Decoupled vibrational spectra for H2O in D2O Ice Ic

Volume

70, number

CHEMICAL

2

DECOUPLED VIBRATIONAL

PHYSICS

LElTERS

I March 1980

SPECTRA FOR H,O IN D,O ICE I,

Gary BITZHAUPT, Wtiiam B. COLLIER, Cynthia THORNTON and J. Paul DEVLIN Department Received

of Chemrstty,

17 December

Oklahoma State Univemty,

Shllwzter,

Oklahoma 74074. USA

1979

Fundamental frcquencres for Hz0 Isolated m D,O Ice I, have been determmed by a combmation of Founer transform infrared difference spectroscopy and Interference enhanced Raman spectroscopy. The results are mterpreted m terms of a ~2 frequency (1732 cm-‘) such that rermt resonance of vt wrth 2~2 causes an enhanced spacmg of v1 (3200 cm-‘) and “3 (3270 cm-t).

1. Introduction Major strides have been made m recent years in analyzing the vrbratronal spectra of the condensed phases of water. The progress has been based on irnproved mfrared [l] and Baman [l-3] data as well as advances in theoretrcal methodology 141. The data base has been expanded recently followmg the discovery that D20 molecules can be isolated in both H20 ice I, and amorphous ice [S]. As a result the values of vl, u2 and y3 for D20 vrbratronally decoupled from the host H20 matrix are known as are the corresponding HOD values [2,3]. These data have been fit to a model by Sceats and Rrce m their evaluation of the bond-bond interaction force constant as well as the Fermi resonance parameter for ui and 2v2 [4]. This modeling also permitted predictions to be made regardmg the v,-v3 band complex for H20 decoupled in D,O amorphous Ice and Ice I,. Early attempts to measure vt and u3 band posrtrons for H20 isolated in D20 ice I, were seemingly unsuccessful [S]. While it was expected that the splitting would be sizeable (WOO cm-t ) and approximately symmetrical about the “OH value for isolated HOD, only a single intense feature, apparently coincident with VOH, was observed. The data suggested that either the deposit was seriously contaminated with HOD or that, for some unfathomed reason, isotopic exchange was rapid r.n a D20 matrix at the deposition temperature (135 K). However, the calculation of

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Sceats and Rice suggested a third alternative, namely that the vt values for decoupled H20 (rgnoring Fermi resonance) are only weakly spht about vOB and that Fermi resonance with 2v2, rather than increasing the splitting as for D20, actually pushes the vi and v3 absorptions into a single feature nearly coincident with vOH -

At the same time that the latter analysis of the H20 in D20 spectra became available, the infrared capabrbties in our laboratory were constderably enhanced through acquisition of a vacuum Fourier transform infrared spectrometer @@lab FTS-20). The accuracy and spectral stripping capabditles of this system are well-suited to a definitive look at the H20 in D20 infrared spectrum. In this paper we report the results of a new study of the H20 in D20 ice I, spectrum together with a consistent interpretation of the decoupled H,O, D20 and HOD spectra.

2. Experimental Samples of H20 isolated in D,O were prepared by conventional matrix isolation procedures except that a crystahine ultrathin film of pure D,O was first deposited at 170 K with the main deposit made at 135 K, permitting expitaxial formation of a crystalline sample. Also, to prevent HOD contaminants from dominating the vt-v3 OH stretching region and the D20 associatron band from completely concealing

