Assessing the i−TTM Model for Halide−Water Dimers: Infrared Spectroscopy and Thermodynamics

Jiarong Zhou, Marc Riera, and Francesco PaesaniUniversity of California, San Diego

Abstract and Background

MB-pol for H2O Interactions

i-TTM for Halide Interactions Infrared SpectroscopyHalide ion hydration plays an important role in solvation mechanics, water−air interfaces, and desalination processes. Properties of these systems are heavily dependent on the interactions between the ionic species and the surrounding water molecules, but experimental studies of these interactions can be difficult. In addition, most ion−water computer models are represented in terms of empirical force fields. A recent development of ab initio−based halide−water potential energy surfaces (PESs) called ion−Thole-type model (i−TTM) can enable a better molecular understanding of small X−(H2O)n clusters (X− = Cl−, Br−, I−). Here we assessed the structural, spectroscopic, and thermodynamic properties of X−(H2O)2 clusters utilizing the i−TTM potential. Infrared spectra and the dissociation enthalpy of the hydrogen bond between the water dimer were investigated at different temperatures through centroid molecular dynamics (CMD) and path integral molecular dynamics (PIMD) simulations in the microcanonical and canonical ensembles.

Thermodynamics

The i-TTM potentials are derived from fits to electronic structure data, and include an explicit treatment of the two-body repulsion, electrostatics, and dispersion energies. Many body effects are represented through classical polarization within an extended Thole-type model. By construction, the i-TTM potentials are compatible with the flexible and fully ab initio MB-pol potential.

The results show that i-TTM can accurately predict thermodynamic and spectroscopic properties of halide−water systems in different environments and conditions.

𝑉 𝑡𝑜𝑡=𝑉𝑤𝑖𝑛𝑡𝑟𝑎+𝑉 𝑤

𝑖𝑛𝑡𝑒𝑟+𝑉 𝑖−𝑇𝑇𝑀

𝑉 𝑖−𝑇𝑇𝑀=𝑉 𝑇𝑇𝑀 ,𝑒𝑙𝑒𝑐+𝑉 𝑇𝑇𝑀 ,𝑖𝑛𝑑+∑𝑖=1

𝑛

(𝑉 𝑖𝑟𝑒𝑝+𝑉 𝑖

𝑑𝑖𝑠𝑝)𝑉 𝑖

𝑑𝑖𝑠𝑝=∑𝑖=1

𝑛− 1

∑𝑗=𝑖+1

𝑛

− 𝑓 (𝑅𝑖𝑗 , 𝛿𝑖𝑗 )𝐶6 ,𝑖𝑗

𝑅 𝑖𝑗6 𝑉

𝑖

𝑟𝑒𝑝

=∑𝑖=1

𝑛− 1

∑𝑗=𝑖+1

𝑛

𝐴𝑖𝑗𝑒𝑏𝑖𝑗 𝑅𝑖𝑗

The energy of a system can be expressed as a many-body expansion of interaction energies. For water clusters, this expansion converges very rapidly with just the 3-body term, suggesting that the PES associated with a system containing N water molecules can in principle be represented as a sum of low-order interactions.

ReferencesAcknowledgements

Center for Aerosol Impacts onClimate and the Environment

http://paesanigroup.ucsd.edu/

The accuracy of the many-body polarizable (MB-pol) model is retained from water dimers to small clusters all the way to the condensed phase as shown by the electronic structure calculation comparisons to the CCSD(T) gold standard values. The open symbols in panel (a) are binding energies calculated using the reference CCSD(T) dimer geometry.

Future Improvements

IHB1 IH FIHB2

A PIMD simulation in the NVT ensemble, which represents constant number of molecules, volume, and temperature, was sampled through 100 different trajectories of 100ps each. The simulations ran at T = 10, 20, 40, 60, 80, 100, 120, 160, 200, 250, and 300K, and the O−X−−O angle was monitored in real-time. In addition, angular scans were performed by calculating the minimum energy of optimized halide−water systems at the MP2/aug-cc-pVTZ level as the O−X−−O angle was varied. The first inflection point (d2E/dθ2 = 0) was chosen as the cutoff angle that separates the populations of the water dimer from the independent water molecules. A van’t Hoffs fit was used to quantify the dissociation enthalpy and entropy of each cluster.

ln𝐾 𝑒𝑞=−∆𝐻𝑅𝑇 +

∆𝑆𝑅

The results show that the dissociation enthalpy of the water dimer is largely reduced in the presence of the halide ions. This reduction has been characterized by a weakening of the intermolecular interaction between the water molecules due to vibrational zero−point effects and a reduction in the intrinsic binding energy.

Cluster ∆H (kcal/mol) ∆S (cal/mol*K)Cl−(H2O)2 0.4756 1.6322

Br−(H2O)2 0.732 2.7198

I−(H2O)2 0.9183 3.0624

A CMD simulation in the NVE ensemble, where E represents constant energy, was sampled at similar conditions. A Fourier-transform of the dipole-dipole time correlation function was taken to generate the infrared spectra. As the temperature increased, the four distinct peaks that represents each OH stretch collapses into broad peaks due to the water molecules becoming indistinguishable. Because of its classical nature, the peaks were red-shifted by around 300 wavenumbers as compared to experimental data for I−(H2O)2 shown.

[1] J. Phys. Chem. B, 2016, 120 (8), pp 1822–1832[2] J. Chem. Theory Comput., 2013, 9 (11), pp 4844–4852[3] J. Chem. Theory Comput., 2013, 9 (12), pp 5395–5403[4] J. Chem. Theory Comput., 2014, 10 (4), pp 1599–1607[5] J. Chem. Theory Comput., 2014, 10 (8), pp 2906–2910[6] J. Phys. Chem. Lett., 2012, 3 (24), pp 3765–3769[7] J. Chem. Phys., 2015, 143, pp 104102[8] J. Chem. Phys., 1984, 80, pp 3726–3741[9] J. Phys. Chem. A, 2015, 119 (10), pp 1859–1866[10] J. Chem. Theory Comput., 2015, 11 (3), pp 1145–1154

This work is supported by CAICE, a NSF Center for Chemical Innovation (CHE-1304527).

• Perform similar simulations and calculations of both thermodynamic and spectroscopic properties of the F−(H2O)2 cluster

• Quantum molecular dynamics simulations in the NVT ensemble for better thermodynamic properties

• Better sampling through replica exchange molecular dynamics (REMD)• Quantum molecular dynamics simulations in the NVE ensemble for more accurate

infrared spectrum• Physically meaningful definition of a cutoff angle to separate the populations of

hydrogen bonded water dimer from the independent water molecules when determining the equilibrium constant