Denise Koch - Academia.edu (original) (raw)

Papers by Denise Koch

Aps Meeting Abstracts, 2004

ABSTRACT We present a theoretical study of the photodissociation dynamics of NaI(H2O)n [n=1-4] cl... more ABSTRACT We present a theoretical study of the photodissociation dynamics of NaI(H2O)n [n=1-4] clusters. The NaI system has been a prototype system for the study of photodissociation dynamics involving curve crossing of covalent and ionic states. A semiempirical valence-bond approach is employed to describe the electronic structure of NaI, while classical potentials are used for the water-water and ion-water interactions. The cluster photodissociation dynamics, including possible nonadiabatic transitions between the NaI excited and ground electronic states, are simulated with the "molecular dynamics with quantum transitions" method. We show that the excited state population decays faster with increasing cluster size, because of the dynamical stabilization of the outer, ionic branch of the excited state potential by solvent molecules. As observed previously for NaI(H2O), the reversed polarity of NaI in the Franck-Condon region of the excited state causes the evaporation of 95reaches the curve crossing region, i.e. within 200 fs of excitation. We discuss possible probe schemes and time-resolved photoelectron spectroscopy in order to monitor the cluster photodissociation in time and make a connection with experiment.

This thesis provides a detailed and thorough theoretical investigation of the solvation structure... more This thesis provides a detailed and thorough theoretical investigation of the solvation structure of ions in water clusters and of solvation effects on photochemically-induced electron transfer processes occurring in seeded aqueous clusters. NaI(H 2 O) n clusters were chosen as a model system for the latter because the electronic structure of NaI is characterized by a curve crossing of ionic and covalent states, and the presence of solvent molecules can significantly affect the NaI electronic structure and photodissociation dynamics due to the differential solvation of these two states. Furthermore, the surface solvation state adopted by large halide ions determines to a great extent the solvation structure of alkali-metal halides in water clusters, and therefore significantly affect their photochemistry. The first-ever rigorous investigation of the solvation thermodynamics of halide-water clusters, presented here, reveals that entropy and polarization drive the ion from a surface to interior solvation structure by cluster size 20 for fluoride, and cluster size 60 for the heavier halides. The outcome of the simulations seems to depend strongly on the choice of model used to describe the system intermolecular interactions, and an array of first-principles simulation methodologies has been designed accordingly. Designing models that allow for solvent polarization and computationally efficient semiempirical methods that can properly describe weak interactions is shown to be essential throughout the thesis. Nonadiabatic simulation techniques were developed, in combination with an hybrid quantum-mechanics/molecular mechanics (QM/MM) model to describe intermolecular interactions, in order to investigate the photodissociation dynamics of NaI(H 2 O) n clusters. Simulation results suggest that the addition of only a few water molecules is sufficient to completely quench the oscillatory NaI dynamics observed in the gas phase, but that the process is dominated by rapid water evaporation. As a result, electron transfer in NaI(H 2 O) n is largely governed by the NaI large-amplitude motion, like in the gas phase, and the solvent only influences the nonadiabatic dynamics by mediating the Nal internuclear separation at which curve crossing occurs. When embedded in an argon matrix, however, the NaI(H 2 O) n nonadiabatic dynamics appears to involve an activationless or activated inverted electron transfer process along the solvent coordinate analogous to what may occur in solution.

Various methodologies to simulate molecular dynamics and complex chemical reaction dynamics are r... more Various methodologies to simulate molecular dynamics and complex chemical reaction dynamics are reviewed. In principle, the most accurate description of the time evolution of a system would be obtained by solving the time-dependent Schrödinger equation for the system of interest. This equation, however, is too difficult to solve for high-dimensional systems, such that various approximations are generally introduced to simulate reaction dynamics. In the first part of these proceedings, first-principles molecular dynamics simulation techniques are presented as an avenue to simulate reactions occurring on a single electronic state realistically, and application of this methodology is illustrated with simulations of the dynamics of the photochemically excited iodide-acetonitrile complex. In the second part, simulations of nonadiabatic processes, i.e., involving more than one electronic state, are discussed. Various techniques available to simulate nonadiabatic dynamics are reviewed, and...

