In order to develop a highly predictive macroscopic model for a system of interest, it is essential to understand it from a molecular level. In an attempt to understand the phase behavior of aqueous hydrogen fluoride, we study the molecular level clustering in this mixture. In this study, this is performed using mPW1B95 [1], a recently developed hybrid meta density functional theory method. Several previous theoretical studies have attempted to describe the acid dissociation of HF in water. In the current investigation we focus on the association patterns that are exhibited in this binary mixture. We study the stable geometries of the clusters of (HF)_n-(H2O)_m clusters with (m+n) up to as high as 18. The cluster geometries are initially optimized with a 6-31+G(d,p) basis set, and the corresponding zero-point energies, enthalpies and free energies are determined. For a higher level of accuracy, a single point energy calculation is performed, for the optimized geometry, using MG3S basis set and the energies for this level of theory are provided. For each cluster, several different structures are investigated, and the most stable structure is reported. The results from this work are compared with the earlier studies on this mixture that were obtained from a different level of theory [2-4]. The most stable association patterns that are obtained from this work will be incorporated in a thermodynamic model that allows formation of cross-clusters in this mixture. The effect of the inclusion of these association patterns in this mixture are studied based on the phase equilibrium data predictions from the macroscopic model.

References:

(1) Y. Zhao and D. G. Truhlar, "Hybrid Meta Density Functional Theory Methods for Thermochemistry, Thermochemical Kinetics and Noncovalent intearctions: The MPW1B95 and MPWB1K Models and Comparitive Assessments for Hydrogen Bonding and van der Waals Interactions", J. Phys. Chem. A, 108, 6908-6918, (2004).

(2) G. M. Chaban, and R. B. Gerber, "Ab initio Calculations of Anharmonic Vibrational Spoctroscopy for Hydrogen Fluoride (HF)_n (n=3,4) and Mixed Hydrogen Fluoride/ Water (HF)_n(H₂O)_n (n=1,2,4) Clusters",Spectrochim Acta A,58, 887-898, (2002).

(3) S. Odde, B. J. Mhin, H. M. Lee and K. Kim, S, "HF(H2O)n Clusters with an Excess Electron : Ab initio study", J. of Chem. Phys., 121, 11083-11087, (2004). (4) S. Re, "Enhanced Stability of Non-Proton-Transferred Clusters of Hydroated Hydrogen Fluoride HF(H2O)n (n=1-7): A Molecular Orbital Study",J. Phys. Chem. A, 105, 9725-9735, (2001).