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the H,O v2 band, the H20 percentage in the D,O matrix was at least 5%, well above a true Isolation level. Deuterated water, “100%” from Merck and Company was used after vacuum degassing while the Hz0 was purified by deionizing, distdling and degassing prior to use. Infrared spectra of the deposits were recorded at 90 K, 135 K and ~155 K using a Digilab FTS-20 spectrometer. The sample temperature was controlled using an Air Products CS-202 closed-cycle cooler v&h automatic thermal regulation. The 155 K temperature was chosen since proton exchange occurred on a convenient time scale (a30 min) so that the decrease of the Isolated H20 bands and the growth of the isolated HOD bands could be followed. Subsequently the HOD absorbance spectrum was converuently scaled and subtracted from the original spectrum to display the “pure” Hz0 in D20 spectrum (with HOD contaminants removed). Alternatively, the final (HOD) spectrum could be subtracted, essentially unscaled, from the initial (H20) spectrum so as to eliminate the D20 matrix bands and to present positive H20 and negative HOD features. The former procedure best displayed the H,O vI-v3 band complex while the latter was most useful in the v2 band region. Isolated H20 spectra have also been produced by subtraction of the pure D20 ice I, spectrum from that of the Hz0 in D,O matrix. Raman spectra in the v1 -v3 region were recorded by the mterference-enhanced mtemal reflection method [6-S] from thin films (20-50 microns) deposited in a glass Dewar cell and cooled with hquid nitrogen. Excitation was via -1 .O W of 4880 A radiation from a Coherent Radiation model 53 argon laser. Since polarization characteristics are retained in the scattered light, when the interference enhanced method is employed, measurements of both IIand 1 scattered hght was useful.

LE-ITERS

1 Max& 1980

note the appearance of the sample spectrum in the u2 region of isolated H,O and HOD prior to, and after, proton exchange occurs. This is possiile through the curves presented in fig. 1. Curves (A) and (B) are the spectra for H,O isolated in D20 (150 K) before and after proton exchange has been thermally promoted. Without additional manipulation of these curves it is apparent that a shoulder near 1730 cm-*, OQthe D20 association band which is centered at 1630 cm-l, weakens while a-definite band appears at 1510 cm-l as a result of proton exchange. That impression is confirmed by curve (C), the difference between the initial and final spectra, since a maximum is apparent at 1732 cm-t and a minimum occurs at =I500 cm-t_ However, the difference spectrum also clearly indicates that isolated H20 has a band near 1570 cm-l which disappears as a result of proton exchange with the D20 matrix. The 1732 and al 570 cm-l bands

3. Results 3.1. Injkred

spectra

The most reassuring approach to an analysis of the vi-v3 infrared band system for Hz0 isolated m D20, a region which is clouded because of overlap with the “OH band of the isolated HOD contaminant, is to first

Fig. 1. Infrared absorbance spectra in the v2 bending mode frequency range for Hz3 isolated in D,O at 2.50 K: (A) orig ind deposit; (B) same sample after proton exchange is thermaIIyeqilllibrated; (C) spectrum (B) subtracted from spec trum (A) after scaling by 1.05 (to account for some convession of D20 to HOD); (0) spectrum (A) with pure D20 spec-

trum subtracted.

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can also be noted in curve (D) wluch results from the difference spectrum between H,O m D,O and pure D,O. The assignment of these features wfl be considered in a later section. The point of emphasis 1s that the proton exchange of Hz0 with the D,O matrut can be clearly discerned, particularly through growth of the I5 10 cm-’ band of HOD. Further, the near absence of that band in the crlginal sample spectrum is testimony that the deposit IS only lightly contaminated with HOD. It follows that the spectrum m the O-H JQ -Q region is most hkely dommated by Hz0 isolated m D,O. It 1s not conceivable that the HOD concentration exceeded that of the H,O in the original D,O matrices and we estimate a 3 : 2 ratio of H,O to HOD wlthin those deposits (i.e. ~25% of H,O present as HOD). The curves

in fig. 2 become very informatlve when studled from that viewpoint. Curve (A) is the original deposit absorbance spectrum wlule curve (B) IS the HOD spectrum that results from nearly complete pro-

