, 112: 15793-15800.
Hanna, G.; Geva, E. “Computational study of the one and two dimensional infrared spectra of a vibrational mode strongly coupled to its environment: Beyond the cumulant and Condon approximations” Journal of Physical Chemistry B, 2008, 112: 12991-13004.
Hanna, G.; Geva, E. “Vibrational energy relaxation of a hydrogen-bonded complex dissolved in a polar liquid via the mixed quantum-classical Liouville method” Journal of Physical Chemistry B, 2008, 112: 4048-4058.
Hanna, G.; Kapral, R. “Quantum-classical Liouville dynamics of proton and deuteron transfer rates in a solvated hydrogen-bonded complex” Journal of Chemical Physics, 2008, 128: 164520(1)-164520(9).
Hanna, G.; Kim, H.; Kapral, R. “Quantum-classical reaction rate theory” In Quantum Dynamics of Complex Molecular Systems, ed. by D. Micha and I. Burghardt (Springer-Verlag, Paris), 2006, 281-305.
Kim H.; Hanna, G.; Kapral, R. “Analysis of kinetic isotope effects for nonadiabatic reactions” Journal of Chemical Physics, 2006, 125: 084509(1)-084509(8).
Hanna, G.; Kapral, R. “Nonadiabatic dynamics of condensed phase rate processes” Accounts of Chemical Research, 2006, 39: 21-27.
Hanna, G.; Kapral, R. “Quantum-classical Liouville dynamics of nonadiabatic proton transfer” Journal of Chemical Physics, 2005, 122: 244505(1)-244505(11).