Molecules with an excess number of hydrogen-bonding partners play a crucial role in fundamental chemical processes, ranging from the anomalous diffusion in supercooled water to the transport of aqueous proton defects and the ordering of water around hydrophobic solutes.Here we show that overcoordinated hydrogen bond environments can be identified in both the ambient and supercooled regimes of liquid water by combining experimental Raman multivariate curve resolution measurements and machine learning accelerated quantum simulations. In particular, we find that OH groups appearing in spectral regions usually associated with non-hydrogen-bonded species actually correspond to hydrogen bonds formed in overcoordinated environments. We further show that only these species exhibit a turnover in population as a function of temperature, which is robust and persists under both constant pressure and density conditions. This work thus provides a new tool to identify, interpret, and elucidate the spectral signatures of crowded hydrogen bond networks.
In the present work, the information gained by an electron for "knowing" about the position of another electron with the same spin is calculated using the Kullback-Leibler divergence (DKL) between the same-spin conditional pair probability density and the marginal probability. DKL is proposed as an electron localization measurement, based on the observation that regions of the space with high information gain can be associated with strong correlated localized electrons. Taking into consideration the scaling of DKL with the number of σ-spin electrons of a system (N(σ)), the quantity χ = (N(σ) - 1) DKLfcut is introduced as a general descriptor that allows the quantification of the electron localization in the space. fcut is defined such that it goes smoothly to zero for negligible densities. χ is computed for a selection of atomic and molecular systems in order to test its capability to determine the region in space where electrons are localized. As a general conclusion, χ is able to explain the electron structure of molecules on the basis of chemical grounds with a high degree of success and to produce a clear differentiation of the localization of electrons that can be traced to the fluctuation in the average number of electrons in these regions.
Raman
multivariate curve resolution is used to decompose the vibrational
spectra of aqueous hydrogen peroxide (H2O2)
into pure water, dilute H2O2, and concentrated
H2O2 spectral components. The dilute spectra
reveal four sub-bands in the OH stretch region, assigned to the OH
stretch and Fermi resonant bend overtone of H2O2, and two nonequivalent OH groups on water molecules that donate
a hydrogen bond to H2O2. At high concentrations,
a spectral component resembling pure H2O2 emerges.
Our results further demonstrate that H2O2 perturbs
the structure of water significantly less than either methanol or
sodium chloride of the same concentration, as evidenced by comparing
the hydration-shell spectra of tert-butyl alcohol
dissolved in the three aqueous solutions.
The
affinity of hydroxide ions for methyl hydration shells is assessed
using a combined experimental and theoretical analysis of tert-butyl alcohol (TBA) dissolved in pure water and aqueous
NaOH and NaI. The experimental results are obtained using Raman multivariate
curve resolution (Raman-MCR) and a new three-component total least
squares (Raman-TLS) spectral decomposition strategy used to highlight
vibrational perturbations resulting from interactions between TBA
and aqueous ions. The experiments are interpreted and extended with
the aid of effective fragment potential molecular dynamics (EFP-MD)
simulations, as well as Kirkwood–Buff calculations and octanol/water
partition measurements, to relate TBA-ion distribution functions to
TBA solubility changes. The combined experimental and simulation results
reveal that methyl group hydration shells more strongly expel hydroxide
than iodide anions, whose populations near the methyl groups of TBA
are predicted to be correlated with sodium counterion localization
near the TBA hydroxyl group.
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