Water-anion hydrogen bonding dynamics: Ultrafast IR experiments and simulations

Abstract

Many of water's remarkable properties arise from its tendency to form an intricate and robust hydrogen bond network. Understanding the dynamics that govern this network is fundamental to elucidating the behavior of pure water and water in biological and physical systems. In ultrafast nonlinear infrared experiments, the accessible time scales are limited by water's rapid vibrational relaxation (1.8 ps for dilute HOD in HO), precluding interrogation of slow hydrogen bond evolution in non-bulk systems. Here, hydrogen bonding dynamics in bulk DO were studied from the perspective of the much longer lived (36.2 ps) CN stretch mode of selenocyanate (SeCN) using polarization selective pump-probe (PSPP) experiments, two-dimensional infrared (2D IR) vibrational echo spectroscopy, and molecular dynamics simulations. The simulations make use of the empirical frequency mapping approach, applied to SeCN for the first time. The PSPP experiments and simulations show that the orientational correlation function decays via fast (2.0 ps) restricted angular diffusion (wobbling-in-a-cone) and complete orientational diffusive randomization (4.5 ps). Spectral diffusion, quantified in terms of the frequency-frequency correlation function, occurs on two time scales. The initial 0.6 ps time scale is attributed to small length and angle fluctuations of the hydrogen bonds between water and SeCN. The second 1.4 ps measured time scale, identical to that for HOD in bulk DO, reports on the collective reorganization of the water hydrogen bond network around the anion. The experiments and simulations provide details of the anion-water hydrogen bonding and demonstrate that SeCN is a reliable vibrational probe of the ultrafast spectroscopy of water.

Ion effect on the dynamics of water hydrogen bonding network: A theoretical and computational spectroscopy point of view

“…The reorientation of water along the OH bond vector is more sensitive to anion than cation due to the direct hydrogen bond between anion and the water hydrogen. Cations, on the other hand, limit the reorientation of the water dipoles, especially for the divalence and trivalence ones (Boisson et al, 2011;Buchner et al, 2004;Buchner & Hefter, 2009;Laage & Hynes, 2007, 2008b; Lin, Auer, & Skinner, 2009;Park & Fayer, 2007;Turton et al, 2008;Wachter et al, 2007;Yamada, Thompson, & Fayer, 2017;R. Zhang & Zhuang, 2013).…”

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

“…For SeCN -, the electric field at the position of the C projected onto the CN bond-vector does accurately predict the C − − − N stretching frequency, but this is largely due to a cancellation of effects. 26 Nevertheless, this phenomenological association of local electric field with vibrational frequency still strongly supports the concept that the vibrational frequency will be sensitive to molecular orientation.…”

“…Theoretical work has shown that both electrostatics beyond the dipole approximation and chemical interactions like charge transfer play important roles in determining the C − − − N stretching frequency. 26,58,59 Nevertheless, if these interactions are vectorial in nature, i.e. they depend on the orientation of the molecule, then they can be incorporated into the conceptual framework of RISD.…”

“…42 Furthermore, a combination of MD simulations and ultrafast infrared spectroscopy developed a spectroscopic map of SeCNin D 2 O that describes the frequency dependence of the nitrile stretch to a hydrogen bonding environment. 26 2D-IR spectroscopy can report two-point correlations in vibrational frequency and orientation. The shapes of 2D-IR spectra and how they change encode the frequency fluctuations of IR chromophores -the frequency fluctuation correlation function (FFCF).…”

Transient Glass Formation around a Quadrupolar Photoexcited Dye in a Strongly H‐Bonding Liquid Observed by Transient 2D‐IR Spectroscopy

“…[18] Likewise,t he longest memory of inhomogeneity of the very strongly H-bonded excess proton in water, [19] was recently found to be only of the order of 200 fs. [18] Likewise,t he longest memory of inhomogeneity of the very strongly H-bonded excess proton in water, [19] was recently found to be only of the order of 200 fs.…”