1 March 1980

ton exchange. If(B) is scaled by the proper factor (I.e. e-25) and subtracted from (A) we should obtain the ZJ~-Y~ band complex for Hz0 in D20 ice I,. From the reasoning above, that factor IS most probably between 0.2 and 0.4 and almost certainly between 0.0 and 0.6. The results of subtracting the HOD band from the original deposit spectrum using a scaling factor of 0.3 and 0.6 are shown in curves (C) and (D) respectively. Comparison of curves (C) and (D) with each other and curve (A) shows that the general form of the ZQ-u3 band complex is unchanged by these subtractions. In each case the dominant peak appears at 3270 cm- 1, a pronounced shoulder can be noted at ~3200 cm-1 and a weak band is visible at 3430 cm-‘. We conclude that each of these three features is produced by the pseudo-isolated H,O molecules. Thus, within the OH stretching region, the effect of proton transfer to form isolated HOD is the loss of the ~3200 cm- 1 band, the splittmg and broadening of the 3430 cm-l shoulder into a weak doublet (3420 and 3470 cm-‘) and the growth of a new band at 3370 cm-l which precisely overlaps the isolated H20 3370 cm- 1 band. The new band is broader than the voH band of isolated HOD reported by Rice et al. [2], as is the H20 band 111the original deposit, a fact that reflects the somewhat greater H20 concentration used tn the present work. 3.2. Raman spectra

3400

cni’

3200

Fig. 2. Infrared absorbance spectrum mode regron for Hz0 Isolated m D20 depont; (B) same sample after proton (C) spectrum (A) after subtracuon of spectrum (A) after subtractlon of(B)

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3c

D

in the OH stretching at 90 K- (A) onginal exchange is equdrbrated; (B) scaled by 0.30, (D) scaled by 0.60.

Although reported orally 191, the I&man spectrum of D20 isolated in H,O Ice I, has never been descnbed m prmt. Fig. 3a shows the II and 1 Raman spectra in the OD stretching region for isolated D20 (-5% in H20) as obtamed by a single internal reflection, at the synchronous angle [lo], from the sample-CsBr prism interface. The important characteristic of this interference enhanced spectrum, in addition to the roughly 40-fold increase m signal to noise ratio over that obtainable by external reflection [6,7], is the retentlon of polarization characteristics by the scattered hght. As a result, the 2367 cm-l band, previously assigned to the v1 symmetric stretching mode from mfrared measurements, is strongly reduced in intensity for the 1 polarized scattered light, wlule the y3 band at 2444 cm-1 is depolarized_ This is consistent with the interpretations given for the infrared spectrum [4,5] and would suggest that the Fknan spectrum

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CHEh¶lCAL PHYSICS LETTERS

Z March 1980

presence of HOD in the sample through the contri%ution of the polarized vOH vibration to the 3270 cm-l band. It is clear from fig, 3a that the OD stretching band (asterisk) for HOD isolated in Hz0 is strongly polarized so it would be surprising if VOHfor HOD ti D20 was not the source of a poiarized Raman component. In fact Scherer and Snyder have shown that these viirations give rise to simikly polarized Raman scattering for ice Ih 131.

3.3. Miscellaneous

Fig. 3. (a) Parallel (A) and perpendicularly CB) polarized Raman spectra for Da0 isolated U-IHz0 at 90 K (* denotes VODfrom HOD contammant). (b) Parallel (C) and Perp~ndicUIarly (D) polarized Raman spectra for Hz0 IsoIated i.n D2O at 90 K. (Curve (E) 1s the adjusted Hz0 in D20 infrared absorbance curve (C) of fig. 2.) for f-I20 isolated in D20 wouId be useful in pinpointing the vz and v3 frequency values for that case as well. The JXaman spectra for Hz0 isolated in D20, using the sampling procedure mentioned above, is presented in fig. 3b for ice I, at 90 IL A comparison of curves (C)(parallel pokuization) and (D) (perpendicular polarization) shows two intense bands in the OH stretching region, one of whxh is strongly polarized (3200 cn~-~) and the second relatively depolarized (3270 cm-r). It is certain that the 3270 cm-* band is a composit with some (depolarized) contribution from H,O and a polarized component from pOH of the isolated HOD contammant. The band position matches the 90 K infrared band frequency for these two vibrations. The two surprising aspects of the isolated H20 Raman spectrum, the relahue weakness of the polarized component (v,), and the partial polarized character of the 3270 cm- 1 band, are both caused by the

