raman spectral studies of aqueous solutions of non-electrolytes: dimethylsulfoxide, acetone and...

Raman Spectral Studies of Aqueous Solutions of Non=electrolytes: Dimethylsulf oxide, Acetone and Acetonitrile?

Surjit Singht and Peter J. KruegerD Department of Chemistry, The University of Calgary, Calgary, Alberta, Canada T2N 1N4

Raman spectra of aqueous solutions of dimethylsulfoxide, dimethylsulfoxide-d6, acetone and acetonitrile are reported in the OH stretching region, and in the stretching regions of the functional groups of these solutes. Raman intensities and frequency shifts of these bands show definite trends with varying concentrations of the solutions. The results obtained are discussed in terms of the mixture model of water, and they are compared with results obtained from Raman spectral studies of aqueous electrolytes. It is proposed that water structure breaking and the formation of solute-solvent complexes when organic solutes are added to water can account for these results.

INTRODUCTION

The Raman spectrum of water in the fundamental OH stretching region has been duscussed recently by Walrafenl.* on the basis of his earlier ~ t n d i e s ~ ~ and those by Bernstein7 et al. Effects of t empera t~ re ,~ .~ pressure and various solute^^'^ on the OH stretching band contours have been discussed. The Raman results are consistent with a mixture model for water in which hydrogen bond breaking occurs with increasing temperature or with the addition of certain electrolytes. The stimulated Raman results,”-” in particular, appear to rule out continuum models for water structure.

Walrafen3 resolved the OH stretching band contour of water into four Gaussian components at 3247, 3435, 3535 and 3622 cm-l. On the basis of the temperature dependence of the intensities of these components and the appearance of an isosbestic point at -3460 cm.-l, the bands at 3535 and 3622cm-’ were assigned to intramolecular valence vibrations of non-hydrogen bonded water and the bands at 3247 and 3435cm-l were assigned to vibrations of lattice water. In another study Murphy and Bernstein7 approximated the OH stretching band contour with a symmetric Gaussian- Lorentzian product function and resolved it into five components at 3215,3400,3455,3545 and 3635 cm-l. To explain the existence of five components in the OH stretching band contour, these authors assumed that liquid water is mainly made up of tetraco-ordinated molecules having approximately Czu symmetry and a small concentration of a species most probably having

t Presented at 6th International Conference on Raman Spectro- scopy, Bangalore, India (1978).

$ CIDA/NRC Research Associate, 1975-1977. Permanent Address: Chemistry Department, Indian Institute of Technology, Madras-600 036, India.

8 Author to whom correspondence should be addressed.

C, symmetry, with three hydrogen bonds. The low frequency bands at 3215 and 3400 cm-I were assigned to a Fermi resonance between the overtone of the bending vibration at 1640 f 5 cm-’ and the symmetric stretching vibration of the tetraco-ordinated (CZu) qolecule. The component at 3455 cm was assigned to the asymmetric stretching frequency of the tetraco- ordinated molecule. The trico-ordinated species with two bonded OH groups were believed to have frequencies close enough to those of the tetraco-ordinated species so that their bands would be obscured by the more intense bands of the latter. The trico-ordinated species with one bonded OH group would have a high frequency band associated with the free OH vibration and a bonded OH vibration at a lower frequency, and the bands at 3635 and 3545cm-’ were assigned to those OH vibrations, respectively. In a recent review Walrafen2 has shown that, on the basis of high pressure experiments,6 dilution g ~ d i e s , ” ~ the effect of the addition of structure-breaking electrolyte^,^*^ and stimulated Raman scattering the Fermi resonance assignments for the bands at 3215 and 3400cm-’ are not correct. He points out, however, that of the four components resolved earlier3 at 3247, 3435, 3535 and 3622 c 6 ’ , the high frequency components at 3622 and 3535cm-’ may be assigned to the non-bonded and hydrogen bonded OH bands of the trico-ordirated species. The low frequency band at 3435cm- was assigned to the symmetric stretching mode of H20

molecules that have Czu symmetry (primarily with reference to an intramolecular motion, that is, to one that is not so strongly coupled to other molecules). The fourth component at 3247 cm-’ was assigned to a vibration involving the in-phase OH stretching motion of an aggregate consisting of a central H 2 0 molecule and its nearest, or perhaps higher, neighbours. This component was therefore deemed to have intramolecular as well as intermolecular character and in this regard it was noted that the frequency difference between the two

