OH/OD Isotopic Shift Factors of Isolated and H‑Bonded SurfaceSilanol GroupsKristina Chakarova, Nikola Drenchev, Mihail Mihaylov, Plamen Nikolov, and Konstantin Hadjiivanov*

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia, 1113, Bulgaria

*S Supporting Information

ABSTRACT: The OH/OD isotopic shift factors (i = νOH/νOD) of isolatedsilanols on SiO2 and [Si]BEA are between 1.3563 and 1.3568, values lower thanthe theoretical shift of 1.3744. However, i of the harmonic OH modes almostcoincides with the theoretical value which indicates that the experimentaldeviations in this case are mainly due to anharmonicity. The anharmonicityslightly decreases when the OH groups participate in weak H-bonding withadsorbed CH4 or CO which should lead to an increase of i. However, contrary tothese expectations, i additionally decreases. This is attributed to the lower acidityof the OD groups as compared to the respective OHs. The value of i is also lowerfor H-bonded silanols as compared to isolated SiOH groups. It is concluded that idepends on the extent of H-bonding which allows easy distinguishing between isolated and H-bonded surface hydroxyls.

1. INTRODUCTION

Most of the dispersed inorganic materials contain surfacehydroxyl groups.1,2 The latter determine, to a high extent, thesurface properties of the solids and consequently theiradsorption and catalytic performances. The surface hydroxylscan be generally divided into two types: isolated and H-bonded.1,2 As a rule, the isolated OH groups are more stable,while the H-bonded hydroxyls can easily recombinate to water,although in some cases they demonstrate a remarkablestability.3

Different kinds of OH groups can be distinguished on thebasis of their IR spectra. H-bonded hydroxyls are characterizedby broader and more intense OH stretching bands than isolatedones and vibrate at lower frequencies. Red shift and broadeningof ν(OH) bands are also caused by the electrostatic field,hydroxyls in zeolite cavities being typical examples.4 However,the ν(OH) position depends on different factors and cannot beused to determine the extent of H-bonding. In many cases thebandwidth is also not applicable because of the possibilities ofsuperimposition of different bands. Therefore, it is of interest toelaborate criteria for distinguishing between isolated and H-bonded hydroxyls and estimation of the strength of the H-bond. In this connection, the different temperature shift of theν(OH) maxima has been noted:5 bands of isolated hydroxylsare shifted to lower frequencies when the temperatureincreases, while the opposite effect takes place with H-bondedOH groups. However, this criterion is also not reliable because,according to Rekik et al.,6 for asymmetrical H-bonds ν(OH) isshifted to lower frequencies with a temperature increase.An often used approach to check the accessibility of OH

groups is the exchange of their protons with deuterium.1,7−33

The most convenient way is to use D2O,5,7−23 but D2 has also

often been applied.24−33 Another reason to perform an H/Dexchange is to obtain high quality spectra.16 Due to the light

scattering, in some cases the OH region is too noisy, and OHspectra are of low quality; sometimes hydroxyls are even notobservable. However, the O−D modes lie at around 1000 cm−1

below the OH modes and are in a region where the noise islimited. Sometimes the H/D exchange is performed to checkfor eventual spectral effects, e.g., Fermi resonance.8,19,20

In the ideal case the bands due to OH groups should beshifted, after exchange with deuterium, by the isotopic shiftfactor which is easily calculated on the basis of the formula34

νν

μμ

= =i OH

OD

OD

OH (1)

where μ is the reduced mass. The force constant is assumed tobe the same for the OH and OD bonds. According to eq 1, theOH/OD isotopic shift factor should be 1.3744.Figure 1 summarizes published literature data on the OH/

OD isotopic shift factor for various surface hydroxyl groups. Itis clearly seen that the experimentally obtained values of i arelower than the theoretical value and, for the OH groupsabsorbing in the 3800−3450 cm−1 region, are typically between1.35 and 1.365,7−33 (see Figure 1). In this wavenumber range iremains almost constant, although a tendency for a slightdecrease with the wavenumber can be seen. Lower values of iare reported for OH groups absorbing below 3450 cm−1, andthey concern H-bonded hydroxyls. A similar situation isobserved with OH groups from adsorbed water,35 but someadditional complications can arise in these cases due to the splitof the OH modes into symmetric and antisymmetric ones. Forthat reason, we shall not discuss here water molecules.

