© Paesani Research Group. All rights reserved.

65. Development of a “first principles" water potential with flexible monomers: Dimer potential energy surface,

      VRT spectrum, and second virial coefficient, V. Babin, C. Leforestier, F. Paesani, J. Chem. Theory Comput. 9, 5395

      (2013). [link]

The development of a “first principles” water potential with flexible monomers (MB-pol) for molecular simulations

of water systems from gas to condensed phases is described. MB-pol is built upon the many-body expansion of

the intermolecular interactions, and the specific focus of this study is on the two-body term (V2B) representing the

full-dimensional intermolecular part of the water dimer potential energy surface. V2B is constructed by fitting

40,000 dimer energies calculated at the CCSD(T)/CBS level of theory and imposing the correct asymptotic

behavior at long-range as predicted from “first principles”. The comparison of the calculated vibration–rotation

tunneling (VRT) spectrum and second virial coefficient with the corresponding experimental results demonstrates

the accuracy of the MB-pol dimer potential energy surface.

64. Many-body convergence of the electrostatic properties of water. G.R. Medders, F. Paesani, J. Chem. Theory

      Comput. 9, 4844 (2013). [link]

The many-body convergence of the dipole moment and the dipole–dipole polarizability of water is

investigated. It is found that, for systems of low symmetry like the water clusters examined here,

simple measures such as dipole magnitudes and average polarizabilities may lead to an incomplete

interpretation of the underlying physics. Alternative metrics are introduced that allow for an

unambiguous characterization of both properties. The convergence of the many-body decomposition

of the total dipole and the polarizability is studied for (H2O)N, with N = 4 - 6 being minimum energy

water clusters and N = 14 being clusters that were extracted from condensed phase simulations.

For these clusters, it is demonstrated that both the total dipole and polarizability are almost entirely

pairwise additive, with three-body terms contributing less than 4% and all higher-order terms being

essentially negligible.

63. Molecular mechanisms of water-mediated proton transport in MIL-53 metal-organic frameworks. F. Paesani,

      J. Phys. Chem. C 117, 19508 (2013). [link]

Metal–organic frameworks have recently been proposed as promising proton conducting materials for

application in fuel cell technologies. Here, molecular dynamics simulations are used to reveal the microscopic

mechanisms associated with water-mediated proton transport in the MIL-53 materials as a function of temperature,

water loading, and pore size. The structure of the hydrated proton is found to resemble that of a distorted Zundel

complex when the framework closes into a narrow-pore configuration. A transition to Eigen-like structures is then

observed at higher water loading when the pores open as a result of the breathing effect. Although the free-energy

barriers to proton transfer at room temperature are lower than in bulk water, proton transport in MIL-53 is largely

suppressed, which is attributed to the low water mobility inside the pores. Faster proton diffusion is found at

higher temperature, in agreement with conductivity measurements.

62. The curious case of the water hexamer: Cage vs. Prism. V. Babin, F. Paesani, Chem. Phys. Lett. 580, 1 (2013).

      [link]

Small water clusters, such as the hexamer, provide a unique opportunity to advance the molecular-level

understanding of water in all its phases. In particular, the water hexamer is the smallest cluster that possesses

several nearly iso-energetic non-planar isomeric forms whose relative stability at low temperatures can be

probed experimentally and investigated theoretically. Here, we report on the equilibrium populations of the

isomers in the temperature range from 30 K to 150 K for both H2O and D2O as predicted by four different

water potentials. The simulations, performed using path-integral molecular dynamics combined with the

replica exchange method, highlight some deficiencies of empirical water models while providing support

for the accuracy of more recent ab initio-based potentials. The theoretical predictions for the cage/prism

isomeric equilibrium upon isotopic substitution suggest that rotational spectra measured for the deuterated

cluster could deliver further insights on the ground-state properties of the water hexamer.

