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The Hydrogen Bonded Networks in Supercritical Water Qiang Sun, Qianqian Wang, and Dongye Ding J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp503474s • Publication Date (Web): 03 Sep 2014 Downloaded from http://pubs.acs.org on September 9, 2014

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The Journal of Physical Chemistry

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The Hydrogen Bonded Networks in Supercritical Water

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Qiang Sun* and Qianqian Wang

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Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, The School of

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Earth and Planetary Sciences, Peking University, Beijing 100871, China

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Dongye Ding

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School of Resources and Environmental Engineering, Shandong University of Technology, Zibo

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255049, China

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ABSTRACT: In this study, the structure of supercritical water (SCW) is investigated by Raman

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spectroscopy and molecular dynamics simulations. It was found that the hydrogen bonding in

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water is closely related to temperature and pressure (or water density). According to the Raman

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spectroscopic study of SCW, the existence of tetrahedral hydrogen bonds in SCW is also affected

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by the density of SCW. In addition, for SCW with critical density (0.322 g·cm-3), we suggest that

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the tetrahedral hydrogen bonding is absent at water critical point (647 K and 22.1 MPa) based on

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Raman evidence. From the dependence of νmax of the Raman OH stretching bands on temperature

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and pressure, the structure of SCW can be divided into three-dimensional and chain (or string)

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hydrogen bonded networks, which correspond to liquid- and gas-like phases, respectively.

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Keywords: Critical point, Tetrahedral hydrogen bonding, Raman spectroscopy, Molecular

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simulations

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INTRODUCTION

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Recently, there has been increasing interest in the structure of water at high temperature and

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pressure, especially in the supercritical domain. This is because the physical and chemical

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properties of sub- and supercritical water (SCW) are remarkably different from those under

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ambient conditions. Additionally, the density of SCW can be controlled between gas- and

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liquid-like values by varying the pressure and temperature. Then, many fundamental properties,

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such as the dielectric constant, viscosity, and ion product, can be continuously varied in SCW.1

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Therefore, SCW can be treated as a highly reactive, polar, and hydrogen bonded solvent that plays

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a crucial role in many chemical, biological, and industrial processes.2,3

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Many experimental and theoretical studies have been conducted to investigate the structure of

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SCW.4–29 According to the neutron scattering data of SCW at 673 K, Postorino et al.4 observed the

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complete disappearance of the peak located at 1.9 Å, which is a direct evidence of hydrogen

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bonding in the oxygen–hydrogen radial distribution function (gOH(r)). Therefore, the authors

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claimed that almost all the hydrogen bonds between water molecules break at this temperature.

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Later, according to the newly derived site–site pair correlation functions from the old data, the

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above view was revised by Soper et al.,5 who suggested a highly distorted arrangement of water

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molecules with a largely reduced degree of hydrogen bonding compared with water at ambient

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conditions. Because O–H vibrations are sensitive to hydrogen bonding, Raman and infrared (IR)

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spectroscopy are widely applied to investigate the structure of SCW. These methods indicate that a

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significant fraction of network water is still present even at high temperatures, pressures, and

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solute concentrations,13 and that supercritical water can be considered as an ideal mixture of small

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water clusters ((H2O)n, n = 1–3) at the chemical equilibrium.20 In addition, according to NMR 2


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studies of SCW, hydrogen bonding persists at supercritical temperatures, and the average number

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of hydrogen bonds per water molecule is greater than one at the supercritical densities.21

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According to the X-ray Raman spectrum of supercritical water at 380 °C and density 0.54 g⋅cm−3,

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Wernet et al.23 suggested that supercritical water is inhomogeneous and can be regarded as a

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heterogeneous system with small patches of bonded molecules in various tetrahedral

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configurations and surrounding non-bonded gas-phase-like molecules. Recently, it has been

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suggested that SCW can be separated into a gas-like phase with short chains (or sheets with local

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planar geometries) and a liquid-like phase with three-dimensional percolating hydrogen bonded

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networks according to whether it is below or above the percolation threshold, respectively.24,25

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Parallel with those experimental efforts, there have been extensive theoretical works to present a

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microscopic understanding of SCW,26-29 which are devoted to investigate the hydrogen bonding

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network and its influence on physicochemical properties of SCW. It was found that hydrogen

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bonds still exist in SCW, but hydrogen bond number decreases with the increase of temperature or

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the decrease of the density. From the above studies, the hydrogen bonding in SCW still remains

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elusive.

