Vibrational Spectroscopy 51 (2009) 213–217

The Raman OH stretching bands of liquid water

Qiang Sun

The School of Earth and Space Sciences, Peking University, Beijing 100871, China

A R T I C L E I N F O

Article history:

Received 16 July 2007

Received in revised form 20 April 2009

Accepted 6 May 2009

Available online 15 May 2009

Keywords:

Water

Raman

Hydrogen bonding

Cluster

A B S T R A C T

In this work, from the discussion on water structure and clusters, it can be deduced that the OH

stretching vibration is closely related to local hydrogen-bonded network for a water molecule, and

different OH vibrations can be assigned to OH groups engaged in various hydrogen bonding. At ambient

condition, the main local hydrogen bonding for a molecule can be classified as DDAA (double donor–

double acceptor), DDA (double donor–single acceptor), DAA (single donor–double acceptor) and DA

(single donor–single acceptor) and free OH vibrations. As for water at 290 K and 0.1 MPa pressure, the

OH stretching region of the Raman spectrum can be deconvoluted into five sub-bands, which are located

at 3014, 3226, 3432, 3572, and 3636 cm�1, and can be assigned to nDAA-OH, nDDAA-OH, nDA-OH, nDDA-OH, and

free OH2 symmetric stretching vibrations, respectively.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

The hydrogen bonding in liquid water holds an important key toits peculiar behavior and has great implications for chemical,biological, and geological processes. Moreover, the OH stretchingvibration is sensitive to the variations of hydrogen bonding andthus the study of OH vibrations can be used as an approach forunderstating the hydrogen bonding. However, due to its complex-ity, there is no generally accepted view on Raman OH stretchingvibration. Based on the asymmetric shape of Raman OH stretchingregion, Ratcliffe and Irish [1] and Walrafen [2] assigned the band at3230–3260 cm�1 to Fermi resonance between OH stretching andovertone of the bending mode, the band at 3450 cm�1 to asymmetric stretching and the band at 3630 cm�1 to an asymmetricstretching of H2O. Later, Walrafen and Chu [3] assigned the band at3250 cm�1 to the in-phase OH stretching motions of a hydrogen-bonded aggregate consisting of a central H2O molecule and itsnearest neighbors. Furthermore, they suggested the band at3400 cm�1 as the OH stretching motions of those H2O moleculesthat lost the phase relationship. Recently, Khoshtariya et al. [4–6]deconvoluted the OD stretching overtone and OH stretchingregions of the IR spectra into five sub-bands and proposed anotherassignment for those individual species.

More recently, studies of water clusters have attracted muchattention both experimentally and theoretically. The studies focuspredominantly on the OH stretching vibration region for freeclusters and clusters connected to benzene and phenol chromo-phores. Moreover, X-ray absorption spectroscopy (XAS) and X-rayRaman scattering (XRS) at the oxygen K edge (O 1s) for water

E-mail address: QiangSun@pku.edu.cn.

0924-2031/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.vibspec.2009.05.002

structure have also been reported [7–11] and proved to be apowerful technique to reveal unique information about thehydrogen bonding in water.

In this work, from the discussion on water structure andclusters, the Raman spectrum of water in the OH stretching regionat ambient temperature is deconvoluted into five sub-bands, andeach sub-band is assigned to an OH group engaged in differenthydrogen-bonded network.

2. Experimental

In this experiment, deionized water, H2O was used as a startingmaterial. The ambient-pressure Raman spectra were measured at275 and 290 K. Raman spectra were collected using a 50� objectiveand confocal micro-Raman system (Renishaw 1000). An Ar+ ionlaser with an excitation wavelength at 514.5 nm was operated at25 mW. The spectrometer with an entrance slit of 50 mm was usedto collect the signals. The resolution is about 1 cm�1. Each Ramanspectrum was recorded for 30 s.

The Raman spectra are analyzed by Jandel Scientific Peakfitv4.04 program. The Raman spectra were firstly smoothed untilnoise diminished, and then corrected the baselines. Later, Gaussianfunctions were used to fit the spectra to minimize the randomdistribution of fitted residuals.