The Journal of Physical Chemistry C, 2007

ABSTRACT In order to better understand the isomerization between HNC and HCN on icy grain (or com... more ABSTRACT In order to better understand the isomerization between HNC and HCN on icy grain (or comet nuclei) surfaces in the interstellar medium in connection with a Strecker synthesis route to glycine, B3LYP/6-31+G(d,p) calculations have been carried out on the mechanisms of direct proton transfer (PT), where water molecules play a purely solvating role, and indirect PT, where water molecules participate in a proton relay mechanism. In the direct PT mechanism, a rather high-energy barrier exists for isomerization of HNC to HCN. In the much more important indirect mechanism, a concerted PT process is possible for isomerization in the presence of several water molecules. The calculations show that three water molecules bound to HNC and HCN give rise to a ring reaction significantly favoring the isomerization, a mechanism previously found for this reaction by Gardebien and Sevin (J. Phys. Chem. A 2003, 107, 3925). Further quite important solvation effects are included in the present work by addition of explicit solvating water molecules, and by a comparison with Polarizable Continuum Model (PCM) solvation. The final calculated free-energy barrier at 50 K is 3.4 kcal/mol for the isomerization of HNC to HCN with three water molecules in a ring acting as a bridge for concerted PT and seven explicit solvating water molecules; PCM solvation of this entire system leads to a further free-energy barrier reduction of 0.8 kcal/mol. The back isomerization of HCN to HNC, however, is unlikely, with an estimated free-energy barrier of 9.5 kcal/mol at 50 K. These results imply that, on icy surfaces in the interstellar medium, the isomerization of HNC to HCN occurs relatively easily, and the implications for the Strecker synthesis of glycine are discussed.

The Journal of Physical Chemistry A, 2006

We have investigated the photodissociation dynamics of NaI(H 2 O) n [n) 1-4] clusters using the m... more We have investigated the photodissociation dynamics of NaI(H 2 O) n [n) 1-4] clusters using the molecular dynamics with quantum transitions method and a quantum mechanics/molecular mechanics description of NaI(H 2 O) n , which involves a semiempirical valence-bond approach to describe the NaI electronic structure and classical solvent-solvent and solute-solvent interaction potentials. Our simulation results show that the NaI(H 2 O) n excited-state population decay upon reaching the NaI curve-crossing region increases with cluster size due to the stabilization of the ionic branch of the NaI excited state by the surrounding water molecules, and the resulting increase in nonadiabatic transition probability. After reaching the curve-crossing region for the first time, however, the excited-state population decay resembles that of bare NaI because of rapid evaporation of 99% and 95% of the water molecules for NaI(H 2 O) and NaI(H 2 O) n [n) 2-4], respectively. This extensive evaporation is due to the reversed NaI polarity in the Franck-Condon region of the NaI first excited state, which causes strong repulsive NaI-H 2 O forces and induces rapid nonstatistical water evaporation, where product water molecules are formed more rotationally than translationally hot. A few water molecules (5% or less) remain transiently or permanently bound to NaI, forming long-lived clusters, when NaI remains predominantly ionic, i.e., remains in the excited state, after reaching the curve-crossing region. To connect simulation results with experiment, we have simulated femtosecond probe signals resulting from two-photon and one-photon excitation to the X and I NaI + probe states. In agreement with experimental findings, the probe signals resulting from the two-photon probe scheme, where excitation occurs from the covalent branch of the excited state, decay exponentially over the NaI first excited-state vibrational period, with very little evidence of long-time dynamics. The one-photon probe scheme (not used for experimental cluster studies) is shown to be less sensitive to solvation, in that excitation energies will remain similar over a range of cluster sizes, as the ionic branch of the excited state and the NaI + probe states are stabilized to the same extent by the presence of water molecules. The resulting probe signals are also more revealing of the NaI(H 2 O) n photodissociation dynamics than the two-photon probe signals, as they may allow monitoring of solvation effects on the NaI nonadiabatic dynamics and of successive evaporation of water molecules. Time-resolved photoelectron spectra provide limited additional information regarding the NaI(H 2 O) n photodissociation dynamics. A key consequence of the rapid water evaporation demonstrated here is that experimentally observed signals may arise from the photodissociation of much larger NaI(H 2 O) n parent clusters.