Received: January 4, 2013Revised: February 15, 2013Published: February 20, 2013

Article

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© 2013 American Chemical Society 5242 dx.doi.org/10.1021/jp400106s | J. Phys. Chem. C 2013, 117, 5242−5248

Concerning surface OH groups, in most cases the reasons forthe deviations of i from the theoretical value have not beendiscussed. However, there are many papers dealing with bulksystems where similar effects have been reported.36−39 Variousmodels have been proposed, and the anomalous isotopic shiftfactor has been associated with the anharmonicity of the OHbond.36,38 In a recent review,37 three main components of theanomalous OH/OD isotopic effects have been underlined: (i)higher anharmonicity of the OH vibrations as compared toOD; (ii) contribution of the bending modes; and (iii)tunnelling effects taking place with strongly hydrogen-bondedsystems. It has also been underlined that the situation withweak H-bond was complicated because in these cases theanharmonicity decreases.In this work we report on the OH/OD isotopic shift factors

of isolated and H-bonded silanols on SiO2 and dealuminatedBEA zeolite ([Si]BEA). Silica was chosen because of therelatively simple spectrum in the OH region and the availabilityof many literature data concerning the hydroxyl coverage of thisoxide.7,9,29 Dealuminated BEA zeolite was used because it ischaracterized by stable H-bonded hydroxyls manifesting intensebands in the IR spectra.3 We demonstrate that weak H-bonds,although reducing the anharmonicity of the OH modes, cause adecrease of the isotopic shift factor.

2. EXPERIMENTAL SECTION2.1. Materials. SiO2 was a commercial Aerosil sample with

a specific surface area of 336 m2 g−1. A tetraethylammoniumBEA (TEABEA) zeolite (Si/Al = 11) was provided by RIPP(China). It was calcined (15 h, 823 K) and then dealuminatedby treatment with 65% HNO3 (4 h, 390 K). The sample thus

prepared will be denoted as [Si]BEA. No residual aluminumwas proven by chemical analysis.Carbon monoxide (>99.5% purity) was supplied by Merck.

Acetonitrile-d3 (99.6 atom % D) originated from AldrichChemical Co., Inc. (USA). Methane (4.5) was provided byMesser Griesheim GmbH (Germany). D2O (isotopic purity of99.9%) was obtained from Cambridge Isotope Laboratories,Inc.

2.2. Material Characterization. The IR measurementswere carried out in the 8000−800 cm−1 spectral region using aNicolet 6700 FTIR spectrometer with wavenumber precision of0.01 cm−1 and equipped with an MCT detector. The spectrawere registered at a spectral resolution of 2 cm−1 andaccumulating up to 256 scans. Self-supporting pellets (≈10mg cm−2) were prepared by pressing sample powders at about100 kPa and were directly treated in the IR cell. The latterallowed measurements between 100 K and ambient temper-ature. Prior to the adsorption experiments, the samples wereactivated by heating for 1 h at 673 K under oxygen andevacuation for 1 h at the same temperature. Deuteroxylationwas achieved by treatment of the samples with D2O vapor (200Pa) at 673 K followed by evacuation at the same temperature.

3. RESULTS3.1. OH and OD Groups on SiO2. The background

spectrum of the activated SiO2 sample (Figure 2, panel A,spectrum a) is typical of silica.7,9,29 A sharp band at 3746 cm−1

tailed to lower frequencies is assigned to silanol groups. Abroad shoulder centered on ca. 3665 cm−1 is due to H-bonded

Figure 1. OH/OD isotopic shift factor vs OH stretching frequency.Squares, isolated OH groups in zeolites (data from Table S1 from theSupporting Information); circles, H-bonded OH groups in zeolites(data from Table S2 from the Supporting Information); triangles, OHgroups in nonzeolitic systems (data from Table S3 from theSupporting Information).

Figure 2. FTIR spectra of activated silica registered at ambienttemperature (a) and at 100 K (b) and of deuteroxylated silica atambient temperature (c) and at 100 K (d). Panel A, ν(OH) region;panel B, ν(OD) region; panel C, 2ν(OH) region; panel D, 2ν(OD)region.