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In this study, in situ Raman spectra of SCW are measured, and the Raman OH stretching bands

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of water are applied to investigate the hydrogen bonding in water. The hydrogen bonding between

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water molecules is closely related to the electron delocalization. In comparison with molecular

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dynamics simulations, quantum chemistry calculations provide a more reasonable approach to

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investigate the water structure. In this work, due to high-efficiency, molecular dynamics

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simulations are conducted to study the structure of SCW.

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EXPERIMENTAL

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In this study, deionized water was produced by a Milli-Q water purification system, and its

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conductivity was 5.5 × 10−6 S/m. The experiments were conducted in an externally heated

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hydrothermal diamond anvil cell (HDAC)30 equipped with low-fluorescence type Ia diamonds

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with culet faces 1 mm across. The temperature was measured by a pair of type-K thermocouples

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attached to both the upper and lower diamond anvils. The pressure at the elevated temperatures

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was determined from the measured liquid–vapor homogenization temperature using the

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International Association for the Properties of Water and Steam 1995 equation of state of water.31

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The errors in the pressure and temperature were estimated to be 5% and ±2 K, respectively.

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Additionally, to investigate the effects of pressure on the structure of ambient water, high-pressure

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experiments were conducted up to 250 MPa at 295 K. The experimental pressure was calculated

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according to the Raman shift of the 464 cm−1 peak of quartz. According to Schmidt and

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Ziemann,32 this pressure estimation method had an uncertainty of ±50 MPa.

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The unpolarized Raman spectra were recorded using a confocal micro-Raman system in a

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backscattering geometry. An Ar+ ion laser was used with an excitation wavelength of 514.5 nm

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and operated at 25 mW. The spectrometer with an entrance slit of 50 µm was used to collect the

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signals. The resolution was about 1 cm−1. Each Raman spectrum was recorded for 30 s with a 20×

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objective.

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The Raman spectra were analyzed by the Jandel Scientific Peakfit v4.04 program. The Raman

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spectra were firstly smoothed until the noise diminished, and the baselines were corrected. Then,

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Gaussian functions were used to fit the Raman OH stretching bands of water.

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MOLECULAR DYNAMICS SIMULATIONS

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In this study, owing to its simplicity and popularity, the well-established extended simple point

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charge (SPC/E) potential33 was used to model the water–water interaction energy. The SPC/E

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potential is known to predict well the temperature and pressure dependent number density in

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liquid to supercritical water while reproducing the structural properties from ambient to

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supercritical conditions.34 The water–water interaction energy is given by

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12 6  σ   σ wt − wt   wt − wt VW −W (r ) = ∑ + 4ε wt − wt   −    rOO   rOO   ij 4πε 0 rij

qi q j

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In this work, the molecular dynamics simulations were performed using the GROMACS

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package (version 4.07).35,36 The simulations were carried out in the NPT ensemble. The simulation

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time was 6000 ps with a time step of 2 fs. Additionally, periodic boundary conditions were applied

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in all three directions. The Nose–Hoover thermostat and Parrinello–Rahman pressure coupling

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were used in the simulations. The Lennard–Jones interactions were truncated at 1.0 nm. The

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particle mesh Ewald method was used to calculate the long-range electrostatics forces.

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RESULTS AND DISCUSSION

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For a single H2O molecule, the vibrational normal modes are 2A1 (including a symmetric

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stretching vibration ν1 at 3657.05 cm−1 and a bending vibration ν2 near 1595 cm−1) and B1

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(anti-symmetric stretching vibration ν3 at 3755.97 cm−1),37 which are all Raman active. When a

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hydrogen bond forms between two water molecules, electron redistribution occurs. For water

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molecular clusters, hydrogen bond formation increases the O–H bond lengths, and it causes a

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20-fold greater reduction in the H–O and O–O distances.38 In addition, it causes a red shift of the

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hydrogen-bonded OH stretching frequency, and the magnitude of the red shift increases with 5



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cluster size.39 Therefore, the hydrogen bonding strength is closely related to the OH stretching

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vibrations.