Chemical Physics Letters, 2002

A quantitative investigation of surface vs. interior solvation in iodide-water clusters was perfo... more A quantitative investigation of surface vs. interior solvation in iodide-water clusters was performed by evaluating the potentials of mean force and structural properties of I À ðH 2 OÞ n clusters (n ¼ 32, 64) from Monte Carlo simulations with both non-polarizable and polarizable model potentials. Simulation results clearly indicate that the iodide ion tends to reside at the surface of a water cluster of size 32, whereas entropy and polarization effects make the interior solvation state more likely for a cluster size of 64. This is consistent with previous analyses of cluster experimental and model data, which suggest a transition from surface to bulk behavior around a cluster size of 60.

… of Physical Chemistry C, 2008

... 10 have demonstrated the reliability of calculations performed with the Becke three parameter... more ... 10 have demonstrated the reliability of calculations performed with the Becke three parameter hybrid functional and the correlation functional of Lee, Yang, and Parr ... direct B and indirect A pathways out of the HCN−imine complex (I1) involve two steps; for B, one has a C−N C ...

The Journal of Physical Chemistry B, 2008

Sodium iodide has long been a paradigm for ionic and covalent curve crossing and ultrafast nonadi... more Sodium iodide has long been a paradigm for ionic and covalent curve crossing and ultrafast nonadiabatic dynamics, and our interest lies in the influence of solvation on this process. The NaI(H2O)n photodissociation dynamics are simulated with the molecular dynamics with quantum transitions method. A quantum mechanics/molecular mechanics (QM/MM) description is adopted for the NaI(H2O)n electronic states, in which a semiempirical valence bond approach is used to describe the NaI electronic structure, and a polarizable optimized potential for cluster simulations model is used to describe solute-solvent and solvent-solvent interactions. In contrast to previous work with a nonpolarizable MM model [Koch et al., J. Phys. Chem. A, 2006, 110, 1438], this approach predicts that the NaI ionic ground- to covalent first-excited-state Franck-Condon energy gaps reach a plateau by cluster size 16, in relatively good agreement with experiment and electronic structure calculations; this allows us to safely extend our previous simulations to larger cluster sizes, i.e., n > 4. The simulations suggest that the disappearance of the two-photon ionization probe signals observed in femtosecond pump-probe experiments of NaI(H2O)n, n >/= 4, is due to the shift of the NaI curve-crossing region toward larger NaI internuclear separations because of solvent stabilization of the NaI ionic state. Further, the latter causes the adiabatic ground and excited states to acquire pure ionic and covalent character, respectively, by cluster 8, resulting in NaI ionic ground-state recombination or dissociation. To make a connection with electron transfer in solution, free energy curves have been generated as a function of a solvent coordinate similar to that of solution theory. Inspection of the free energy curves together with the results of excited-state simulations reveal that the electron-transfer process in clusters is not governed by the collective motion of the solvent molecules, as in solution, but that it rather proceeds along the NaI internuclear separation coordinate, as in the gas phase. In fact, solvation in small clusters mainly influences the nonadiabatic dynamics by modulating the NaI internuclear separation at which the ionic and covalent curve-crossing region occurs. Furthermore, the simulations show that electron transfer does not occur in the inverted regime, as predicted by the free energy curves, because of the extreme nonequilibrium nature of the NaI(H2O)n photodissociation process, and the rate of electron transfer increases with cluster size and increasing solvation. Overall, this work demonstrates the importance of including polarization in realistic excited-state simulations of NaI(H2O)n relaxation.