3. Discussion

3.1. Liquid water structure

As for water molecules, apart from the universal van der Waals’interaction, a specific interaction, hydrogen bonding also exists,and many of the unique properties of water are attributed to the

Q. Sun / Vibrational Spectroscopy 51 (2009) 213–217214

result of three-dimensional hydrogen bonding network formedbetween water molecules. It is well known that the hydrogen bondin liquid water arises as a result of electrostatic interactionbetween a hydrogen atom and some excess of negative charge on aneighboring oxygen atom belonging to another molecule [12].Therefore, three kinds of hydrogen bond criteria can be con-ceivably classified as geometric [13], energetic [14] and hybrid [15]ones.

Although the structure of liquid water has been intensivelyinvestigated, the results remain controversial. Most of the modelscan be divided into two categories: (a) the mixture/interstitial and(b) the distorted hydrogen bond (continuum) [16]. For the former,the mixture models postulate the simultaneous existence of two ormore relatively long-lived structures in the liquid, such as the‘‘flickering-cluster’’ model proposed by Frank and Wen [17].Different and discrete combinations of hydrogen-bonded mole-cules are assumed to coexist as evidenced by the existence ofisosbestic points which are well known in the spectroscopy ofreversible chemical reactions [18]. For the latter, currently themost favored model, is based on the assumption that the structurerelaxes on a time scale that is also observed in other liquids.Moreover, water is thought to exist as a continuous network ofmolecules interconnected by somewhat distorted hydrogen bonds[14,19].

Much work has been done to understand the detaileddescription of the hydrogen bond network in liquid water. Resultsfrom X-ray and neutron scattering experiments and ab initio

molecular dynamics (MD) simulations have suggested a locallytetrahedral liquid structure, wherein (on average) each watermolecule is hydrogen-bonded to four nearest neighbors via twodonor and two acceptor bonds [20]. This view was recentlychallenged by Wernet et al. [7] who used X-ray absorptionspectroscopy and X-ray Raman scattering to probe the moleculararrangement in the first coordination shell of liquid water. Bycomparison with bulk and surface of ordinary hexagonal ice (ice Ih)and calculated spectra, they reported that a dominant fraction ofwater molecules was in configurations with only two stronghydrogen bonds; one donating and one accepting with theconfigurations connected via a three-dimensional weakly hydro-

Fig. 1. (a) The dependence of OH stretching frequencies on cluster size. Different symbo

[24,26], n3 [24,26], free-OH [24,26,27,53], PD-OH [24,26,27,54,55], DA-OH [24], DDA-O

different colors are applied to differentiate free gas-phase and water complexes trapped

stretching region can be fitted with five Gaussian sub-bands (Table 1); the fitted resid

gen-bonded network [7,9]. Such a structure is in contrast to thetraditional perspective which suggests that most molecules are inlocal tetrahedral configurations, not too different from ice.

3.2. Deduction from water clusters

So far, most experimental measurements on water clustershave been conducted by IR spectroscopy. As for water, the normalmodes are IR and Raman active; this means that the frequenciesare the same. However, due to the difference in selection rules, theshapes and intensities are very different. This should be notedwhen IR measurements are applied to study the Raman spectrumof water.

For a single H2O molecule, the vibrational normal modes are:2A1 (including a symmetric stretching vibration n1 at3657.05 cm�1 and a bending vibration n2 near 1595 cm�1) + B1

(anti-symmetric stretching vibration n3 at 3755.97 cm�1) [21,22],they are all both Raman (and IR) active. In water dimer, whichconsists of two nonequivalent water molecules, the hydrogenbonding can be differentiated into the PD (proton donor) and PA(proton acceptor). The four major absorption bands can beobserved at 3745, 3735, 3660 and 3601 m�1, respectively [23–28], which are assigned to the excitation of the symmetric (n1) andasymmetric (n3) stretching vibrations in the nonequivalent dimerconstituents (the PA and PD molecule). As for trimer, tetramer andpentamer, theoretical calculations have demonstrated that theyshould be quasi-planar structures, where each H2O can interactwith neighboring molecules by DA interaction [29–33]. Addition-ally, experimental measurements have demonstrated that thebonded OH vibrations are located at 3533, 3416 and 3360 cm�1,and free OH vibrations are located at 3724, 3714 and 3714 cm�1,respectively [24,25]. Because water hexamer represents a transi-tion from cyclic to three-dimensional geometry, it has beenextensively studied by theoretical methods [34–44]. Recent ab

initio studies have established that several of these isomers (chair,boat, and cage) are very close in energy, and their relativestabilities have repeatedly been shown to sensitively depend onthe level of calculations. With the increase of cluster size (n > 6),theoretical calculations have demonstrated that the clusters have

ls are applied to discriminate OH stretching vibrational modes of water clusters, n1

H [24], DAA-OH [24], and different structures of hexamers [53–57]. Additionally,

in various macroscopic rare-gas matrices. (b) At 290 K and 0.1 MPa, the Raman OH

uals are also shown in the figure.

