The Theoretical Studies of Interactions of the OH 2 (H 2 O) n Clusters Evolution Toward the Hydroxide Anion Hydration* Rafał Roszak, Robert W. G ora, and Szczepan Roszak* The studies of evolution of nature of interactions are presented for the OH  (H 2 O) n (n ¼ 1–4) clusters in the context of the hydroxide anion hydration. The structure, thermodynamics, charge distribution, and interaction energy partitioning are presented for anions in the gas phase and aqueous solution. The many-body contributions to the interaction energies are considered for increasing size of complexes. The smallest clusters that are capable of reproducing the thermodynamics of hydroxide anion hydration were elucidated and the simplified procedure for calculations of hydroxide anion hydration is proposed and tested. V C 2012 Wiley Periodicals, Inc. DOI: 10.1002/qua.24227 Introduction The hydroxide anion (OH  ) complexes with water belong to the most important species in nature due to their essential role in chemical and biochemical reactions in aqueous solu- tion. The AOH fragment plays an important role in many proc- esses and its proper description is crucial for their modeling. The OH  (H 2 O) n complexes as well as OH  properties in the bulk environment were intensively studied, both experimen- tally and theoretically. The main problem regarding the XAOH ¼ X þ þ OH  reaction is a poor convergence of hydration properties with an increasing number of involved water mole- cules. Interaction energy of OH  with its first hydration shell accounts for only half of the total hydration energy in bulk water causing considerable problems in the modeling strategy. This problem is especially visible when the specific interactions with water molecules are important for the studied mechanism. The experimental data for OH  (H 2 O) n complexes in the gas phase are available from mass spectrometric, [1–3] electron scat- tering, [4] dissociative photodetachment, [5] and infrared spectro- scopic [6,7] measurements. The measured properties of OH  in water include enthalpy and free energy of hydration. [8–11] The solvation environment of OH  in water has been investigated using FTIR and neutron-diffraction experimental approaches. [12,13] The studies of gas-phase clusters were sup- ported by theoretical predictions and results of ab initio calcu- lations are available. [6,7,12,14,15] The absolute hydration free energy was modeled using ab initio methods combined with the continuum solvation models [12,16] and molecular dynamics simulations. [17–20] The available results indicate the shell structure of formed complexes both in gas-phase as well as in aqueous environ- ment with an apparent magic numbers of 3 (the first solvation shell) and 17 (the shell needed to enclose OH  inside the water cluster). Other shell closings were also observed includ- ing one suggested for n ¼ 4. Because state-of-the-art ab initio calculations with 17 water molecules are not feasible, the hybrid ab initio/continuum solvent model approaches consti- tute probably the best computational strategy which is assumed throughout this study. To elucidate the overall pic- ture of cluster evolution from bare OH  anion to bulk aqueous solution the interaction energy partitioning is performed in this work, focusing on nonadditivities of interaction energy components. The calculations provide the fundamental under- standing of interaction energies. The solvent effects are included in these studies for elucidated clusters capable of reproducing the thermodynamic data and a computational protocol for OH  hydration is proposed. Methods of Calculations The geometry optimization of studied complexes has been performed applying the second-order Møller-Plesset perturba- tion theory (MP2). [21] The harmonic vibrational frequencies were obtained at the same level of approximation. Addition- ally, single point calculations of electronic energy were per- formed applying the coupled cluster method utilizing single and double substitutions and including triple excitations noni- teratively [CCSD(T)] [22] which were supplemented by the MP2 thermochemistry. The presented results were obtained using Dunning’s triple-zeta correlation-consistent basis set aug- mented with diffuse functions (aug-cc-pVTZ). [23,24] The solvent effects were included in calculations applying the conductor- like polarizable continuum model (C-PCM) [25] as implemented in Gamess (US). The vibrational frequencies and thermody- namic properties of the studied complexes were determined within the ideal gas, rigid rotor, and harmonic oscillator approximations. [26] R. Roszak, R. W. G ora, S. Roszak Institute of Physical and Theoretical Chemistry, Wrocław University of Technology, Wybrzez˙e Wyspia  nskiego 27, 50-370 Wrocław, Poland E-mail: szczepan.roszak@pwr.wroc.pl *This work was dedicated to Prof. Ilya Kaplan, the pioneer of many-body studies, on the occasion on his 80th birthday. Contract grant sponsor: Polish Ministry of Science and Higher Education. V C 2012 Wiley Periodicals, Inc. International Journal of Quantum Chemistry 2012, DOI: 10.1002/qua.24227 1 WWW.Q-CHEM.ORG FULL PAPER