it has been shown that v2 for HOD in D20 apparently occurs at 1 S 10 cm- * (fe. 1). Scherer and Snyder report a value of 1495 for HOD in a SO : SO mixture of H20 and D20. A new measurement of the HOD band that emerges in an Hz0 matrix upon proton exchange with isolated D20 gives a v2 value of 1465 cm- 1 for that case. Clearly the HOD bending mode couples quite strongly with v2 of the particular ice matrix and is forced up in frequency in the D,O case and down for the H,O medium (i.e. away from the mat& v2 value). A similar repulsion has been reported for the v3 nitrate ion mode for MCNO$ ion pairs isolated in H200rD,0 matrices [I 1 J. Thus, this coupling with the matrix modes is certainly not unexpected. A comment is necessary about proton exchange rates at the 1 SS K sampling temperature. An early series of proton transfer measurements, designed to reveal the activation energy for the proton exchange in H20 ice I,, indicated a half life of ~30 min at 175 K and extrapolated to a half life of nearly 10 h at 155 K [ 121. After that series of measurements our vacuum system was exposed to HF during a second series of’ measurements designed to show the effect of HF dopant on the activation energy for proton transfer m ice I,. That series yielded inconsistent values for ~~12 as a function of temperature and, subsequently, half hves at 155 K were reduced to the 30 min range, Extended efforts to establish the source of inconsistencies have led to some extremely interestiug observations, including the separation of the proton transfer and molecular rotation steps in the generation of isolated HOD starting from isolated D,O. These results, and what they imply with respect to the earlier series of measurements, will be the subject of a separate article. 297

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4. Discussion From the infrared and Raman results presented in the earlier sections it is now possible to present the

complete set of fundamental tnbrational frequencies for DzO, Hz0 and HOD isolated in ice I, as given m table 1. The combined infrared and Raman data for Hz0 in D,O seems to require identification of v, with the polarized shoutder at 3200 cm-1 while v3 stands clearly at 3270 cm-l. Thus, the pattern in the stretching region, when the zJOHmterference IS removed, is remarkably simllar to that found for D20 in Hz0 ice I, Hnth a u1 -ZJ~ sphttmg (70 cm-l) the same withm experimental errot. The extra feature (3430 cm-l) in the stretching region, by analogy with the published assignments for sinuI.lr features for isolated HOD 133, can be assigned to a ~tret~~g-tr~slation~ combinatton mode, although a contribution from 2v, is posable. We have confirmed the Sceats-Rice prediction that 2~~of Isolated Hz0 IS coincident with “OH of HOD [4] but, whereas they have suggested that vl would aIs0 cortverge on VoH because of Fermi resonance with a lower energy 2v2 state *, our data show a surprisingly large v3-pI spacmg. The source of this * Actually the calculated mfrared spectrum for isolated Ha0 presented by Sceats and Rtcc has a shoulder near 3225 cm-’ which they have not discussed nor attempted to relate to the v1 mode.

Table 1 Compfete set of observed and cakulated a) fundamental frequencies (cm-‘) for HzO, DzO and HOD Isolated m appropnate tee I, matrices H20 in D20

D,O in Hz0

HOD (matnx)

Y, (obr) ~2 cobs.) vj fobs.)

3200 1732 3270

2367 1230 2444

3270 2418

vt kale.) v2 kdc.) “J kalc.1

3233 1716 3284

2339 1252 2406

3260 2372 1502

1488

(D20)

fH,O) I

1510 0320) 14”

‘H20’

a) Calculated from fo ce constants adjusted to fit Hz0 and D20 observed freq tiencres (km = 5.95 mdynfA, k_, = 0.81 mdynfA. kw = 0.06 mdynfll, i&~ 0.0) with km*and kra constramed to tnrtat values.