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178 JOURNAL OF RAMAN SPECTROSCOPY, VOL. 13, NO. 2, 1982 @ Wiley Heyden Ltd, 1982

RAMAN SPECTRAL STUDIES OF AQUEOUS SOLUTIONS OF NON-ELECTROLYTES

components at 3435 land 3247 cm-' was not far from the balue of 170 cm- assigned to the 0 -0 stretching mode of 0 -H . * 0 units.I4

Walrafen*>' has also reported detailed studies on the effect of electrolytes on the contour of the OH stretching band of water. These studies definitely favoured the mixture model for water. On the basis of these studies it was concluded that: (i) Monatomic anions like I-, Br- and C1- break down the water structure and the OH band contour contains an intense Raman component arising from vibrations of partially covalent X- . . - H-0 units, in addition to components from HzO. The frequencies corresponding to the X- - - H-0 species are near to, and the components are spectrally unresolved from those of the principal hydrogen bonded components of the spectrum of H20. (ii) The F- ion produces a structural enhancement which results from interactions between the slightly polariz- able F- ion and water; the F- ion probably substitutes for HzO and interacts with the protons of four neighbouring water molecules, (iii) The polyatomic anions of alkali metal salts of extremely strong acids give rise to overt splittings in the OH stretching contours which arise from the breakdown of the hydrogen bonded structure and from the presence of anion-water interactions that are primarily electrostatic, as opposed to linear partially covalent hydrogen bonded linkages with monatomic ions. These anion-water interactions give rise to sharp components that fall within the non- hydrogen bonded region of the spectrum of H20. (iv) The polyatomic anions of strong and moderately strong acids give rise to noticeable changes in OH band contours, but the high frequency OH stretching components, however, are generally not as high in frequency as those induced by salts of extremely strong acids. A breakdown of water structure occurs and anion-water interaction is also present. (v) The tetraalklammonium ions exert a strong structure-making effect on water as evidenced by a weakened OH stretching component.

Walrafen3 in his studies on aqueous electrolytes also mentioned that the Raman spectra of aqueous solutions of dimethylsulfoxide (DMSO), KC1 and KBr are similar. In all the three cases the 3225 cm-' band decreases in intensity and the band at 3450cm-I increases in intensity whereas in the case of aqueous solutions of NaC104 both the 3225 and 3450cm ' bands decrease in intensity and another band around 3500 cm-' appears. It was suggested that structure- breaking of water occurs in all the four cases considered above, resulting in an intensity decrease in the 3225 cm-l band. The intensity increase of the 3450 cm-' band in C1-, Br- and DMSO solutions was explained by the existence of linear hydrogen bonds of the type X- * . - H-0 and S = 0 . - H-0, thus increasing the concentration of hydrated water responsible for the increase in the intensity of the 3450 cm-' band. Since linear hydrogen bonds are not predicted for the CIO4- ion in water, even though structure- breaking of water occurs, when NaC104 is added the 3450 cm-' band for hydrated water decreases and another band appears in the non-hydrogen bonded region, which was assigned to the OH bond involved in the purely electrostatic complexes formed between C1OC and HzO. Its intensity increases with increasing concentration of NaCI04.

