Graduate Student Sarah Johnson will present her research to the department.
Although hydrogen fluoride (HF) is a prototypical weak acid, estimates of the point at which proton transfer becomes energetically favorable in binary (HF)m(H2O)n clusters varies widely. For a single HF molecule (i.e. m=1), values at which proton transfer becomes energetically competitive ranges from n=6 to n≥10 depending on the electronic structure methods employed and structures considered. In this work, (HF)m(H2O)n clusters, where m+n≤8, are optimized with resolution-of-the-identity second-order Møller-Plesset perturbation theory (RI-MP2) with a series of Dunning’s correlation consistent basis sets with diffuse functions on non-hydrogen atoms (aug-cc-pVXZ for O and F, cc-pVXZ for H where X=D, T, denoted haXZ). More than 190 unique structures have been identified, and harmonic vibrational frequencies confirm that each one corresponds to a minimum on the potential energy surface (i.e. no imaginary frequencies, ni=0). For the lowest-lying RI-MP2/haTZ minimum associated with each value of m and n, explicitly-correlated coupled-cluster single point energy computations that include all single and double substitutions as well as a perturbative treatment of connected triple substitutions (CCSD(T)-F12) are used to estimate the relative electronic energies of the species near the complete basis set (CBS) limit. The smallest cluster for which the lowest-energy structure exhibits proton transfer is (HF)2(H2O)4. Most of the proton transfer structures characterized share a common hydrogen bonding topology. Specifically, the proton donor(s) tend to accept two hydrogen bonds, while the proton acceptor(s) tend to donate two hydrogen bonds.