Computational Characterization of Ar(H2O)6 Clusters and the Energetic Perturbations Induced by the Binding of a Single Argon Atom to Water Hexamer Isomers

This work systematically examines the interactions between a single Ar atom and the low-lying water hexamer isomers through full geometry optimizations and subsequent harmonic vibrational frequency computations performed using MP2 and the 2-body:Many-body (2b:Mb) methods with a triple-ζ correlation consistent basis set augmented with diffuse functions on the heavy atoms (cc-pVTZ for H and aug-cc-pVTZ for O and Ar; denoted as haTZ). CCSD(T)/haTZ and CCSD(T)-F12/haTZ-F12 single-point computations were also employed to examine the relative energetics of the resulting Ar(H2O)6 structures, while SAPT2 + 3(CCD)/haTZ computations were used to analyze contributions of electrostatics, exchange repulsion, induction, and dispersion to the Ar binding preferences. More than two dozen low-lying Ar(H2O)6 stationary points were identified for the Prism, Cage, Book1, Book2, Bag, Glove, Ring, Boat1, and Boat2 water hexamer isomers. Each level of theory consistently shows that Ar binding nonuniformly changes the relative energies of the different water hexamer topologies, producing a substantially denser manifold of low-energy structures. The 2b:Mb/haTZ harmonic zero-point vibrational energy (ZPVE) corrections are nearly identical with and without Ar, indicating that the changes in relative energy induced by Ar binding are driven primarily by electronic rather than harmonic vibrational contributions. When the 2b:Mb/haTZ harmonic ZPVE corrections are combined with the CCSD(T)-F12/haTZ-F12 electronic energies, five Ar(H2O)6 structures form a closely spaced manifold spanning approximately 0.5 kcal mol–1, with Book1 having the lowest nominal ZPVE-corrected energy. The CCSD(T)-F12/haTZ-F12 interaction energies show that Ar consistently binds by more than 1 kcal mol–1 to the water hexamers, approaching 1.7 kcal mol–1 when Ar interacts with the Book1 isomer of (H2O)6. SAPT interaction energies for the compact Ar(H2O)6 isomers with three-dimensional hydrogen-bond networks are comparable to those recently reported for Ar binding to the face of cyclic water tetramers, whereas the more open Ar(H2O)6 isomers with primarily two-dimensional hydrogen-bonding arrangements exhibit interaction energies that slightly exceed those for Ar binding to the face of cyclic water pentamers.