Fig. 2. The local hydrogen-bonded networks for a water molecule. Hydrogen

bonding is drawn in dashed line. (a) DDAA, DDA, DAA, and DA can be expected to be

the main hydrogen bonds in liquid water at ambient condition. (b) At ambient

temperature, structures DD, AA, D and A can be ignored.

Q. Sun / Vibrational Spectroscopy 51 (2009) 213–217 215

three-dimensional conformers [45–47]. Fig. 1(a) shows thedependence of OH vibration frequency on cluster size. Incomparison with free gas data, slight shifts of OH stretchingvibration to lower frequencies are reported in macroscopic rare-gas matrices [24]; they are also marked in the figure. From thefigure, it can be deduced that the OH vibration is closely related tolocal hydrogen bonding for a water molecule. In this work, localhydrogen bonding means the interactions between a watermolecule with neighboring molecules, or the hydrogen-bondednetwork in the first coordination shell of the molecule. Aftercooperative effects on hydrogen bonding are correctly estimated, itmay be reasonable to investigate the OH stretching vibration inlocal hydrogen bonding network. From theoretical calculations onsmall ring water clusters (n = 2–6), the effects can represent up to20–30% of their total binding energy and affect the OH stretchingvibrational spectra [48,49]. Additionally, from Fig. 1(a), thecooperative effects can only affect the DA-OH vibration for thesmall water clusters (n < 6), the effects become weaker withcluster size, just like the systematic contraction trends of DA-OHvibration.

Fig. 3. Intensity-normalized difference of Raman spectra measured at 275 and 290 K. Du

sub-bands. Based on this, it is reasonable to fit the Raman spectrum of liquid water in

From the above discussion, the following conclusions can bederived. (a) The OH vibration is closely related to the localhydrogen-bonded network for a water molecule. (b) Different OHbands can be assigned to OH engaged in corresponding localhydrogen-bonded network. (c) The DDAA hydrogen bonding(tetrahedral structure) tends to form in large water clusters,therefore DDAA-OH cannot be measured in small water clusters.

3.3. Interpretation of the Raman OH stretching region

For a water molecule, the local hydrogen-bonded network canbe differentiated by the participation of the molecule with theneighboring molecule to form hydrogen bonds either as protondonor (D), proton acceptor (A), or their combinations. From thetheoretical studies on water clusters [34–36], local hydrogenbonding for a molecule can be discriminated as DDAA, DDA, DAA,DA, DD, AA, D and A (Fig. 2). Because DD and AA structures haveslightly repulsive two-body components between the end frag-ments [50], they are disfavored. D and A can exist in dimers formedby two nonequivalent water molecules, which may be present insupercritical water. From the Boltzmann distribution, it isreasonable to ignore D and A at ambient temperature. Therefore,it can be concluded that the main local hydrogen bonding can beexpected to be DDAA, DDA, DAA and DA at ambient temperature.This implies that liquid water comprises rings or chains besidesthree-dimensional hydrogen bonding, and is supported by recentXAS studies on water structure [7–11]. Correspondingly, theRaman OH stretching vibration should be deconvoluted into fivesub-bands, DDAA-OH, DDA-OH, DAA-OH, DA-OH and free OH.Additionally, this deconvolution can be demonstrated by thenormalized intensity difference of water between 275 and 290 K(Fig. 3 and Table 1), which is frequently utilized to observe ormeasure weak changes of band profiles caused by small variationsof physical or chemical nature. In this work, the OH stretchingregion in the Raman spectrum of water at 290 K and 0.1 MPa isfitted with five Gaussian sub-bands (Fig. 1(b)) that are located at3014, 3226, 3432, 3572, and 3636 cm�1, respectively. Thereasonability of this deconvolution can also be evidenced by therandom distribution of fitted residuals as shown in the figure.

For a single water molecule, the vibrational modes include asymmetric stretching vibration n1, a bending vibration n2 and ananti-symmetric stretching vibration n3, which are all Raman active.

e to the obvious asymmetry, the difference spectrum can be deconvolved into five

the OH stretching region with five sub-bands.