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According to our recent study on the dependence of the OH vibration frequency on the size of

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water clusters,40,41 when three-dimensional hydrogen bonded networks form, the OH stretching

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vibrations are mainly dependent on the local hydrogen-bonded networks (local hydrogen bonding

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refers to the interactions between a water molecule with neighboring molecules or with the

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hydrogen-bonded networks in the first coordination shell of the molecule). This has been

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confirmed by a Raman spectroscopic study of the effects of increasing pressure on water

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structures.42 From our recent study,42 with increasing pressure up to 400 MPa at 293 K, the Raman

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OH stretching bands slightly move to lower wavenumber. However, high pressure does not

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obviously change the profile of the Raman OH stretching band. Based on our interpretation of the

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Raman OH stretching bands, this indicates that high pressure has no obvious effects on the first

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shell of a water molecule. These results are in agreement with other studies.43,44

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From the above analysis, it is reasonable to assign different OH vibrational frequencies to

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different local hydrogen bonding motifs. For a water molecule, the local hydrogen-bonded

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networks can be differentiated by whether the molecule forms hydrogen bonds as a proton donor

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(D), proton acceptor (A), or a combination of both with neighboring molecules. Under ambient

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conditions, the main local hydrogen bonding motifs for a water molecule can be classified as

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double donor–double acceptor (DDAA), double donor–single acceptor (DDA), single

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donor–double acceptor (DAA), and single donor–single acceptor (DA).40,41 At ambient conditions,

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the Raman OH stretching band of water can be deconvoluted into five sub-bands located around

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3041, 3220, 3430, 3572, and 3636 cm−1, which can be assigned to the νDAA-OH, νDDAA-OH, νDA-OH, 6


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νDDA-OH, and free OH symmetric stretching vibrations, respectively.41

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To date, many experimental and theoretical studies have been performed to investigate the

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hydrogen bonding in ambient water. However, there still remains strong debate on the structure of

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liquid water.45–47 This study focuses on the structure of SCW. Therefore, much attention is paid to

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the dependence of vmax on temperature and pressure and the Raman spectral changes of tetrahedral

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hydrogen bonding (3220 cm−1 sub-band for ambient water).

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For water from ambient to supercritical conditions, the in situ Raman OH stretching bands of

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water are shown in Figure 1. With increasing temperature, the Raman OH stretching bands move

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to higher wavenumbers. Simultaneously, the increase of temperature results in the decrease of the

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intensity of the 3220 cm−1 sub-band (tetrahedral hydrogen bonding), and the Raman OH stretching

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bands become increasingly symmetric. These results are in agreement with other Raman

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spectroscopic studies of water at high temperature.13–16 This indicates that increasing temperature

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results in the decrease of hydrogen bonding strength. Because tetrahedral hydrogen bonding is the

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main three-dimensional hydrogen bonding, and taking into consideration the dependence of OH

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vibrations on the size of water clusters,40,41 it is expected that the disappearance of tetrahedral

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hydrogen bonds may lead to the structural transformation from three-dimensional to chain or

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string (νDA-OH) hydrogen bonding in SCW.

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In this study, the νmax is used to investigate the spectral changes of the Raman OH stretching

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bands of water. The effect of temperature on νmax of the Raman spectra is shown in Figure 2. With

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increasing temperature along the gas–liquid equilibrium line, νmax moves to high wavenumber. For

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SCW, the shift of νmax of the Raman spectra is affected by both pressure and temperature.

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Therefore, the hydrogen bonding in SCW should be closely related to the temperature and 7



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pressure (or SCW density). Additionally, in reference with νmax in the equilibrium region, the shift

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of νmax of SCW at 25 MPa is more obvious than that at 100 MPa (Figure 2). This is in agreement

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with other Raman spectroscopic studies of νmax of SCW.13 From this, the structural changes in

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water critical region may be different from those of SCW apart from the critical region.

