Effects of electronic structure on the hydration of PbNO3+ and SrNO3+ ion pairs

Abstract

Hydration of PbNO₃⁺ and SrNO₃⁺ with up to 30 water molecules was investigated with infrared photodissociation (IRPD) spectroscopy and with theory. These ions are the same size, yet the IRPD spectra of these ion pairs for n = 2–8 are significantly different. Bands in the bonded O–H region (∼3000–3550 cm⁻¹) indicate that the onset of a second hydration shell begins at n = 5 for PbNO₃⁺ and n = 6 for SrNO₃⁺. Spectra for [PbNO₃]⁺(H₂O)_2–5 and [SrNO₃]⁺(H₂O)_3–6 indicate that the structures of clusters with Pb(II) are hemidirected with a void in the coordinate sphere. A natural bond orbital analysis of [PbNO₃]⁺(H₂O)₅ indicates that the anisotropic solvation of the ion is due to a region of asymmetric electron density on Pb(II) that can be explained by charge transfer from the nitrate and water ligands into unoccupied p-orbitals on Pb(II). There are differences in the IRPD spectra of PbNO₃⁺ and SrNO₃⁺ with up to 25 water molecules attached. IR intensity in the bonded O–H region is blue-shifted by ∼50 cm⁻¹ in nanodrops containing SrNO₃⁺ compared to those containing PbNO₃⁺, indicative of a greater perturbation of the water H-bond network by strontium. The free O–H stretches of surface water molecules in nanodrops containing 10, 15, 20, and 25 water molecules are red-shifted by ∼3–8 cm⁻¹ for PbNO₃⁺ compared to those for SrNO₃⁺, consistent with more charge transfer between water molecules and Pb(II). These results demonstrate that the different electronic structure of these ions significantly influences how they are solvated.