The Bending Mode of Water: A Powerful Probe for Hydrogen Bond Structure of Aqueous Systems

Abstract

Insights into the microscopic structure and dynamics of the water's hydrogen-bonded network are crucial to understand the role of water in biology, atmospheric and geochemical processes, and chemical reactions in aqueous systems. Vibrational spectroscopy of water has provided many such insights, in particular using the O-H stretch mode. In this Perspective, we summarize our recent studies that have revealed that the H-O-H bending mode can be an equally powerful reporter for the microscopic structure of water and provides more direct access to the hydrogen-bonded network than the conventionally studied O-H stretch mode. We discuss the fundamental vibrational properties of the water bending mode, such as the intermolecular vibrational coupling, and its effects on the spectral lineshapes and vibrational dynamics. Several examples of static and ultrafast bending mode spectroscopy illustrate how the water bending mode provides an excellent window on the microscopic structure of both bulk and interfacial water.

Figure 1

(a) Time averaged probability distribution of the O–H stretch frequency vs hydrogen bond strength. Reproduced from ref (5). (b) A scatter plot of the experimentally determined H–O–H bending frequency vs averaged O–H stretch frequency of water. The data points were experimentally obtained by measuring the frequencies of water in various media. The blue and red lines represent the relations of eqs 1 and 2, respectively.

Figure 2

(a) Measured IR and Raman spectra of the O–H stretch vibration. The data are reproduced from refs ( and 15), respectively. (b) Computed IR and Raman spectra of the O–H stretch vibration based on the frequency-mapping technique, with the intermolecular intramode coupling and Fermi resonance (H₂O) and without any vibrational coupling (HOD in D₂O). The data are reproduced from refs ( and 17), respectively. (c) Measured IR and Raman spectra. The data are reproduced from ref (18). (d) Computed IR spectra of the H–O–H bending vibration based on the frequency-mapping technique, including the intermolecular intramode coupling (H₂O) and without any vibrational couplings (dilute H₂O in D₂O). The data are reproduced from ref (7). All data are normalized to the peak maximum.

Figure 3

(a) Anisotropy decay for various H₂O/D₂O mixture. The hot ground state was subtracted from the transient absorption signal. Traces are offset by increments of 0.4. Symbols show experimental data, and solid lines show fits using a single-exponential decay. (b) Comparison of the anisotropic decay times for the bending mode and the stretch mode as a function of H₂O/O–H fraction. The stretch mode data were taken from ref (21). Both bending and stretching mode data are approximated by single exponentials. The lines serve to guide the eye. The bending mode data are reprinted from ref (18).

Figure 4

2D-IR spectra in the H–O–H bending mode region as a function of waiting time for pure H₂O. (a) Experimental data of Tokmakoff and co-workers (Reproduced with permission from ref. Copyright 2017 AIP Publishing), (b) experimental data of Kuroda and co-workers (Reproduced with permission from ref (33). Copyright 2014 the Royal Society of Chemistry.), and (c) simulation data of Saito and co-workers (Reproduced with permission from ref (32). Copyright 2013 AIP publishing.). The ω₁ and ω₃ indicates pump and probe frequencies, respectively.

Figure 5

(a, b) SFG spectra at the (a) negatively charged (H₂O/DPPG) and (b) positively charged (H₂O/DPTAP) interfaces with various ion concentrations at ssp polarization combination (s-, s-, and p-polarization for the SFG signal, visible pulse, and IR pulse, respectively). The black lines represent the fit. The pink and blue shaded regions indicate H–O–H bending mode and C=O stretch mode contributions, respectively. (c, d) The interfacial Im(χ_bend⁽²⁾) (black line) and the bulk Im(χ_bend⁽³⁾Φ(c)) (colored lines) spectra obtained from the fit of the spectra for the (c) water/DPPG and (d) water/DPTAP interfaces. The data were reproduced with permission from ref (56). Copyright 2019 American Chemical Society.

Figure 6

(a) SFG intensity spectra of the H–O–H bending mode at the water–air interface with 1/0, 2/1, 1/1, 1/2, and 0/1 H₂O/D₂O mixtures at ssp polarization combination. Solid lines are fits to the data. The data are reprinted from ref (10) by permission of the PCCP Owner Societies. (b) Normalized Im(χ_bend⁽²⁾) spectra of neat H₂O and H₂O in D₂O at the water–air interface. The data are constructed from the fits of the SFG intensity spectra in panel a. The dashed line data is obtained from [H₂O] = 25%. The solid and striped shaded regions indicate the uncertainty of each measurement.^, (c) Normalized Im(χ_str⁽²⁾) spectra of neat H₂O and HOD in D₂O at the water–air interface. The data are reproduced from refs (57) (blue) and (59) (red). The dashed line data is obtained from an H₂O/D₂O mixture with [OH] = 25%.

Figure 7

IR spectra in the water bending mode range of alcohol–water mixtures at various temperatures. These solutions have the same composition of 1/0.2 molar ratio (alcohol/water). The legend indicates the temperature of the system in units of °C. The data is reproduced with permission from ref (61). Copyright 2019 American Chemical Society.

Figure 8

(a) The H–O–D bending mode spectra at HSA–water interface with various H₂O/D₂O mixture ratios. (b) The Im(χ⁽²⁾) spectra of HSA and water contributions from the fits (solid lines in panel a). Shaded regions in blue represent the H–O–D bending mode contributions of isotopically diluted water. (c) Amplitude of the H–O–D contribution vs H₂O fraction. Theoretical prediction denoted by the black broken line is obtained from the equation of [HOD]²/[H₂O][D₂O] = 3.86.⁻ These data are reproduced from ref (10) by permission of the PCCP Owner Societies.