Though some account of the effect of the addition of DMSO on the band contour of water is given by Walrafen3 and its similarity with the effect of added C1 and Br- has been proposed, we do not find any comprehensive study dealing with the effect of non-electrolytes on the Raman OH band contours of water. If the above similarity is true all organic solutes with N, 0, S, or X atoms which are capable of forming linear hydrogen bonds should therefore behave in a similar fashion as DMSO. No account is available in the literature as to whether hydrophobic and hydrophilic solutes (or typical aqueous and non-aqueous mixtures) as defined by

behave in a similar fashion or a different fashion on the basis of Raman spectral results. With these objectives in mind we have now undertaken these Raman spectral studies of aqueous solutions of non- electrolytes. We report here the behaviour of aqueous solutions of dimethylsulfoxide, dimethylsulfoxide-d6, acetone and acetonitrile with respect to their Raman spectra in the OH stretching region and in the stretching regions of the functional groups of the solutes ( v ~ = ~ , VC=O and vCGN, respectively).

EXPERIMENTAL

Dimethylsulfoxide (Fisher 'Certified'), dimethylsulfoxide-d6 (Merck, Sharp and Dohme-99.5% d), acetone (Baker 'Instra-Analysed'), and acetonitrile (Matheson, Coleman and Bell 'Spectroquality') were stored over molecular sieves and were used without further purification. Water was double distilled from KMn04 and KZCr207. Raman spectra were recorded with samples at room temperature, using a Jarrell Ash MSlOl spectrometer equipped with a SSR 105 photon counter and a Coherent Radiation CR-3 Argonion laser. All the spectra were recorded employing the 5 14 nm exciting line, operating at 800 mW. The spectra were generally recorded with a 0.1 s time constant and 0.5 or 1 cm-ls-' monochromator speed and were cali- brated using well-known Neon and CC14 lines. All the intensities were reproducible to within *2% and the frequencies to within k0.5 cm-'.

RESULTS

DMSO-water system

It was proposed pre~iously""~ that DMSO can alter the structure of water by acting as a proton acceptor. This behaviour may account for its cryoprotective properties. In an early Raman spectral study of aqueous DMSO solutions, Lindberg et ~ 2 1 . ' ~ reported that the intensity and frequency of the Raman band due to the S=O stretching mode decreased continuously with increasing water content of the mixture, but that no band splitting caused by hxdrogen bond formation was observed. Safford et af.* have reported results on inelastic neutron scattering, x-ray diffraction and infrared absorption spectra of aqueous DMSO solutions. It was found that in dilute aqueous solutions DMSO causes an enhancement and sharpening of the

JOURNAL OF RAMAN SPECTROSCOPY, VOL. 13, NO. 2, 1982 179

S. SINGH AND P. J. KRUEGER

intermolecular frequencies of the water structure associated with a small and co-operative perturbation in the orientation of many H20 molecules. As the concentration of DMSO is increased the existing water structure is broken down owing to the formation of hydrogen bonded and thermoabile DMSO-H20 complexes. In the absence of vSo splitting in aqueous DMSO solutions Lindberg et UZ.” concluded that molecular interactions in this case are primarily of a very strong dipole-dipole and possibly partly ionic type. Safford et aLZ0 report that infrared spectra of aqueous DMSO solutions at 1 “C showed a uso band splitting at mole fractions of DMSO > 0.8 indicating partially covalent linear hydrogen bonded complex formation of the type S=O * - - H-0. Brink and Falk,*’ on the basis of their infrared spectral studies of aqueous solutions of DMSO, found that the OH band profiles of HzO do not change on addition of DMSO up to a concentration of 0.2 mole fraction of DMSO and concluded that there was no indication of either hydrogen bond breaking or of any pronounced tendency towards clatherate or iceberg formation in dilute DMSO solutions.