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For water at ambient temperature, the primary effect of increasing pressure is to modify the

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second water shell around a central water molecule, pulling it inwards, while changes to the first

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shell are relatively small.43,44 To investigate the effect of pressure on the structure of SCW, the

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dependence of νmax on pressure is shown in Figure 3. From the figure, the effect of increasing

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pressure on νmax is closely related to the density of SCW, and SCW can be divided into a gas-like

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phase and a liquid-like phase. For high-density (liquid-like) SCW, there is a slight decrease of νmax

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with increasing pressure. This is similar to the effect of pressure on water at ambient temperature.

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However, for low-density (gas-like) SCW, νmax significantly decreases with increasing pressure.

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This means that the hydrogen bonded networks in SCW are closely related to the SCW density,

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and there is an obvious structural difference between low- and high-density SCW.

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From the above dependence of νmax on pressure, different hydrogen bonded characteristics can

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be expected in SCW with different density. For SCW with high density, similar to ambient water,

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its structure can be regarded as three-dimensional hydrogen bonded networks. However, from the

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OH vibrations of molecular clusters, low-density SCW may be composed of dimer (H2O)2 and

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trimer (H2O)3 clusters, as described by Tassaing et al.17,20 Therefore, for gas-like SCW,

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three-dimensional hydrogen bonding is lost, and hydrogen bonding is present in the small

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molecular clusters as chains (or strings) of hydrogen bonds. From the above explanation of the

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Raman OH stretching bands, the tetrahedral hydrogen bonded network is the predominant 8


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three-dimensional hydrogen bonding motif in water. Therefore, it is reasonable to regard

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tetrahedral hydrogen bonding as the structural characteristic to differentiate the different structural

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types of SCW. Additionally, the critical region of SCW located at the transition between the liquid-

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and gas-like phases may correspond to the structural transformation from three-dimensional to

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chain (or string) hydrogen bonding.

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According to the dependence of OH vibrations on water molecular clusters (H2O)n,40,41 OH

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vibrations are closely related to the structural characteristics of hydrogen bonding. When the size

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of the clusters is less than the hexamer (n < 6), only chain (or string) hydrogen bonding is

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expected in the small clusters, and the increase of cluster size leads to an obvious decrease of

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DA-OH stretching vibrations. However, when three-dimensional hydrogen bonding occurs (n ≥ 6),

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different local hydrogen bonding motifs correspond to different OH vibrational frequencies, and

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the increase of cluster size does not affect the OH vibration frequency. Thus, the disappearance of

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three-dimensional hydrogen bonded networks undoubtedly plays an important role in the νmax of

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the Raman OH vibrations. Therefore, the obvious shift of νmax in the water critical region, as

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shown in Figure 2, can be attributed to the structural transformation from three-dimensional to

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chain (or string) hydrogen bonding. This is also in agreement with the above discussion on the

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water critical region, and corresponds to structural transformation from three-dimensional to chain

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(or string) hydrogen bonding in SCW.

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From the Raman OH stretching bands of water, tetrahedral hydrogen bonding (νDDAA-OH

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sub-band) is the main three-dimensional hydrogen bonded network. Therefore, it is important to

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investigate the structural changes of tetrahedral hydrogen bonds in SCW. The Raman OH

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stretching bands of SCW with densities from 0.11 to 0.62 g⋅cm−3 at 673 K are shown in Figure 4. 9



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No tetrahedral hydrogen bonding sub-band is observed in SCW with low density (0.11 and 0.19

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g⋅cm−3). However, the tetrahedral hydrogen bonding sub-band is observed in SCW when the

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density is higher than the critical density. Additionally, the increase of the SCW density increases

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the intensity of the tetrahedral hydrogen bonding sub-band. Therefore, as with hydrogen bonding

203

in SCW, the existence of tetrahedral hydrogen bonded networks in SCW is also closely related to

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the water density.

