Insights into the Solvation and Mobility of the Hydroxyl Radical in Aqueous Solution

• Published in: Journal of chemical theory and computation Vol. 7; no. 11; pp. 3725 - 3732
• Main Authors: Codorniu-Hernández, Edelsys, Kusalik, Peter G
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 08-11-2011
• Subjects: Condensed Matter, Interfaces, and Materials
• ISSN: 1549-9618; 1549-9626
• DOI: 10.1021/ct200418e

A detailed description of the local solvation structure and mobility of hydroxyl radicals (OH*) in aqueous solution near ambient conditions is provided by Car–Parrinello molecular dynamics simulations. Here, we demonstrate that for HCTH/120 and BLYP functionals, smaller systems (i.e., 31·H2O–OH*) are contaminated by system size effects, being biased for the presence of a three-electron two-centered hemibond structure between the oxygen atoms of a water molecule and the radical. Radial and spatial distribution functions of relatively large 63·H2O–OH* systems reveal the existence of a 4-fold coordinated “inactive” OH* structure with three H-bond donating neighbors and a strongly coordinated H-bond accepting neighbor. The local hydration structure around the radical exhibits more H-bond ordering than has been predicted by recent simulations employing classical force fields. Local structural fluctuations can end with spontaneous H-transfer reactions from the nearest H-bond donor water molecule, facilitated by the formation of an “active” OH* state, resembling the proton transfer mechanism of hydrated OH– (i.e., slight polarization of the (H3O2)* complex). A comparison of the free energy barriers for the H-transfer reaction obtained by both DFT functionals and for both system sizes is also provided, demonstrating that this can be a very rapid process in water.