In Figs. 1-3 the variations in Raman band profiles of the S=O and OH stretching modes are shown as a

h

I075 850

R a m a n shift, AV (cm-’)

Figure 1. Rarnan S=O stretching bands in aqueous DMSO solutions. Mole fractions of DMSO are: A, 1.0; B, 0.75; C, 0.50; D, 0.27; E, 0.02.

function of the concentration of DMSO in water. The S=O stretching band broadens with increasing concentrations of water and the high frequency component of the broad band vanishes at higher water concentrations and the band eventually seems to move towards low frequency. This trend can be well observed in Fig. 2 where in pure DMSO (curve A) an asymmetric band is observed which shows up as a broad band in curve B whereas the high frequency component decreases in relative intensity in curve C. In curve E the low frequency component has increased dramatically and the high frequency component seems to be vanishing. this situation at 0.5 mole fraction changes,gradually to curve F where only one band is obseryed-the low frequency component at 0.06 mole fraction of DMSO. As is evident from both Figs. 1 and 2 the intensity of the S=O stretching mode continues to decrease as the concentration of water increases. This can be observed with reference to the neighbouring band at -950 cm-’ assigned to SCH3 in DMSO. The 3225 cm-’ band seems to decrease in intensity in comparison to the 3450 cm-’ component of the OH band contour of H,O when the concentration of DMSO is increased, as is evident from Fig. 3. (Although the precise frequencies of the two broad components of the OH band contour in the Raman spectrum of water change as a function of the environment, they will be referred to as the 3225 and 3450 cm-’ components throughout for the sake of sim- plicity. The exact frequencies recorded are found in Tables 1-3.) As the intensity of the high frequency band increases the free OH band at 3625cm-’ becomes obscured and no comment can be made regarding its intensity from direct observation of the overall band profiles.

The Raman spectral data on aqueous DMSO solutions over the mole fraction range 0.005-0.84 of DMSO are summarized in Table 1. Since the intensities of the bands depend on instrumental parameters such as slit width, laser power, collecting optics and alignment of the sample, the intensities given in the Table for the S=O stretching mode are those relative to the intensities of the SCH3 band at 950cm-’ under the same experimental conditions. Of course in doing so it is assumed that the intensity of the 6CH3 band does not change to a great extent with a variation in the concentration of water, which, though not completely true, seems to be a satisfactory assumption in the absence of any better simple method for the measure- ment of absolute intensities. This will hereafter be referred to as the internal standard for intensity measurements. It is important to choose an internal standard which is relatively inert under the experimental conditions, i.e. in this case SCH3 is not expected to change its intensity too much when the concentration of water is increased. Secondly, the reference band should be in the neighbourhood of the band whose intensity is to be measured since in that case the sensitiv- ity of the photomultiplier tube for the two bands will only vary slightly and also the scattering parameter, (vo-u,) , where vo refers to the incident light and v, to the vibrational frequency, will not vary too much for the two bands.

The intensity of the S=O stretching band decreases with increasing concentration of water, as reported by Lindberg ef u1.,19 but contrary to their observations

4

180 JOURNAL OF RAMAN SPECTROSCOPY, VOL. 13, NO. 2, 1982


1050 1000 950

Raman s h i f t , A; (cm-')

Figure 2. Raman S=O stretching bands in aqueous solutions of DMSO on an expanded scale. Mole fractions of DMSO are: A, 1.0; B, 0.75; C, 0.65; D, 0.57; E, 0.50; F, 0.06.

US=O is found to split into two bands as mentioned above. The band positions and the apparent intensity ratios (where possible) are given in Table 1. The 1042cm-' band in DMSO itself is not symmetrical because of self-association,22 the low frequency component being due to the S=O stretching mode in the associated species. When a small amount of water is added (mole fraction of DMSO -0.84) the band broadens and an apparent shift to lower frequency is observed. At a somewhat higher concentration of water (mole fraction of DMSO -0.75 or 0.65) the formation of the new band is clearly visible. With increasing concentration of water the low frequency band becomes more distinct and its frequency falls, whereas the intensity of the high frequency band decreases. At 0.27 mole fraction DMSO, when the high frequency component has essentially disappeared, the band becomes symmetrical. When the concentration of water is increased still further the band shows little shift beyond 0.06 mole fraction DMSO. The formation of the low frequency band due to the S=O-..H-O species and the disappearance of the non-bonded S=O stretching

band can be seen clearly from the apparent peak height ratios of low frequency to high frequency components given in the Table and from the increase in apparent A u ~ / ~ ( S O ) values (from 36 to 54 cm-') when the concentration of DMSO decreases from 1 to 0.5 mole fraction, which then decreases to 26 cm-' as the mole fraction is further reduced to 0.005.