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From the above, for SCW in critical region, the obvious shift of νmax should be closely related to

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the disappearance of tetrahedral hydrogen bondings, which leads to the structural transformation

207

from three-dimensional to chain (or string) hydrogen bonds. Therefore, it is necessary to

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investigate the structural behaviors of tetrahedral sub-band at water critical point. Figure 4 shows

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the Raman OH stretching bands of SCW at 646 K and 0.39 g⋅cm-3, and 647 K and 0.11 g⋅cm-3.

210

From the figure, no tetrahedral hydrogen bonds can be found in the gas-like SCW, and only weak

211

tetrahedral sub-band can be observed in the SCW with density slightly higher than critical density.

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After carefully reexamining other Raman spectra of SCW at critical point,13 and considering the

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dependence of hydrogen bonding on SCW density, we suggest that the critical point may

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correspond to the disappearance of tetrahedral hydrogen bondings in SCW. Therefore, the critical

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point should connect to a known continuous phase transition in water just described in Alphonse et

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al. study.48

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In this work, to investigate the structure of SCW, molecular dynamics simulations were also

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carried out to obtain the radial distribution functions (RDFs) gHH, gOH, and gOO, which are shown

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in Figure S1 in the Supporting Information. For ambient water, the simulated radial distribution

220

functions are in good agreement with the experimental results of Soper.49 With increasing 10


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temperature from 295 to 647 K, the nearest neighbor hydrogen-bonded OH and HH peaks in the

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gOH and gHH functions become gradually smeared, clearly indicating that the hydrogen bonding

223

interactions between nearest neighbor water molecules considerably weaken. In addition, the

224

increase of temperature also leads to broadening of the first OO peak, and lowers the intensity of

225

the second peak at 4.5 Å, which is the characteristic peak of the tetrahedral arrangement of the

226

water molecules at ambient conditions. At the critical point, a well-resolved peak at 1.9 Å can still

227

be detected in gOH, which is the signature of the presence of hydrogen bonding, and the tetrahedral

228

hydrogen bonding in gOO is hardly distinguishable. For SCW at 673 K, inspection of the site–site

229

radial distribution functions confirms that SCW at 0.11 g·cm−3 is gas-like, with all three RDFs

230

slowly converging to their asymptotic values without oscillations. This is different from the RDFs

231

of liquid-like SCW.

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To measure the local structure in SCW, the hydrogen bonding number (average number of

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hydrogen bonds per water molecule, nHB) was calculated from the molecular dynamics simulations.

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Despite the fundamental role of hydrogen bonding in molecular processes, there is still not a

235

robust, self-consistent, and universally accepted definition of a hydrogen bond.50,51 In this work,

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the geometrical definition of hydrogen bonding is used to determine the hydrogen bonds in water.

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This definition makes use of both the oxygen–oxygen distance (rOO) and the ∠OOH angle

238

between two water molecules. For two water molecules to be considered hydrogen bonded, we

239

used 3.5 Å and 30° for the upper bounds of rOO and ∠OOH, respectively, based on a previous

240

study.52 From Figure 5, with increasing temperature, the decrease of nHB is closely related to the

241

water density. At the critical point, the hydrogen bonding number was determined to be 1.536.

242

From the above discussion on the structure of SCW, this value may be used to discriminate the 11



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type of SCW (gas-like or liquid-like).

244

From the above, the structure of SCW can be divided into three-dimensional and chain (or

245

string) hydrogen bonded networks, which correspond to liquid- and gas-like phases, respectively.

246

The critical region of SCW located at the transition between the liquid- and gas-like phases may

247

correspond to the structural transformation from three-dimensional to chain (or string) hydrogen

248

bonding. Additionally, the tetrahedral hydrogen bonding is the primary three-dimensional

249

hydrogen bonding in water, and can be regarded as the structural characteristic to differentiate the

250

different structural types of SCW.

251 252

CONCLUSIONS

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In this work, Raman spectroscopy and molecular dynamics simulations were used to investigate

254

the structure of supercritical water. We found that the hydrogen bonding in water is closely related

255

to temperature and pressure (or water density). According to the Raman spectroscopic study of

256

SCW, we suggest that the water critical point may correspond to the disappearance of tetrahedral

257

hydrogen bonding in SCW. From the dependence of νmax of the Raman OH stretching bands on

258

temperature and pressure, the structure of SCW can be divided into three-dimensional and chain

259

(or string) hydrogen bonded networks, which correspond to liquid- and gas-like SCW, respectively.