Aps Meeting Abstracts, 2004

ABSTRACT We present a theoretical study of the photodissociation dynamics of NaI(H2O)n [n=1-4] cl... more ABSTRACT We present a theoretical study of the photodissociation dynamics of NaI(H2O)n [n=1-4] clusters. The NaI system has been a prototype system for the study of photodissociation dynamics involving curve crossing of covalent and ionic states. A semiempirical valence-bond approach is employed to describe the electronic structure of NaI, while classical potentials are used for the water-water and ion-water interactions. The cluster photodissociation dynamics, including possible nonadiabatic transitions between the NaI excited and ground electronic states, are simulated with the "molecular dynamics with quantum transitions" method. We show that the excited state population decays faster with increasing cluster size, because of the dynamical stabilization of the outer, ionic branch of the excited state potential by solvent molecules. As observed previously for NaI(H2O), the reversed polarity of NaI in the Franck-Condon region of the excited state causes the evaporation of 95reaches the curve crossing region, i.e. within 200 fs of excitation. We discuss possible probe schemes and time-resolved photoelectron spectroscopy in order to monitor the cluster photodissociation in time and make a connection with experiment.

This thesis provides a detailed and thorough theoretical investigation of the solvation structure... more This thesis provides a detailed and thorough theoretical investigation of the solvation structure of ions in water clusters and of solvation effects on photochemically-induced electron transfer processes occurring in seeded aqueous clusters. NaI(H 2 O) n clusters were chosen as a model system for the latter because the electronic structure of NaI is characterized by a curve crossing of ionic and covalent states, and the presence of solvent molecules can significantly affect the NaI electronic structure and photodissociation dynamics due to the differential solvation of these two states. Furthermore, the surface solvation state adopted by large halide ions determines to a great extent the solvation structure of alkali-metal halides in water clusters, and therefore significantly affect their photochemistry. The first-ever rigorous investigation of the solvation thermodynamics of halide-water clusters, presented here, reveals that entropy and polarization drive the ion from a surface to interior solvation structure by cluster size 20 for fluoride, and cluster size 60 for the heavier halides. The outcome of the simulations seems to depend strongly on the choice of model used to describe the system intermolecular interactions, and an array of first-principles simulation methodologies has been designed accordingly. Designing models that allow for solvent polarization and computationally efficient semiempirical methods that can properly describe weak interactions is shown to be essential throughout the thesis. Nonadiabatic simulation techniques were developed, in combination with an hybrid quantum-mechanics/molecular mechanics (QM/MM) model to describe intermolecular interactions, in order to investigate the photodissociation dynamics of NaI(H 2 O) n clusters. Simulation results suggest that the addition of only a few water molecules is sufficient to completely quench the oscillatory NaI dynamics observed in the gas phase, but that the process is dominated by rapid water evaporation. As a result, electron transfer in NaI(H 2 O) n is largely governed by the NaI large-amplitude motion, like in the gas phase, and the solvent only influences the nonadiabatic dynamics by mediating the Nal internuclear separation at which curve crossing occurs. When embedded in an argon matrix, however, the NaI(H 2 O) n nonadiabatic dynamics appears to involve an activationless or activated inverted electron transfer process along the solvent coordinate analogous to what may occur in solution.

Various methodologies to simulate molecular dynamics and complex chemical reaction dynamics are r... more Various methodologies to simulate molecular dynamics and complex chemical reaction dynamics are reviewed. In principle, the most accurate description of the time evolution of a system would be obtained by solving the time-dependent Schrödinger equation for the system of interest. This equation, however, is too difficult to solve for high-dimensional systems, such that various approximations are generally introduced to simulate reaction dynamics. In the first part of these proceedings, first-principles molecular dynamics simulation techniques are presented as an avenue to simulate reactions occurring on a single electronic state realistically, and application of this methodology is illustrated with simulations of the dynamics of the photochemically excited iodide-acetonitrile complex. In the second part, simulations of nonadiabatic processes, i.e., involving more than one electronic state, are discussed. Various techniques available to simulate nonadiabatic dynamics are reviewed, and...