Table 1 also gives the apparent peak height ratios of the 3450 cm-' and 3225 cm-' components of the OH band contours when the concentration of DMSO is increased from 0 to 0.85 mole fraction. This obs:rvation is similar to the results reported by Walrafen in that the 3225 cm-' band decreases in intensity on addition of DMSO. In the last columns of Table 1 and in Fig. 4 the Raman spectral data and the OH band contours for aqueous DMSO-d6 solutions are given. Since the inter- ference of the neighbouring bands due to CH3 groups (as in the DMSO-water system) is not present the band profiles can be observed more clearly. The variation of the peak height ratios for these components as a function of the DMSO-d6 concentration is more reliable in this case as there is no appreciable contribution from


S . SINGH AND P. J. KRUEGER

Ramon rhllt, A; Icm-'1

Figure3. Raman OH stretching bands in aqueousDMS0 solutions. Molefractionsof OMS0 are: A, 0.0; B, 0.02; C,O.O6; D,0.15; E,0.50; F, 0.57.

the residual CH3 stretching mode of DMSO impurity. From this set of spectra it is rather more clear that on addition of DMSO-d6 not only does the intensity of the low frequency component (3225 cm-') of the OH band decrease but this component also moves toward high frequency (Fig. 4 and Table 1). The high frequency component also seems to move toward higher frequency, but this shift is not so appreciable.

Acetone-water system

In preliminary on the acetone-D20-water system it was shown that the band width of the 2600 cm-' band increases on addition of acetone. We report here detailed studies of the effect of the addition of water on the fundamental C=O stretching band and the effect of the addition of acetone on the OH band contour of water. With increasing concentration of water the C=O stretching frequency keeps decreasing,

as shown in Fig. 5 . Intensities of this band are given in Table 2 in terms of the ratios of the peak heights of the C=O stretching band and the neighbourjng CH3 bending mode at -1450 cm-'. The half bandwidths for solutions of various concentrations of acetone are also shown in Table 2. The intensity of the C=O stretching mode does not change appreciably, contrary to the finding for the S=O stretching band in the DMSO- water system where a uniform decrease in the S=O stretching intensity with increasing concentration of water is observed. The hul,z values increase from 16.0 to 19.5 cm-' as the concentration of acetone decreases from mole fraction 1.0 to 0.02. As observed in the case of the DMSO-water system, the half bandwidth first increases as the concentration of water rises in the solutions and then the values show a decreasing trend showing the formation of the band due to interaction between the solute and solvent.

In Fig. 6 'are shown the OH band contours of water with varying concentration of acetone. The 3225 cm-'



Table 1. Raman spectral data for the DMSO-water system (v and A V ~ , ~ in cm-*) DMSO DMSO-d,

Mole fraction of DMSO

0.00 0.005 0.01 0.02 0.06

0.1 5

0.27

0.50

0.57 0.65

0.75 0.84 1 .oo

k.0

1.15 1.16 1.18 1.22

-

1.38

1.51

1.95

2.1 5 2.45

2.87 3.05 3.70

40

1012 101 1 1012 101 1

-

1013

1016

1022

1022 1024

1025 ? -

4 C J

- - - - -

-

-

1040

1041 1040

1040 1039 1042

hv,,,(SO)