Table 1Fitting results of the Raman spectrum of water in the OH stretching region at 275

and 290 K, under 0.1 MPa pressure.

Mode 275 K 290 K

Frequency FWHM Intensity

(%)

Frequency FWHM Intensity

(%)

DAA-OH 3004 145 1.1 3014 130 0.8

DDAA-OH 3227 228 40.9 3226 219 38.6

DA-OH 3431 219 48.6 3432 228 53.6

DDA-OH 3565 144 5.6 3572 129 4.3

Free OH 3633 110 3.6 3636 96 2.7

Q. Sun / Vibrational Spectroscopy 51 (2009) 213–217216

For the bending mode, its Raman intensity is weak, therefore theovertone is usually much weaker. Additionally, according toaccurate depolarization ratio measurements, Walrafen et al. haveascribed the band around 3250 cm�1 to OH stretching vibrationengaged into tetrahedral hydrogen bonding [3]. For n3 vibration,studies have shown that strong power laser (>5 W) is needed inorder to excite it [3]. Therefore, the Raman OH stretching bandshould be the results of OH symmetric stretching vibrationengaged in hydrogen bonding.

The 3014 cm�1 component is assigned to DAA-OH stretchingvibration. This assignment is in agreement with the Gottingengroup’s works [51]. From their works on free gas-phase clusters,they derived that the OH vibration related to single donor–doubleacceptor (DAA) should lie from 2950 to 3100 cm�1. It should benoted that DAA is obviously different from DDA. As already notedin the past, the DAA bonds manage to optimize the single, linearhydrogen bond geometry much better than the DDA molecules,which have to accommodate two bent bonds. Therefore, O� � �Odistances emanating from bonds between DDA and DAA moleculestend to be longer (2.8 A) than bonds emanating from DAA to DDA(2.6 A), and DDA is weaker [52].

The 3226 cm�1 sub-band can be ascribed to the DDAA-OHstretching vibration. This assignment is in accordance withWalrafen et al.’s works [3], they ascribed the band to the in-phaseOH stretching motions of a hydrogen-bonded aggregate consistingof a central H2O molecule and its nearest and farther neighbors.

The 3432 cm�1 band is assigned to DA-OH stretching vibration.It follows from Fig. 1 that DA is weaker than DAA and stronger thana DDA hydrogen bonding. Therefore, the 3432 cm�1 component isascribed to DA-OH stretching vibration. Additionally, the coop-erative effects can affect the DA-OH vibration for the small waterclusters (n < 6), the effects become weaker with cluster size.

From Fig. 1, it can be found that the sub-band around3572 cm�1 should be assigned to DDA-OH stretching vibration.This is in accordance with the work of Steinbach et al., whodeduced that the OH stretching vibration related to a doubledonor–single acceptor bond should be at about 3550 cm�1 [51]. Incomparison with DAA, the DDA is weaker.

The 3636 cm�1 sub-band is assigned to free OH2 symmetricstretching vibration. As for the 3700 cm�1 vibration of waterclusters, it actually results from the free n3 vibration. IR measure-ments of free monomer H2O suggested that the intensity of anti-symmetric stretching vibration at 3755.97 cm�1 was much strongerthan that of symmetric stretching vibration at 3657.05 cm�1.However, Raman data showed that the intensity alterations forthe two OH stretching modes can be observed. Additionally, theintensity alterations were also reported in the IR and Ramanmeasurements of (H2O)2, (H2O)3, (H2O)4 and (H2O)5 [31]. Therefore,the sub-band is assigned to free OH2 symmetric stretching vibration.

4. Conclusions

In this work, from discussion on water structure and clusters,the OH stretching vibration is closely related to local hydrogen

bonding, and this can be applied to investigate the OH stretchingregion in the Raman spectrum of liquid water. At ambienttemperature, the main local hydrogen bonding network can beexpected to comprise DDAA, DDA, DAA, DA and free OH bonds.Correspondingly, as for water at 290 K and 0.1 MPa, the Ramanspectrum in the OH stretching region can be deconvoluted intofive sub-bands, which are located at 3014, 3226, 3432, 3572, and3636 cm�1, and can be assigned to nDAA-OH, nDDAA-OH, nDA-OH,nDDA-OH, and free OH2 symmetric stretching vibrations, respec-tively.

Acknowledgements

The two anonymous reviewers are greatly appreciated for theircareful examination of the manuscript and providing me goodsuggestions to revise the paper.

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