t March 1980

large spacing seems to be revealed by an analysis of the v2 region for isolated H20. Of the two observed isolated H20 features in this region (fig. l), at 1732 cm-l and 1570 cm- I, only the 1732 cm-l value can reasonably be asslgned as the bendmg fundamental. This conclusion flows from (a) a review of the D,OHOD-H20 2~2frequency pattern, (b) an application of the product rule and (c) an approximate normal mode analysis. With respect to (a) a comparison of the gascrystal frequency shifts for DzO and HOD, using the values in table 1 (above) and table 2 of ref. [13], show blue-shifts of 52 cm-l and 85 cm-l respectively. An exttapolatlon to a shift value of 118 cm-’ for Hz0 would give an isolated Hz0 “2 frequency of 17 13 cm-1 _ Siarly, application of the product rule to D,O vl (2392) and v2 (1230) values and the pt value of H20 (3270 cm-l) yleIds a ~2 value for H20 of 1707 cm-*, where the v1 values corrected for Fermi resonance [4] have been used. The product rule, usmg uncorrected u1 values (table l), would give 1723 cm-l for p2 of isolated HZO. A correction for anharmonicity would lower these predicted v2 values somewhat. VIbratIonal frequencies for Hz0 have also been calculated using three different sets of force constants, with the fist two sets fitted to the D,O observed frequencies corrected for Fermi resonance. For set I (krr = 6.15 mdyn/& k,, = 0.79 mdyn/A), the interaction constants were set equal to zero. For set II (k,, = 6.16 mdyn/A, k,, = 0.78 mdynli%), krrt(0.06 mdyn/Q was borrowed from ref. [4] and constrained to that value. Both calculations place v2 for Hz0 near 1700 cm-t (1689 cm-1 and 1686 cm-l respectively). For set III, the value of krrtwas constrained to the set II v&e, whrle I& and k,, were adjusted to the observed uncorrected isolated Hz0 and D20 frequencies, simultaneously. The set III force constants and frequencres are presented in table 1 which shows that the same force constants that give v2 for HOD (1502 cm-‘) close to the average observed value (1488 cm-‘) place v2 for H,O above 1700 cm-t. It should be noted that the isolated H20 uz frequency may be increased sigmficantly through resonant interaction with the D,O matrix v2 mode so the value of 2% appropriate for estimating the Fermi resonance effect becomes d~~cuit to evaluate.

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CHEMICAL PHYSICS LETTERS

1 March 1980

Acknowledgement

[51 G. Rltzhaupt, C. Thornton and J.P. Devlin, Chem Whys.

We are grateful to the National Science Foundation for support of this research through project grant (CHE 770653) and through a grant for purchase of the FTS-20 spectrometer (CHE 78-01764).

t’51P.C. LI and J.P. Devlin, J. Chen Phys 59 (1973) 547.

References [II E. Wballey, Can. J. Chen 55 (1977) 3429. [2] T.C. Sivakumar, D. Schuh, M.G. Sceats and S.A. Rice, Chem Phys. Letters 48 (1977) 212. [3] J.R Scherer and RG. Snyder, J. Chen Phys. 67 (1977) 4794. 141 M.G. Sceats, M. Stavola and S.A. Rice, J. Chem. Phys. 71 (1979) 983.

Letters 59 (1978) 420.

I71 P. Huber-W&hli and HsH, Ciinthard, Spectrochim. Acta 34A(1978) 1253. 181 L. D’Hooge and J.M. Vigoureux, Chem. Phys, Letters 65 (1979) 500. 191 C. Ritzhaupt, C. Thornton, MS. Khatka!e and J9. Devlin, Paper MH6, Thirty-Fourth Symposius on Molecular Spectroscopy, Columbus, Ohio, June 1979. P.K. Tlen, k. Ulrich and RJ. Martin. AppL Phys Lettes 14 (1969) 291. G. Ritzhaupt and J.P. Devhn, J. Phys Chem 79 (1975) 2265. G. Ritzhaupt and J.P. Deviin. Chem Phys, Letters 65 (1979) 592 J.R Scherer. in. Advances in infrared and Raman spectroscopy, VoL 5, eds RJ.H. Clark and RE. Hester

(Heyden, London, 1978) ck 3.

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