The Journal of Physical Chemistry C, 2007

ABSTRACT In order to better understand the isomerization between HNC and HCN on icy grain (or com... more ABSTRACT In order to better understand the isomerization between HNC and HCN on icy grain (or comet nuclei) surfaces in the interstellar medium in connection with a Strecker synthesis route to glycine, B3LYP/6-31+G(d,p) calculations have been carried out on the mechanisms of direct proton transfer (PT), where water molecules play a purely solvating role, and indirect PT, where water molecules participate in a proton relay mechanism. In the direct PT mechanism, a rather high-energy barrier exists for isomerization of HNC to HCN. In the much more important indirect mechanism, a concerted PT process is possible for isomerization in the presence of several water molecules. The calculations show that three water molecules bound to HNC and HCN give rise to a ring reaction significantly favoring the isomerization, a mechanism previously found for this reaction by Gardebien and Sevin (J. Phys. Chem. A 2003, 107, 3925). Further quite important solvation effects are included in the present work by addition of explicit solvating water molecules, and by a comparison with Polarizable Continuum Model (PCM) solvation. The final calculated free-energy barrier at 50 K is 3.4 kcal/mol for the isomerization of HNC to HCN with three water molecules in a ring acting as a bridge for concerted PT and seven explicit solvating water molecules; PCM solvation of this entire system leads to a further free-energy barrier reduction of 0.8 kcal/mol. The back isomerization of HCN to HNC, however, is unlikely, with an estimated free-energy barrier of 9.5 kcal/mol at 50 K. These results imply that, on icy surfaces in the interstellar medium, the isomerization of HNC to HCN occurs relatively easily, and the implications for the Strecker synthesis of glycine are discussed.

The Journal of Physical Chemistry A, 2006

We have investigated the photodissociation dynamics of NaI(H 2 O) n [n) 1-4] clusters using the m... more We have investigated the photodissociation dynamics of NaI(H 2 O) n [n) 1-4] clusters using the molecular dynamics with quantum transitions method and a quantum mechanics/molecular mechanics description of NaI(H 2 O) n , which involves a semiempirical valence-bond approach to describe the NaI electronic structure and classical solvent-solvent and solute-solvent interaction potentials. Our simulation results show that the NaI(H 2 O) n excited-state population decay upon reaching the NaI curve-crossing region increases with cluster size due to the stabilization of the ionic branch of the NaI excited state by the surrounding water molecules, and the resulting increase in nonadiabatic transition probability. After reaching the curve-crossing region for the first time, however, the excited-state population decay resembles that of bare NaI because of rapid evaporation of 99% and 95% of the water molecules for NaI(H 2 O) and NaI(H 2 O) n [n) 2-4], respectively. This extensive evaporation is due to the reversed NaI polarity in the Franck-Condon region of the NaI first excited state, which causes strong repulsive NaI-H 2 O forces and induces rapid nonstatistical water evaporation, where product water molecules are formed more rotationally than translationally hot. A few water molecules (5% or less) remain transiently or permanently bound to NaI, forming long-lived clusters, when NaI remains predominantly ionic, i.e., remains in the excited state, after reaching the curve-crossing region. To connect simulation results with experiment, we have simulated femtosecond probe signals resulting from two-photon and one-photon excitation to the X and I NaI + probe states. In agreement with experimental findings, the probe signals resulting from the two-photon probe scheme, where excitation occurs from the covalent branch of the excited state, decay exponentially over the NaI first excited-state vibrational period, with very little evidence of long-time dynamics. The one-photon probe scheme (not used for experimental cluster studies) is shown to be less sensitive to solvation, in that excitation energies will remain similar over a range of cluster sizes, as the ionic branch of the excited state and the NaI + probe states are stabilized to the same extent by the presence of water molecules. The resulting probe signals are also more revealing of the NaI(H 2 O) n photodissociation dynamics than the two-photon probe signals, as they may allow monitoring of solvation effects on the NaI nonadiabatic dynamics and of successive evaporation of water molecules. Time-resolved photoelectron spectra provide limited additional information regarding the NaI(H 2 O) n photodissociation dynamics. A key consequence of the rapid water evaporation demonstrated here is that experimentally observed signals may arise from the photodissociation of much larger NaI(H 2 O) n parent clusters.

