Manifolds of low energy structures for a magic number of hydrated sulfate: SO42-(H2O)24.

Low energy structures of SO42-(H2O)24 have been obtained using a combination of classical molecular dynamics simulations and refinement of structures and energies by quantum chemical calculations. Extensive exploration of the potential energy surface led to a number of low-energy structures, confirmed by accurate calibration calculations. An overall analysis of this large set was made after devising appropriate structural descriptors such as the numbers of cycles and their combinations. Low energy structures bear common motifs, the most prominent being fused cycles involving alternatively four and six water molecules. The latter adopt specific conformations which ensure the appropriate surface curvature to form a closed cage without dangling O-H bonds and at the same time provide 12-coordination of the sulfate ion. A prominent feature to take into account is isomerism via inversion of hydrogen bond orientations along cycles. This generates large families of ca. 100 isomers for this cluster size, spanning energy windows of 10-30 kJ mol-1. This relatively ignored isomerism must be taken into account to identify reliably the lowest energy minima. The overall picture is that the magic number cluster SO42-(H2O)24 does not correspond to formation of a single, remarkable structure, but rather to a manifold of structural families with similar stabilities. Extensive calculations on isomerization mechanisms within a family indicate that large barriers are associated to direct inversion of hydrogen bond networks. Possible implications of these results for magic number clusters of other anions are discussed.

Infrared Spectra of Deprotonated Dicarboxylic Acids: IRMPD Spectroscopy and Empirical Valence-Bond Modeling.

Experimental infrared multiple-photon dissociation (IRMPD) spectra recorded for a series of deprotonated dicarboxylic acids, HO2 (CH2 )n CO 2 - (n=2-4), are interpreted using a variety of computational methods. The broad bands centered near 1600 cm-1 can be reproduced neither by static vibrational calculations based on quantum chemistry nor by a dynamical description of individual structures using the many-body polarizable AMOEBA force field, strongly suggesting that these molecules experience dynamical proton sharing between the two carboxylic ends. To confirm this assumption, AMOEBA was combined with a two-state empirical valence-bond (EVB) model to allow for proton transfer in classical molecular dynamics simulations. Upon suitable parametrization based on ab initio reference data, the EVB-AMOEBA model satisfactorily reproduces the experimental infrared spectra, and the finite temperature dynamics reveals a significant amount of proton sharing in such systems.

Strategy for Modeling the Infrared Spectra of Ion-Containing Water Drops.

Hydrated ions are ubiquitous in environmental and biological media. Understanding the perturbation exerted by an ion on the water hydrogen bond network is possible in the nanodrop regime by recording vibrational spectra in the O-H bond stretching region. This has been achieved experimentally in recent years by forming gaseous ions containing tens to hundreds of water molecules and recording their infrared photodissociation spectra. In this paper, we demonstrate the capabilities of a modeling strategy based on an extension of the AMOEBA polarizable force field to implement water atomic charge fluctuations along with those of intramolecular structure along the dynamics. This supplementary flexibility of nonbonded interactions improves the description of the hydrogen bond network and, therefore, the spectroscopic response. Finite temperature IR spectra are obtained from molecular dynamics simulations by computing the Fourier transform of the dipole moment autocorrelation function. Simulations of 1-2 ns are required for extensive sampling in order to reproduce the experimental spectra. Furthermore, bands are assigned with the driven molecular dynamics approach. This method package is shown to compare successfully with experimental spectra for 11 ions in water drops containing 36-100 water molecules. In particular, band frequency shifts of the free O-H stretching modes at the cluster surface are well reproduced as a function of both ion charge and drop size.

Vibrational mode assignment of finite temperature infrared spectra using the AMOEBA polarizable force field.

The calculation of infrared spectra by molecular dynamics simulations based on the AMOEBA polarizable force field has recently been demonstrated [Semrouni et al., J. Chem. Theory Comput., 2014, 10, 3190]. While this approach allows access to temperature and anharmonicity effects, band assignment requires additional tools, which we describe in this paper. The Driven Molecular Dynamics approach, originally developed by Bowman, Kaledin et al. [Bowman et al. J. Chem. Phys., 2003, 119, 646, Kaledin et al. J. Chem. Phys., 2004, 121, 5646] has been adapted and associated with AMOEBA. Its advantages and limitations are described. The IR spectrum of the Ac-Phe-Ala-NH2 model peptide is analyzed in detail. In addition to differentiation of conformations by reproducing frequency shifts due to non-covalent interactions, DMD allows visualizing the temperature-dependent vibrational modes.

Hydration of the sulfate dianion in cold nanodroplets: SO4(2-)(H2O)12 and SO4(2-)(H2O)13.

The structures, energetics and infrared spectra of SO4(2-)(H2O)12 and SO4(2-)(H2O)13 have been investigated by a combination of classical polarizable molecular dynamics and static quantum chemical calculations. Snapshots extracted from MD trajectories were used as inputs for local DFT optimization. Energies of the most stable structures were further refined at the ab initio level. A number of new low energy structures have thus been identified. The most stable structures of SO4(2-)(H2O)12 have the sulfate on the surface of the water cluster, while it may be slightly more burried in SO4(2-)(H2O)13, however still with an incomplete first hydration shell. Differences in the infrared spectra arise in part from mixing of sulfate stretching and water librational modes in the 900-1100 cm(-1) region, leading to some sensitivity of the IR spectrum to the structure. Second shell water molecules however do not generate signatures that are specific enough to relate spectra to structures straightforwardly, at least in this frequency range. Thus the emergence of a new band at 970 cm(-1) in the SO4(2-)(H2O)13 spectrum cannot be taken as a clue as to the number of water molecules which is necessary for a cluster to close the first hydration shell of sulfate. This number is at least 14 and possibly larger. However the density of low energy isomers is large enough that individual structures may loose meaning at all but the lowest temperatures.