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Flexible models of the radical and water molecules including short-range interaction of hydrogen atoms have been employed in molecular dynamic simulation to understand mechanism of •OH hydration in aqueous systems of technological importance. A key role of H-bond connectivity patterns of water molecules has been identified. The behavior of •OHaq strongly depends on water density and correlates with topological changes in the hydrogen-bonded structure of water driven by thermodynamic conditions. Liquid and supercritical water above the critical density exhibit the radical localization in cavities existing in the solvent structure. A change of mechanism has been found at supercritical conditions below the critical density. Instead of cavity localization, we have identified accumulation of water molecules around •OH associated with the formation of a strong H-donor bond and diminution of non-homogeneity in the solvent structure. For all the systems investigated, the computed hydration number and the internal energy of hydration ΔhU showed approximately linear decrease with decreasing density of the solvent but a degree of radical-water hydrogen bonding exhibited non-monotonic dependence on density. The increase in the number of radical-water H-acceptor bonds is associated with diminution of extended nets of four-bonded water molecules in compressed solution at ∼473 K. Up to 473 K, the isobaric heat of hydration in compressed liquid water remains constant and equal to -40 ± 1 kJ mol-1. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Self-diffusion (D w) and D OH coefficients comparable up to 470 K. • Above 470 K D OH by 30% lower due to enhanced complexation of OH with H 2 O. • Continuous lifetime ∼10−14 s calculated for both H-donor and H-acceptor bond. • Intermittent lifetime 4 ps (373 K) drops to 1 ps (673 K) exceeding translational time. • Conclusions on OH aq translational dynamics depend on model potential used. Abstract MD simulations using flexible models were performed to show how velocity autocorrelation functions, diffusion (D OH) and self-diffusion (D w) coefficients, continuous and intermittent lifetimes of H-donor and H-acceptor bonds depend on: i) transformations in the hydrogen-bond network (HBN) of water below the critical point, and ii) decreasing density at the supercritical isotherm (∼670 K). Contrary to simulations neglecting short-range hydrogen-oxygen interactions, we show that complexation of OH aq with H 2 O is enhanced by breakage of the continuous HBN, resulting in the increase of OH aq hydrodynamic radius and the D OH values smaller than D w by 30%. Difference between D OH and D w increases to 65% at 670 K below the critical density. It is explained by enhanced aggregation of water molecules around OH aq , earlier recognized as the self-trapping mechanism. Except near-ambient conditions, the intermittent lifetime of H-bonds is longer than the translational time, indicating that OH aq is complexed with water and moves as a molecular aggregate. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Power spectrum of OH aq stretching vibrations computed vs temperature and density. • Decomposition to components assigned to H-bonded complexes OH aq -(H 2 O) nHB=0,1,2,3. • Hydrogen-bond network effect on fundamental frequencies of band-components shown. • Statistics of OH-water H-bonds assessed from partial area of band-components. Abstract The effect of water temperature and density on stretching vibrations power spectrum of OH aq was studied from ambient to supercritical conditions by molecular dynamics simulation. Decrease in the height, red-shift of the maximum and broadening of the spectrum were found with increasing temperature and decreasing water density. Decomposition of power spectra into Gaussian bands assigned to the radical monomer and hydrogen-bonded complexes OH aq -(H 2 O) nHB=1,2,3 showed equal spacing of components and the red-shift relative to the monomer band, linearly increasing with the decreasing number of solvent H-bonds. The radical-water H-bond statistics resulting from band-contribution showed the significant increase in the bond number between 373 and 573 K, associated with breakage of the continuous hydrogen-bond network. At 673 K the degree of hydrogen bonding was significantly controlled by density. We found that the most optimized radical-water complexes were formed when fluid density decreased below the critical value. [ABSTRACT FROM AUTHOR]