Chemical Physics Letters, 2002

A quantitative investigation of surface vs. interior solvation in iodide-water clusters was perfo... more A quantitative investigation of surface vs. interior solvation in iodide-water clusters was performed by evaluating the potentials of mean force and structural properties of I À ðH 2 OÞ n clusters (n ¼ 32, 64) from Monte Carlo simulations with both non-polarizable and polarizable model potentials. Simulation results clearly indicate that the iodide ion tends to reside at the surface of a water cluster of size 32, whereas entropy and polarization effects make the interior solvation state more likely for a cluster size of 64. This is consistent with previous analyses of cluster experimental and model data, which suggest a transition from surface to bulk behavior around a cluster size of 60.

… of Physical Chemistry C, 2008

... 10 have demonstrated the reliability of calculations performed with the Becke three parameter... more ... 10 have demonstrated the reliability of calculations performed with the Becke three parameter hybrid functional and the correlation functional of Lee, Yang, and Parr ... direct B and indirect A pathways out of the HCN−imine complex (I1) involve two steps; for B, one has a C−N C ...

The Journal of Physical Chemistry B, 2008

Sodium iodide has long been a paradigm for ionic and covalent curve crossing and ultrafast nonadi... more Sodium iodide has long been a paradigm for ionic and covalent curve crossing and ultrafast nonadiabatic dynamics, and our interest lies in the influence of solvation on this process. The NaI(H2O)n photodissociation dynamics are simulated with the molecular dynamics with quantum transitions method. A quantum mechanics/molecular mechanics (QM/MM) description is adopted for the NaI(H2O)n electronic states, in which a semiempirical valence bond approach is used to describe the NaI electronic structure, and a polarizable optimized potential for cluster simulations model is used to describe solute-solvent and solvent-solvent interactions. In contrast to previous work with a nonpolarizable MM model [Koch et al., J. Phys. Chem. A, 2006, 110, 1438], this approach predicts that the NaI ionic ground- to covalent first-excited-state Franck-Condon energy gaps reach a plateau by cluster size 16, in relatively good agreement with experiment and electronic structure calculations; this allows us to safely extend our previous simulations to larger cluster sizes, i.e., n > 4. The simulations suggest that the disappearance of the two-photon ionization probe signals observed in femtosecond pump-probe experiments of NaI(H2O)n, n >/= 4, is due to the shift of the NaI curve-crossing region toward larger NaI internuclear separations because of solvent stabilization of the NaI ionic state. Further, the latter causes the adiabatic ground and excited states to acquire pure ionic and covalent character, respectively, by cluster 8, resulting in NaI ionic ground-state recombination or dissociation. To make a connection with electron transfer in solution, free energy curves have been generated as a function of a solvent coordinate similar to that of solution theory. Inspection of the free energy curves together with the results of excited-state simulations reveal that the electron-transfer process in clusters is not governed by the collective motion of the solvent molecules, as in solution, but that it rather proceeds along the NaI internuclear separation coordinate, as in the gas phase. In fact, solvation in small clusters mainly influences the nonadiabatic dynamics by modulating the NaI internuclear separation at which the ionic and covalent curve-crossing region occurs. Furthermore, the simulations show that electron transfer does not occur in the inverted regime, as predicted by the free energy curves, because of the extreme nonequilibrium nature of the NaI(H2O)n photodissociation process, and the rate of electron transfer increases with cluster size and increasing solvation. Overall, this work demonstrates the importance of including polarization in realistic excited-state simulations of NaI(H2O)n relaxation.