Denise M. Koch

On the transition from surface to interior solvation in iodide–water clusters

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−(H2O)n clusters (n=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.

Mechanisms of translational and rotational energy transfer in (N2)n cluster–surface scattering

Molecular dynamics simulations were employed to investigate the dynamics of surface-induced (N2)n cluster fragmentation. The calculated translational and rotational state distributions of the monomer products of (N2)n clusters scattered off a hard surface indicate that the translational states follow a single Maxwell–Boltzmann distribution, whereas the rotational state distributions are best represented by a sum of two distinct Boltzmann distributions, in agreement with previous experimental findings obtained with a graphite surface. Analysis of the scattering dynamics provides insight into a molecular-level explanation for the differing behaviors of energy transfer to the translational and rotational modes of the monomer products. Our simulation results indicate that translational excitation of scattered products depends on the instantaneous cluster temperature at which the monomers evaporate. The obtention of two rotational distributions indicates that two rotational excitation mechanisms occur during t...

Molecular dissociation and vibrational excitation in the surface scattering of (N2)n and (O2)n clusters

Theoretical studies have predicted that the extreme conditions produced within a cluster during cluster-surface scattering could catalyze multicenter reactions with large activation barriers. However, recent experimental results did not reveal vibrational excitation or molecular dissociation in the scattering of molecular van der Waals clusters on a graphite surface. Building on our previous investigations of translational and rotational excitation, we carried out a detailed study of the mechanisms of energy transfer to the vibrational degrees of freedom of the products of (N2)n and (O2)n cluster-surface scattering by means of molecular dynamics simulations. Our results indicate that the monomer product vibrational energy distributions are best fit by a sum of two Boltzmann distributions, which suggests that two distinct thermal-like processes of vibrational excitation may be occurring during cluster scattering. The cold component of the distribution was shown to involve monomers originating from the clus...

Importance of Polarization in Quantum Mechanics/Molecular Mechanics Descriptions of Electronic Excited States: NaI(H2O)n Photodissociation Dynamics as a Case Study†

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.

Accurate ab initio potential for the Na+⋯I• complex

High-level ab initio calculations employing the multireference configuration interaction and coupled clusters methods with a correlation-consistent sequence of basis sets have been used to obtain accurate potential energy curves for the complex of the sodium cation with the iodine atom. Potential curves for the first two electronic Lambda-S states have very different characters: the potential for the 2pi state has a well depth of approximately 10 kcal/mol, while the 2sigma state is essentially unbound. This difference is rationalized in terms of the anisotropic interaction of the quadrupole moment of the iodine atom with the sodium cation, which is stabilizing in the case of the 2pi state and destabilizing in the case of the 2sigma state. The effects of spin-orbit coupling have been accounted for with both ab initio and semiempirical approaches, which have been found to give practically the same results. Inclusion of spin-orbit interactions does not affect the X(omega = 32) ground state, which retains its 2pi character, but it results in two omega = 12 spin-orbit states, with mixed 2sigma and 2pi characters and binding energies roughly half of that of the ground spin-orbit state. Complete basis set (CBS) extrapolations of potential curves, binding energies, and equilibrium geometries were also performed, and used to calculate a number of rovibronic parameters for the Na+...I* complex and to parameterize model potentials. The final CBS-extrapolated and zero-point vibrational energy-corrected binding energy is 10.2 kcal/mol. Applications of the present results for simulations of NaI photodissociation femtosecond spectroscopy are discussed.

Large-scale first-principles molecular dynamics simulations: application to the microsolvation of biologically-relevant ions in aqueous clusters

First-principles molecular dynamics simulations are employed to investigate the solvation structure of the biologically-relevant paradigm guanidinium-ion in water clusters, Gdn+(H2O)n. In these simulations, the intermolecular interactions are evaluated along the trajectories with the self-consistent-charge density-functional tight-binding (SCC-DFTB) model with the on-site third-order correction and modified effective Coulomb interaction, an approximate model of density-functional theory. For Gdn+(H2O)21, the simulated probability distribution of the ion-to-water cluster centre-of-mass distances and water angular coordinates suggests that Gdn+ is primarily localised at the surface of the water cluster, with the surface of the ion relatively devoid of water molecules. Estimates of the computational resources required for first-principles molecular dynamics simulations of larger Gdn+(H2O)n (n≤100) indicate that, with a modern supercomputer, such simulations can readily be performed in a matter of days.