Coordination structure and charge transfer in microsolvated transition metal hydroxide clusters [MOH]+(H2O)1–4

Abstract

Infrared vibrational predissociation spectra of transition metal hydroxide clusters, [MOH]⁺(H₂O)_1–4·D₂ with M = Mn, Fe, Co, Ni, Cu, and Zn, are presented and analyzed with the aid of density functional theory calculations. For the [MnOH]⁺, [FeOH]⁺, [CoOH]⁺ and [ZnOH]⁺ species, we find that the first coordination shell contains three water molecules and the four ligands are arranged in a distorted tetrahedral geometry. [CuOH]⁺ can have either two or three water molecules in the first shell arranged in a planar arrangement, while [NiOH]⁺ has an octahedral ligand geometry with the first shell likely closed with five water molecules. Upon closure of the first coordination shell, characteristic stretch frequencies of hydrogen-bonded OH in the 2500–3500 cm⁻¹ region are used to pinpoint the location of the water molecule in the second shell. The relative energetics of different binding sites are found to be metal dependent, dictated by the first-shell coordination geometry and the charge transfer between the hydroxide and the metal center. Finally, the frequency of the hydroxide stretch is found to be sensitive to the vibrational Stark shift induced by the charged metal center, as observed previously for the smaller [MOH]⁺(H₂O) species. Increasing solvation modulates this frequency by reducing the extent of the charge transfer while elongating the M–OH bond.