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hydroxyls. In the overtone region there is a band at 7325 cm−1,also tailed to lower frequencies, which is assigned to the2ν(OH) modes (first overtone) of the silanol groups9 (Figure2, panel C, spectrum a). Its integral intensity is ca. 1.5% of theintensity of the 3746 cm−1 band. No feature corresponding tothe overtone of H-bonded hydroxyls is distinguished. Bands atca. 4563 and 4530 cm−1 are also recorded (Figure S1 from theSupporting Information, panel A, spectrum a). They are due tothe combinations of the OH stretchings with the δ(OH)modes. The appearance of two bands in this region has beendebated, and it was recently proposed that this was due to theexistence of two surface modes with bending character arisingfrom the coupling of the bending Si−OH modes with Si−O−Sistretchings.9

Exchange with D2O leads to almost full disappearance of theband at 3746 cm−1 and appearance of a corresponding ODband at 2762 cm−1 (Figure 2, panels A and B, spectra c). Thecalculated OH/OD isotopic shift factor is 1.3563 whichpractically coincides with the reported isotopic shift factor ofSiOH groups on SiO2

7 and on H-ZSM-5.20,21 For convenience,the X-axes scale of panel B corresponds to the X-axes scale ofpanel A, shifted by the experimental isotopic shift factor(1.3563) which allows easy comparison of the spectra.It is seen (Figure 2, panel A, spectrum c) that part of the H-

bonded hydroxyls resisted isotopic exchange; i.e., they wereinaccessible. This makes it difficult to calculate the isotopic shiftfactor of the H-bonded hydroxyls in this case. A weak band at3370 cm−1 is also seen after exchange (Figure S1 from theSupporting Information, panel B, spectrum c) and is attributedto the ν(SiO−D) + δ(Si−O−D) combination mode. Theappearance of one band confirms that the silanols are from onefamily. In the overtone region, a band at 5434 cm−1 is detected(Figure 2, panel D, spectrum c). Note that the experimentalisotopic shift factor for the overtone modes is different(1.3481), and the X-axes scales of panels C and D from Figure2 are considered with it. The integral intensity of the bandattributed to the 2ν(O−D) modes is about 1% of the intensityof the band at 2762 cm−1 characterizing the fundamentalvibrations. This indicates a lower anharmonicity of the ODoscillator as compared to OH.Lowering temperature to 100 K leads to small changes in the

positions of the bands. The band at 3746 is shifted to 3748cm−1 (Figure 2, panel A, spectrum b) and the overtone at 7325to 7329 cm−1 (Figure 2, panel C, spectrum b). The OD band at2762 cm−1 is shifted to 2763 cm−1 (Figure 2, panel B, spectrumd) and the overtone from 5434 to 5436 cm−1 (Figure 3, panelD, spectrum d). No changes were detected in the ODcombination band (Figure S1 from the Supporting Information,panel B, spectrum d), while the two OH combination bandsbecame better resolved and two maxima, at 4573 and 4514cm−1, were well distinguished (Figure S1 from the SupportingInformation, panel A, spectrum b).3.2. Complexes of OH and OD Groups on SiO2 with

CH4, CO, and CD3CN. Three bases of different strength havebeen adsorbed on activated silica and on the deuteroxylatedsample to follow the shift of the OH and OD groups afterformation of H- and D-bonds. Because CH4 and CO are weakbases, their adsorption was performed at 100 K, while theadsorption of CD3CN was performed at ambient temperature.Panels A and B of Figure 3 show the changes in the OH and

OD regions, respectively, occurring after adsorption of CH4,CO, and CD3CN on SiO2. It is seen that in all cases the silanolbands are shifted to broader and more intense bands at lower

frequencies (see Table 1). It is clear that the higher the shiftvalue, the broader and the more intense the new band. It is also

seen that the CD3CN-shifted OD band appears at awavenumber lower by 24 cm−1 than the value expected onthe basis of the isotopic shift factor (2528 cm−1). The samephenomenon, but less pronounced, was observed with the ODbands when complexes with CH4 and CO were formed. Also,the intensity increase after complexation with CD3CN isdefinitely lower for the OD band as compared to the OH one.For brevity, here we will not discuss in detail the changes of

the spectra of adsorbed CO and methane. We shall only notethat the carbonyl band of the SiOH···CO and SiOD···CO

Figure 3. Difference FTIR spectra of silica (A, C) and deuteroxylatedsilica (B, D) after adsorption of CH4 (a, d); CO (b, e); and CD3CN(c, f). Equilibrium pressure of CO = 500 Pa, CH4 = 1500 Pa, andCD3CN = 400 Pa. Temperature of adsorption is 293 K for CD3CNand 100 K for CH4 and CO. Panel A, ν(OH) region; panel B, ν(OD)region; panel C, 2ν(OH) region; panel D, 2ν(OD) region.

