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Progress in Natural Science 19 (2009) 1651–1654

Short communication

Raman OH stretching vibration of ice Ih

Qiang Sun *, Haifei Zheng

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

Received 12 November 2008; received in revised form 10 June 2009; accepted 18 June 2009

Abstract

From our recent research on ice Ih (hexagonal ice), we deduce that free OH groups exist at the ice surface and that dangling OH influ-ences the interior of the proton-disordered ice. Additionally, exploiting recent research by others on water molecular clusters, we proposea new interpretation for the Raman OH stretching vibration of ice Ih. We deconvolute the Raman OH stretching vibration of ice Ih intofour sub-bands, with each sub-band being ascribed to OH vibrations engaged in different types of hydrogen bonding.� 2009 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science inChina Press. All rights reserved.

Keywords: Raman; Hydrogen bonding; Ice Ih

1. Introduction

The structure of ice Ih, the normal form of ice, has beenextensively explored ever since the pioneering work ofBragg (1922), and it has long been known that the oxygenatoms in ice occupy a regular tetrahedral network. How-ever, the hydrogen atoms are disordered over four possiblesites around each oxygen atom. Individual molecules candeviate from their lattice sites and molecular angles mightchange in response to the arrangements of neighboringmolecules, and this intrinsic disorder results in a broaden-ing in the atomic distributions. Thus, a precise descriptionof the molecular geometry of H2O is not possible [1,2].

The vibrational spectra of crystalline ice Ih have been theobject of numerous studies, with the stretching mode regionin particular receiving significant attention [3–9]. IR, paral-lel-polarized, and perpendicular-polarized Raman bandsare structured and quite different from each other, so theassignment of the OH stretching vibration has long been atopic of discussion. Whalley summarizes the efforts priorto 1977 to assign the different spectral features [4]. The lowest

1002-0071/$ - see front matter � 2009 National Natural Science Foundation o

and Science in China Press. All rights reserved.

doi:10.1016/j.pnsc.2009.06.010

* Corresponding author. Tel.: +86 10 62758103.E-mail address: Qiangsun@pku.edu.cn (Q. Sun).

frequency parallel Raman peak and the central IR peak wereassigned to global in-phase symmetric (m1) and antisymmet-ric (m3) stretching vibrations, respectively. To analyze theremaining features, Whalley assumed a similarity with a sim-ple ordered ferroelectric model of ice, and interpreted bandshapes in terms of longitudinal optical–transverse optical(LO–TO) splitting in the corresponding collective m1 and m3

vibrations. Rice et al. [6] performed extensive calculationsinvestigating how ice spectra are shaped by the interplaybetween inter- and intramolecular coupling. In contradic-tion to Whalley, they argue against a significant role of theLO–TO splitting in shaping the spectra. In addition, anotherinterpretation of the stretching vibration spectra of proton-disordered ice Ih is proposed by Buch and Devlin [7]. Theysuggest a tetrahedral model for the disordered ice structurecomposed of four oscillating dipoles in a tetrahedralarrangement around an O atom. In the current publication,we propose a new interpretation of the Raman OH stretch-ing vibration band based on our recent research on ice Ih.

2. Experimental

We prepared a transparent sample of ice (size about2 cm � 2 cm � 2 cm) by slowly freezing deionized water

f China and Chinese Academy of Sciences. Published by Elsevier Limited

Fig. 1. Due to the effects of dangling OH groups, three hydrogen-bondedenvironments for a water molecule can be drawn, which are thetetrahedral hydrogen bonding (DDAA), double donator–single acceptorhydrogen bonding (DDA), and single donor–double acceptor hydrogenbonding (DAA). Hydrogen bonding is shown with a dashed line.

1652 Q. Sun, H. Zheng / Progress in Natural Science 19 (2009) 1651–1654

at 0.01 K/s–263 K and keeping it for 24 h in a refrigerator,after which the ice was placed into a liquid nitrogen con-tainer. The temperature was monitored using a Ni90Cr10–Ni95Al5 thermocouple. We recorded the Raman OHstretching vibration of ice Ih and water at 270 and 275 K.

