Hydrogen bonding and Raman, IR, and 2D-IR spectroscopy of dilute HOD in liquid D ₂ O

Abstract

We present improvements on our previous approaches for calculating vibrational spectroscopy observables for the OH stretch region of dilute HOD in liquid D2O. These revised approaches are implemented to calculate IR and isotropic Raman spectra, using the SPC/E simulation model, and the results are in good agreement with experiment. We also calculate observables associated with three-pulse IR echoes: the peak shift and 2D-IR spectrum. The agreement with experiment for the former is improved over our previous calculations, but discrepancies between theory and experiment still exist. Using our proposed definition for hydrogen bonding in liquid water, we decompose the distribution of frequencies in the OH stretch region in terms of subensembles of HOD molecules with different local hydrogen-bonding environments. Such a decomposition allows us to make the connection with experiments and calculations on water clusters and more generally to understand the extent of the relationship between transition frequency and local structure in the liquid.vibrational spectroscopy ͉ water W ater is ubiquitous in science and nature (1), so it is natural that a tremendous amount of effort has been expended trying to describe and understand the structure and dynamics of its liquid state. Vibrational spectroscopy, both IR and Raman, provides an excellent probe of the local structure in water, because a local mode's vibrational frequency is exquisitely sensitive to the local mode's molecular environment. Actually, the cleanest information about local structure in water comes from the vibrational spectroscopy not of neat water, but rather of dilute HOD in either H 2 O or D 2 O, because in these cases, respectively, the OD or OH local-mode stretch is almost completely decoupled from the other stretches in the liquid, thus functioning well as a local chromophore. IR and Raman spectra on these systems have been measured by many (2-9).Valuable information about local dynamics in liquid water can also be obtained from vibrational spectroscopy experiments, in this case of the subpicosecond time-domain variety. On this time scale a local mode's vibrational frequency is continually changing because of molecular dynamics. The resulting dynamic frequency fluctuations, also known as spectral diffusion, can be measured by transient vibrational hole-burning and three-pulse echoes (5,6,(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22). In particular, these experiments provide information about both the short-time (stretching) and longtime (making and breaking) aspects of intermolecular hydrogen bonds (23-35).We and others have developed methods for the theoretical calculation of steady-state and ultrafast vibrational spectroscopy observables (7,(23)(24)(25)(26)(27)(36)(37)(38)(39)(40). In our approach, the single vibrational mode of interest, for example, the OH stretch of HOD (when it is immersed in D 2 O), is treated quantum mechanically, whereas all other degrees of freedom (the bath) are treated classically. Thus we are making the adiabatic app...

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Evaluation of Thiol Raman Activities and pKa Values Using Internally Referenced Raman-Based pH Titration

“…32 This conclusion is also consistent with the observation that hydrogen bonding induces a red-shift of the O-H stretch in water. [40][41] The two thiols in 1,2-BDT are both deprotonated in the 1 M NaOH solution ( water have, to our knowledge not been reported before. Only one 1,2-BDT thiol pKa value (6.8) was reported and that is for 1,2-BDT dissolved in dimethyl sulfoxide.…”

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“…These results are obtained using the SPC model but we show in Supplemental Figs. S2-S4 that the essential structure of the conformation-space-network does not depend on the water model (i.e., SPC, SPC/E, TIP4P, TIP5P), the length of the MD trajectory (from 10 ps to 1 ns), nor the hydrogen-bond criterion (i.e., the one introduced by Skinner et al [24,25] versus a simple geometry cut-off criterion, see Material and Methods).…”

“…The hydrogen bond definition that we employ is based on the work of Skinner et al [24,25] who found an empirical correlation between the occupancy N of the O-H σ * orbital (electron density which is donated by the hydrogen-bond acceptor) from electronic structure calculations and the geometries observed in MD simulations. They parametrized the orbital occupancy in terms of the H· · · O intermolecular distance r and the angle ψ that the O-H ray makes with the out-of-plane unit vector of the acceptor molecule:…”

“…molecules that hydrogen-bond to it, using a criterion for hydrogen-bonding which is based on an orbital-mixing point of view, and which has been introduced recently by Skinner et al [24,25] (if not stated otherwise). From these waters of the first solvation shell, the search expands in a tree-like manner ( Fig.…”