Table 1. Shift of SiO-H and SiO-D Stretching Modes, FullWidth at the Half Maximum (fwhm) of the Shifted Band,and Changes in the Integral Absorbance

complex

shift of OH/ODmodes relative toisolated groups

fwhm of theOH(OD)band

increase of theOH extinction

coefficient, A/A0a

SiO-H−CH4 −34 cm−1 13.5 cm−1 2.4SiO-D−CH4 −24 cm−1 9.7 cm−1 2.3SiO-H−CO −87 cm−1 24.9 cm−1 4.2SiO-D−CO −62 cm−1 16.8 cm−1 4.2SiO-H−CD3CN −318 cm−1 101.5 cm−1 13.1SiO-D−CD3CN −210 cm−1 69.8 cm−1 10.8

aA, integral absorbance of the shifted OH(OD) band; A0, integralabsorbance of the OH(OD) band of isolated groups.

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species was detected at 2155 cm−1; i.e., CO was not able todistinguish between OH and OD groups.Consider in more detail the spectra of adsorbed CD3CN. It is

known34 that the C−N stretching modes (2263 cm−1 in liquidphase) are sensitive to the acidity of the hydroxyls to whichCD3CN is bound: the higher the frequency, the higher theacidity. Note that deuterated acetonitrile is usually used toavoid the split of the C−N modes due to Fermi resonance.The spectra of CD3CN adsorbed on SiO2 are presented in

Figure S2 from the Supporting Information, panel A. Underequilibrium pressure of 800 Pa the C−N band is detected at2276.4 cm−1 (Figure S2 from the Supporting Information,panel A, spectrum a). The band decreases in intensity with thedecrease of the equilibrium pressure and is slightly shifted tolower wavenumbers (2275.0 cm−1). The spectra of CD3CNadsorbed on deuteroxylated SiO2 are very similar. However, acareful analysis shows that, compared to the SiO2 sample: (i)the C−N bands are slightly less intense under the sameequilibrium pressures and (ii) they appear at slightly lowerfrequencies. The latter difference is small but systematic whencomparing spectra with the same intensities of the C−N band(see Figure S2 from the Supporting Information, panels B andC). These two observations allow concluding that CD3CN is aprobe sensitive enough to detect the differences between theacidities of the SiOH and SiOD groups.Panels C and D from Figure 3 present the spectra in the

overtone regions. It is evident that the relative intensities of theshifted bands are much lower as compared to the situation inthe region of the fundamental vibrations. Moreover, theintensities of the shifted bands fade with the basicity of theadsorbed molecule, and they are practically not observed afteradsorption of CD3CN. The intensity weakening of the firstovertone of the H-bonded OH oscillator is consistent withliterature data.40

3.3. OH and OD Groups on [Si]BEA. To check in moredetail the situation with the H-bonded hydroxyls, we studied a[Si]BEA sample. It is well-known that the BEA zeolite is easilydealuminated which results in the formation of stable H-bonded silanols believed to form hydroxyl nests.3 The spectrumof activated [Si]BEA is shown in Figure 4, panel A, spectrum a.A strong band at 3732 cm−1 is assigned to internal silanols. Ashoulder around 3745 cm−1 evidences some isolated silanols atthe external zeolite surface (see the inset in Figure 4A). Thereis also a very intense band centered on 3525 cm−1. On the basisof the literature data,3 we assign it to H-bonded hydroxyls fromsilanol nests. The respective terminal silanols are characterizedby a band at ca. 3710 cm−1 appearing as a shoulder (see theinset in Figure 4A). Lowering temperature leads to a small blueshift of the band due to isolated, internal, and terminal silanols(to 3738 cm−1), while the band due to H-bonded silanols isred-shifted to 3509 cm−1 and simultaneously increases inintensity (Figure 4, panel A, spectrum b).The spectra of [Si]BEA registered after D-exchange are

presented in Figure 4, panel B. Bands at 2752 and 2609 cm−1

are detected at ambient temperature and shifted, at 100 K, to2755 and 2598 cm−1, respectively. Note that the band due toD-bonded deuteroxyls appears at frequencies higher by 11−13cm−1 than expected on the basis of the isotopic shift factor forisolated silanols.The spectra in the overtone regions are shown in Figure 4,

panels C and D. A band at 7306 cm−1 (7310 cm−1 at 100 K) isattributed to the overtone of the silanol band. The respectiveovertone of the SiOD band is registered at 5419 cm−1 (5422

cm−1 at 100 K). Note that no overtones of the H-bondedsilanols (D-bonded SiOD groups) were registered.The data obtained for the ν(OH), 2ν(OH), ν(OD), and