The Raman spectra were obtained using a confocalmicro-Raman system (Renishaw 1000). The excitationwavelength was the 514.5 nm line of an Ar+ ion laser oper-ating at 25 mW. The ±1 cm resolution spectra wererecorded with single scans, accumulation times of 10 s, aslit width of 50 lm, and an ocular of 50.

The Raman spectra were analyzed using the Jandel Sci-entific Peakfit v4.04 program. We first smoothed theRaman OH vibration spectra to reduce the noise and cor-rect for the baselines. Then, the Raman OH vibration spec-tra were fitted with Gaussians until we achieved a randomdistribution of fitted residuals.

3. Discussion

Ice Ih has the hexagonal structure (space group P63/mmc)and the water molecules occupy C3v symmetry sites. Sincethe molecules have C2v symmetry, their orientations mustbe disordered to give these average symmetries, leading tothe conclusion that ice Ih is proton disordered. Thus, whilethe O atoms form a periodic pattern, the H atoms are ran-domly arranged within the constraints of the so-called icerules. The ordinary hexagonal ice Ih is composed of stacked,buckled hexagonal bilayers, and corresponds to a nearly per-fect tetrahedral network of hydrogen bonds. This structure isfavorable in the interior, but not at the surface because sur-face formation is associated with numerous dangling H andO atoms with unsaturated hydrogen bond coordination.Currently, the exact H-bonding environment at the ice Ih sur-face still raises questions. However, there is a consensus thata large fraction (50% or more) of molecules in the first half-bilayer of the ice Ih surface have one free OH group, whereasthe other half is H-bonded to the second half-bilayer. Thedangling motion enhances the rotational motion of the watermolecules. The vibrational density of states shows a couplingbetween the rotational vibration and the lattice vibration ofwater molecules in the surface layer, and the vibrational cou-pling causes a distortion of the ice lattice. Through thehydrogen-bonding network, the distortion is transmittedinto the interior of the crystal [10]. In addition, simulationshave been performed to model ice adsorbate layers on metalsin the coverage range of 2–4 bilayers and a new kind of defectin the ice structure, an internal dangling OH bond, isreported [11].

In recent years, X-ray absorption spectroscopy (XAS)has been employed to investigate the structure of water inthe gas, liquid, and ice forms [12]. In principle, XAS is sen-sitive to local hydrogen-bonding patterns, because itprobes primarily the instantaneous electronic arrangementin the first coordination shell. From XAS experiments [12],the gas-phase spectrum exhibits well-separated peaks corre-sponding to O 1 s excitations into the antibonding OH 4a1

and 2b1 molecular orbitals at low energies (534 and536 eV). The spectrum from the ice surface is similar tothat of liquid water; both spectra have a peak in the pre-edge region (around 535 eV), a dominant main-edge(537–538 eV), and less intensity compared with bulk icein the post-edge region (540–541 eV). In combination withthe XAS of the ice surface with NH3 [13], Wernet et al.assigned intensities in the pre- and main-edge regions towater molecules with one uncoordinated OH group,whereas the intensity in the post-edge region was assignedto fully coordinated molecules. It should be noted thatthe bulk ice spectrum is dominated by intensity in thepost-edge region; however, a weak main-edge structure isalso detected [12–15]. From these data, we deduce thatthe effects of dangling OH cannot be ignored when study-ing the spectroscopy of ice Ih.

From the previous discussion, the hydrogen-bondedenvironment of a water molecule can be deduced (seeFig. 1). The normal form of ice Ih has an oxygen atomictetrahedral arrangement around one O atom. From theBernal–Fowler ice rules [1,2], the central water moleculecan accept two hydrogen atoms belonging to other mole-cules and form two hydrogen bonds. The molecule can alsodonate its two hydrogen atoms to form hydrogen bondswith oxygen atoms from other molecules. Due to three-dimensional hydrogen bonding, a tetrahedron aroundone water molecule tends to form. However, due to theeffects of the dangling OH group, two other three-dimen-sional hydrogen bonding structures can also form, espe-cially near the ice Ih surface. One of the structures is awater molecule that accepts two hydrogen atoms butdonates a hydrogen atom, and the other is a water mole-cule that donates two hydrogen atoms but accepts onehydrogen atom. From this, we conclude that four hydro-gen-bonded environments may exist around one watermolecule in ice Ih. These are the tetrahedral hydrogenbonding (DDAA), the double donor–single acceptorhydrogen bonding (DDA), the single donor–double accep-tor hydrogen bonding (DAA), and free OH. Therefore, theRaman OH stretching vibration of ice Ih may be deconvo-luted into four sub-bands.