61. Systematic study of structural and thermodynamic properties of HCl(H2O)n clusters from semiempirical replica

      exchange simulations, W. Lin, F. Paesani, J. Phys. Chem. A 117, 7131 (2013). [link]

The structural and thermodynamic properties of HCl(H2O)n clusters with n = 4 - 10 are studied using

Born-Oppenheimer replica exchange molecular dynamics simulations with the PM3-MAIS semiempirical

Hamiltonian. Independently of the cluster size, the simulations predict that HCl exists in the dissociated form

in all low-energy isomers. Different local structures are identified within the clusters due to the presence of the

dissociated proton, including Zundel, Eigen, Eigen-like, H7O3+, and intermediate Zundel - Eigen configurations.

As the cluster size increases, several groups of isomers are identified, whose relative stabilities vary as a function

of temperature. A detailed analysis of the heat capacity indicates that the melting behavior of HCl(H2O)n clusters

is strongly size-dependent. In particular, melting is observed in clusters with n = 7 - 10 in the temperature range

T = 100 - 150 K. By contrast, melting is not observed in clusters with n = 4 - 6.

60. Chemically cross-linked metal-organic frameworks. C.A. Allen, J.A. Boissonnault, J. Cirera, R. Gulland, F. Paesani,

      S.M. Cohen, Chem. Comm. 49, 3200 (2013). [link]

In this study, we report the synthesis of canonical isoreticular metal–organic frameworks (IRMOFs) containing

interligand crosslinks. Chemically crosslinking two molecules of 2-amino-1,4-benzene dicarboxylic acid (NH2-BDC)

gives ligands that readily form IRMOF-1 analogs, producing crosslinked MOFs that may be designed to have

novel properties. In general, the properties of these MOFs are similar to the parent IRMOF-3; however, a notable

difference in guest entrapment was observed with IRMOF-3-AM4XL. The robust inclusion of DMF in this MOF

suggests that the physical properties of the MOF can be substantially altered by even very small changes in

tether length. This observation, combined with the ability to incorporate additional chemical functionality into

the tethering group, opens up a new subclass of MOFs with even more highly tailored chemical and physical

properties solely within the pores.

59. Negative ion photoelectron spectroscopy reveals thermodynamic advantage of organic acids in facilitating

      formation of bisulfate ion clusters: Atmospheric implications. G.-L. Hou, W. Lin, S. H. M. Deng, J. Zhang,

      W.-J. Zheng, F. Paesani, X.-B. Wang, J. Phys. Chem. Lett. 4, 779 (2013). [link]

Recent lab and field measurements have indicated critical roles of organic acids in enhancing new atmospheric

aerosol formation.  We report a combined negative ion photoelectron spectroscopic and theoretical investigation

of molecular clusters formed by HSO4– with succinic acid (SUA), HSO4–(SUA)n along with HSO4–(H2O)n and

HSO4–(H2SO4)n. It is found that one SUA molecule can stabilize HSO4– by ~39 kcal/mol, three times the

corresponding value that one water molecule is capable of (~13 kcal/mol). Molecular dynamics simulations and

quantum chemical calculations reveal the most plausible structures of these clusters and attribute the stability of

these clusters to the formation of strong hydrogen bonds. This work provides direct experimental evidence

showing significant thermodynamic advantage by involving organic acid molecules to promote formation and

growth in bisulfate clusters and aerosols.

58. A critical assessment of two-body and three-body interactions in water. G.R. Medders, V. Babin, F. Paesani,

      J. Chem. Theory Comput. 9, 1103 (2013). [link]

The microscopic behavior of water under different conditions and in different environments

remains the subject of intense debate. A great number of the controversies arise due to the

contradictory predictions obtained within different theoretical models. Relative to conclusions

derived from force fields or density functional theory, there is comparably less room to dispute

highly correlated electronic structure calculations. In this study, a detailed analysis of the two-

and three-body water interactions evaluated at the CCSD(T) level is carried out to quantitatively

assess the accuracy of several force fields, DFT models, and ab initio based interaction potentials

that are commonly used in molecular simulations. On the basis of this analysis, a new model,

HBB2-pol, is introduced which is capable of accurately mapping CCSD(T) results for water dimers

and trimers into an efficient analytical function. The accuracy of HBB2-pol is further established

through comparison with the experimentally determined second and third virial coefficients.