2ν(OD) modes of free and H-bonded silanols are summarizedin Table 2. The detection of the overtones allows calculatingthe harmonic OH and OD frequencies and the anharmonicitycoefficients which are also presented in Table 1. Details on thecalculations are presented elsewhere.29,37 Briefly, the anharmo-nicity constant, X12, is calculated by the formula

ν ν= −X12

(2 )12 OH OH (2)

The harmonic frequency, νOH,harm, can be easily obtainedaccording to the equation

ν ν= + X2OH,harm OH 12 (3)

It is seen that the anharmonicity of the O−H group slightlydecreases when weak H-bonds are formed. Unfortunately, dueto the negligible intensities of the OH/OD overtone bands ofthe complexes of CD3CN with OH/OD groups, theanharmonicity in this case cannot be calculated.

4. DISCUSSION4.1. Isotopic Shift Factor of Isolated Silanols. Usually

the spectra of isotopically labeled molecules are well predictedby theory, and the experimentally observed isotopic shift factorspractically coincide with the calculated ones. However, the bigdifference between the masses of hydrogen and deuterium isthe reason for important deviations in the case of H/D

Figure 4. FTIR spectra of activated [Si]BEA registered at ambienttemperature (a) and at 100 K (b) and of deuteroxylated [Si]BEA atambient temperature (c) and at 100 K (d). Panel A, ν(OH) region;panel B, ν(OD) region; panel C, 2ν(OH) region; panel D, 2ν(OD)region.

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exchange. The results presented here, as well as a huge amountof published literature data (see Figure 1), indicate that the shiftof the isolated surface OH groups is about 1.355, a value lowerthan the theoretically predicted one (1.3744). This phenom-enon has not been widely discussed in the literature. Recently,an empiric equation has been proposed24

ν ν= +0.709( ) 103.7OD OH (4)

It was also noted that the H-bonded hydroxyls deviated fromthe equation.It is normal to suppose, as Zhdanov et al.29 did, that a

decrease of the isotopic shift factor is related to an increase ofanharmonicity. To check this hypothesis, we calculated theisotopic shift factors of the harmonic frequencies (see Table 2).It is seen that, taking into account the harmonic frequencies,the isotopic shift factor for isolated silanols (1.372−1.373)becomes very close to the theoretical value. Similar is thesituation with the free hydroxyl radicals. Thus, the OH and ODstretching modes in this case are at 3568.0 and 2632.1 cm−1,respectively,41 which corresponds to an isotopic shift factor of1.3556. However, the harmonic OH and OD frequencies are at3735.2 and 2720.9 cm−1, respectively, and in this case theisotopic shift factor is 1.3728. Therefore, we can conclude thatthe deviations of the isotopic shift of isolated OH groups aremainly due to anharmonicity, as generally accepted.4.2. Acidity of OH and OD Groups and Isotopic Shift

of H-Bonded Hydroxyls. Let us now consider the situationwith the H-bonded hydroxyls on SiO2. The data presented inTable 2 show that weak H-bonding leads to a decrease of theOH anharmonicity. This is in agreement with data from theliterature concerning weak H-bonds in bulk systems.42−44

However, this could be due simply to the shift of the bandmaxima because the anharmonicity decreases with the wave-number.37 The effect of the wavenumber can be estimatedcomparing the anharmonicity of the modes of isolated SiOHand SiOD groups (see Figure 5). It can be assumed that, at firstapproximation, the dependence is linear. Thus, the dotted linein Figure 5 shows the anharmonicity coefficients of OH/ODgroups which are expected if they depend only on thewavenumber. It is clearly seen from the figure that the effectof H-bonding on the anharmonicity is more pronounced as thesimple effect of the wavenumber change. Therefore, one canconclude there is another reason weak H-bonding results in amore harmonic oscillator. Probably, the O−H part of theasymmetric potential wells formed in linear complexes is closerto the Morse type potential curve. However, deviations fromlinearity could cause even an increase of the anharmonicity.