The Raman OH vibration band of ice Ih at 270 K and0.1 MPa is shown in Fig. 2, while Fig. 3 shows the water

Fig. 2. The Raman OH stretching vibration band of ice Ih at 270 K and0.1 MPa. After baseline correction, the band can be fitted into fourGaussian sub-bands located at 2988, 3150, 3350, and 3506 cm�1. Thefitted residuals are also shown in the figure.

Fig. 3. The Raman OH stretching vibration band of water at 275 K and0.1 MPa. After baseline correction, the band can be fitted into fiveGaussian sub-bands located at 3043, 3211, 3408, 3537, and 3632 cm�1.The fitted residuals are also shown.

Q. Sun, H. Zheng / Progress in Natural Science 19 (2009) 1651–1654 1653

band at 275 K. Obvious spectral changes are detected dur-ing the melting of ice to water. Bergren et al. [5] report localmaxima at 3050, 3150, 3370, and 3500 cm�1 in ice Ih at150 K. We fit four Gaussian functions to the Raman OHvibration spectra of ice Ih, with peaks located at 2988,3150, 3350, and 3506 cm�1 (see Fig. 2). This approach issupported by the random distribution of the residuals ofthe fit, and it is also in accordance with the previous discus-sion on hydrogen bonding. In addition, the same logicapplies to the Raman OH stretching vibration of water,which may be fit using five Gaussian sub-bands (see Fig. 3).

The normal vibration mode for single H2O molecules is2A1 (which includes a symmetric stretching vibration m1 at3657.05 cm�1 and a bending vibration m2 near 1595 cm�1) +B1 (an antisymmetric stretching vibration m3 at 3755.97

cm�1) [16,17], and they are all Raman-active. In addition,studies have shown that a strong power laser (>5 W) isneeded to excite the m3 vibration. Therefore, we concludethat the stretching band is due to the OH symmetric stretch-ing vibration m1.

Recently, small water clusters have been the subject ofextensive experimental and theoretical research, and manymeasurements on the OH stretching vibration of free gas-phase water and of water complexes trapped in macro-scopic rare-gas matrices have been reported. These studiesprovide some general guidance concerning the significanceof individual OH vibrational modes. Therefore, by com-paring with the Raman OH vibration of liquid water, wecan understand and assign each of the sub-bands intro-duced earlier.

The 3506 cm�1 component is assigned to the free OHvibration. Comparing with the Raman OH vibration ofwater, the most striking change is the loss of intensity at3150 cm�1. In melting from ice Ih to water, the importantstructural change is expected to be the disappearance ofthe periodic pattern due to the O atom. As a result, weattribute the 3150 cm�1 component to tetrahedral hydro-gen bonding around a water molecule. From the researchon free gas-phase clusters [18,19], Buck et al. conclude thatthe OH vibration related to the 3-coordinated single DAAshould lie between 2950 and 3100 cm�1, and that the OHvibration related to 3-coordinated DDA should appearnear 3350 cm�1. This is because the DAA molecular bondsoptimize the single linear hydrogen-bond geometry muchmore than is the case for the DDA molecules, which haveto accommodate two bent bonds [20]. From these results,we attribute the 2988 cm�1 component to the OH vibrationrelated to DAA hydrogen bonding, and the 3350 cm�1

component is attributed to the OH vibration related toDDA hydrogen bonding.

4. Conclusions

From our recent studies on the structure of ice Ih, wededuce that free OH groups exist at the ice surface and thatdangling OH influences the interior of ice Ih. From theseresults, and exploiting recent research by others on watermolecular clusters, we propose a new interpretation forthe Raman OH stretching vibration of ice Ih. The RamanOH vibration of ice Ih at 270 K can be fitted using fourGaussian sub-bands centered at 2988, 3150, 3350, and3506 cm�1, which we assign to mDAA–OH, mDDAA–OH, mDDA–OH, and the free OH vibration, respectively.

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