260

Therefore, the critical region of SCW is located at the structural transition between the

261

three-dimensional and chain (or string) hydrogen bonded networks.

262 263

ASSOCIATED CONTENT

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* Supporting Information 12


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The fitted equations of the νmax of supercritical water and the site–site radial distribution functions

266

of simulated water. This information is available free of charge via the Internet at

267

http://pubs.acs.org.

268 269

AUTHOR INFORMATION

270

Corresponding Author

271

*E-mail: [email protected]

272

Tel: +86 136 9358 6882

273

Notes

274

The authors declare no competing financial interest.

275 276

ACKNOWLEDGEMENTS

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The reviewers are greatly appreciated for providing good suggestions to revise the paper. This

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work is supported by the National Natural Science Foundation of China (Grant Nos. 41073048

279

and 41373057).

280 281 282 283 284 285 286 13



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REFERENCES

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(1) Shaw, R.W.; Brill, B.; Clifford, A.; Eckert, A.; Franck, E.U. Supercritical Water-A Medium for Chemistry. Chem. Eng. News 1991, 69, 26–39 (2) Akiya, N.; Savage, P. Roles of Water for Chemical Reactions in High-temperature Water. Chem. Rev. 2002, 102, 2725–2750 (3) Bröll, D.; Kaul, C.; Krämer, A.; Krammer, P.; Richter, T.; Jung, M.; Vogel, H.; Zehner, P. Chemistry in Supercritical Water. Angew. Chem. Int. Edit. 1999, 38, 2998–3014 (4) Postorino, P.; Tromp, R.H.; Ricci, M.A.; Soper, A.K.; Neilson, G.W. The Interatomic Structure of Water at Supercritical Temperatures. Nature 1993, 366, 668–670

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(5) Soper, A.K.; Bruni, F.; Ricci, M.A. Site-site Pair Correlation Functions of Water from 25 to

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400 °C: Revised Analysis of New and Old Diffraction Data. J. Chem. Phys. 1997, 106,

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247–254

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(6) Bellissent-Funel, M.-C.; Tassaing, T.; Zhao, H.; Beysens, D.; Guillot, B.; Guissani, Y. The

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Structure of Supercritical Heavy Water as Studied by Neutron Diffraction. J. Chem. Phys.

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1997, 107, 2942–2949

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(7) Tassaing, T.; Bellissent-Funel, M.-C.; Guillot, B.; Guissani, Y. The Partial Pair Correlation Functions of Dense Supercritical Water. Europhys. Lett. 1998, 42, 265–270 (8) Soper, A.K. The Radial Distribution Functions of Water and Ice from 220 to 673 K and at Pressures up to 400 MPa. Chem. Phys. 2000, 258, 121–137

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(9) Otomo, T.; Iwase, H.; Kameda, Y.; Matubayasi, N.; Itoh, K.; Ikeda, S.; Nakahara, M. Partial

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Pair Correlation Functions of Low-density Supercritical Water Determined by Neutron 14


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419

Figure 1. Raman OH stretching bands of water up to 1073 K and 383 MPa. The homogeneous

420

density is determined from the measurement of the homogeneous temperature, and the pressure is

421

determined by the equation of state of water.31

422 423 424

Figure 2. Dependence of νmax of the Raman OH stretching bands of water on temperature. The

425

fitted lines are shown to guide the eye.

426 427 428

Figure 3. The effect of pressure on νmax of SCW at 673, 773, and 873 K. The νmax of ambient

429

water up to 250 MPa is also shown. The solid lines are fitted lines, and the fitted equations of

430

SCW are listed in the Supporting Information.

431 432 433

Figure 4. Raman OH stretching bands of SCW at 673 K. The inset shows the Raman spectra of

434

water at the critical temperature.

435 436 437

Figure 5. Hydrogen bonding number per water molecule (nHB) as a function of density derived

438

from the simulations. The fitted lines are shown to guide the eye.

439 440 20


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