Taking into account the above considerations, one canexpect that the OH/OD isotopic shift factor should approachthe theoretical value with the decrease of the anharmonicity.However, the experimental results revealed just the oppositetendency (see Figure 1). Below we will discuss the reasons forthis.Recent measurements have shown that the OH bond is 3%

longer as compared to the OD bond; i.e., the O−D bond isstronger.45 This reflects a lower acidity of OD groups.46,47

Consequently, the intermolecular hydrogen bond in water isshorter by ca. 4% as compared to D2O; i.e., the H-bond isstronger than the D-bond. The difference in the acidities of theOH and OD groups in our SiO2 sample, although very small,was also revealed by the spectra of adsorbed CD3CN.Therefore, the shift of the O−D modes induced by

adsorption of weak bases (and corrected by the isotopic shiftfactor) should be smaller, as compared to the shift of the OHmodes. This was exactly what we have observed. Hence, itappears that the effect of the acidity on i is essential.

Table 2. Fundamental and Overtone OH and OD Modes of Silanol Groups, Harmonic Frequencies, Anharmonicity Constants,and OH/OD Isotopic Shift Factors

sampletype of OHgroups

ν(OH),cm−1

ν(OD),cm−1 iexp

2ν(OH),cm−1

2ν(OD),cm−1

X12 (OHanharmonicity),

cm−1harmonic

ν(OH), cm−1garmonic

ν(OD), cm−1 iharm

SiO2 SiOH 3746 2762 1.3563 7325 5434 83.5 3913 2852 1.3720SiO2 SiOHa 3748 2763 1.3565 7329 5436 83.5 3914 2853 1.3719SiO2 SiOH···CH4

a 3714 2739 1.3560 7265 5391 81.5 3877 2826 1.3719SiO2 SiOH···COa 3660 2700 1.3556 7170 5321 75 3810 2779 1.3710SiO2 SiOH···CD3CN 3429 2552 1.3437 - - - - - -[Si]BEA SiOHa 3738 2755 1.3568 7310 5422 83 3904 2843 1.3732[Si]BEA SiOH···COa 3644 2689 1.3552 7126 5287 81 3806 2780 1.3691[Si]BEA SiOH···Olatt.

a 3509 2598 1.3507 - - - - - -aMeasured at 100 K.

Figure 5. Plot of anharmonicity constants of some SiO-H and SiO-Dmodes observed with SiO2 vs the wavenumber.

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Discuss now the H-bonded hydroxyls on the activated[Si]BEA sample. By analogy with the OH groups interactingwith different weak bases, one should expect a lower OH/ODisotopic shift factor as compared to isolated silanols. This waswhat we have experimentally observed. Here we note that, dueto negligible intensity of the overtone bands, no definiteconclusions on the anharmonicity could be drawn, and we donot exclude the decrease of i to be partly due to enhancedanharmonicity.Finally, we would like to note that surface complexes

represent good model systems for investigations. At first, theyare well-defined, and normally there is no hindering from othermolecules, as is the case for solutions. In addition, one canperform all experiments at low temperate thus reducing thetemperature effect. From this point of view the present study ispioneering, and we realize that additional investigations arenecessary to develop the potential of using the isotopic shiftfactor for characterization of surface hydroxyls.

5. CONCLUSIONS•The experimentally obtained OH/OD isotopic shift factor ofisolated silanols is 1.3563−1.3568 and is lower than thecalculated one on the basis of the harmonic oscillator model(1.3744). This deviation is mainly due to the anharmonicity.•Formation of weak H-/D-bonds leads to a decrease of the

anharmonicity of the OH/OD groups which should cause anincrease of the OH/OD isotopic shift factor. However, due tothe lower acidity of the OD groups as compared to therespective OHs, the isotopic shift factor is even lower than withthe isolated OH groups. This allows distinguishing betweenisolated and H-bonded hydroxyls as well as estimation of thestrength of the H-bond.

■ ASSOCIATED CONTENT*S Supporting InformationTables (Tables S1−3) and FTIR spectra (Figures S1−2). Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +3592 9793598. E-mail: kih@svr.igic.bas.bg.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Bulgarian Scientific Fund(grant DCVP 02/2).

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