26 26 27 28

-

30

33

54

54 49

44 41 36

I&//&#+

1.10 1.16 1.19 1.25 1.28

1.34

1.50

1.94

2.20 2.64

- - -

Mole fraction of DMSO-d6

0.00

0.07 0.1 1

0.17 0.23

0.35

0.51

0.68

UbH

3385

3400 3400

3405 3405

3410

3410

3420

3235 1.22 3240 1.28

3250 1.29 3255 1.36

3260 1.60

3270 2.06

3270 2.41

iso is the intensity of the S=O band relative to that of the neighbourin-q 6CH3 band at -950 cm-' (peak heights). I& and /kH are the intensities (peak heights) of the 3450 and 3225 cm using the baseline of the overall OH band envelope. /Lo and / g o are the intensities (peak heights) of the low and high frequency S=O stretching components, respectively. A v , , ~ ( S O ) is the half bandwidth of the overall SO stretching band envelope.

components, respectively, of the hydrogen bonded OH bands

component decreases in intensity as the concentration of acetone is increased, as observed in the case of the DMSO-water system. However, the band disappears completely at a much lower concentration of acetone (mole fraction -0.64) in the case of the acetone-water system than in the DMSO-water system, where it is still present at concentrations as high as 0.84 mole fraction DMSO. Further, when the coycentration of acetone is increased the 3450 cm- component broadens at first, showing the formation of a new band at higher frequency, but with a further increase in the concentration of acetone the intensity of the 3450 cm-' band also decreases and a higher frequency component at -3500 cm-' keeps on increasing in intensity. This can be observed clearly when the structure of the 3450cm-' band is followed in curves A-G; in G the low frequency component of the 3450 cm-' band seems to have almost vanished and the high frequency component at -3500cm-' is beginning to sharpen. The -3620 cm-' component assigned to the free OH stretching mode in pure water is found to shift to higher frequency (to -3680 cm-') for solutions having increased concentrations of acetone. The approximate positions and relative intensities of the various components of the OH band contours are also given in Table 2. Because of different instrument settings the intensities of the same band in different solutions cannot be compared; they have been given in arbitrary units. However the relative intensities (i.e, ratio of the 3450 cm-' to the 3225 cm-I component) can be compared to determine the effect of the addition of solute. As is clear from the values given in the last column of Table 2 the relative intensity of the 3225cm-' band

decreases with increasing concentration of acetone. After the 3225 cm-' bandlalmost vanishes the relative intensity of the 3500cm- band is given with respect to the 3450cm-' band, and it is noteworthy that the value continues to increase with increasing concentration of acetone.

Acetonitrile-water system

In Fig. 7 the trends in the C=N stretching band are shown as a function of different concentrations of acetonitrile. The values of the frequency, intensity and half band width of the C r N band for the various solutions are given in Table 3. With increasing concentration of water the C=N band moves towards higher frequencies. This is contrary to the observations made for the S=O and C=O stretching bands where the bands move to lower frequencies with increasing concentration of water. Similar observations have been made earlierz4 and it has been reported that the C r N band moves towards higher frequencies on complexa- tion. Several explanations have been given for this con- troversial frequency shift, which include kinematic coupling, change in hybridization, bond electron repul- sion, and stronger (T bonding with slightly weaker T

bonding on complex formation. The intensity values given in the Table are in terms of ratios of the peak heights of the C E N band to the neighbouring cornbina- tion band at -2300 cm-'. Similar to the trends found for the S=O stretching mode in the DMSO-water system, the C-N band intensity also decreases as the concentration of water increases. The Aul jZ values, similar to the previous two cases, also first increase and



E

I

4000 3700 3400 3100

Roman shift, A 5 (cm-'1

Figure 4. Raman OH stretching bands in aqueous DMSO-d6 solutions. Mole fractions of DMSO-d, are: A, 0.0; 6.0.17: C, 0.35; D, 0.51; E, 0.68. Other solute bands are marked with a dot.

then decrease as the concentration of water increases in the aqueous solutions, again showing the emergence of a new band owing to the formation of a solute-solvent complex. In Fig. 8 the changes in the OH band contours of water with varying concentrations of acetonitrile are shown. The changes in band shapes seem to be similar to those found in the acetone-water system. The 3225 cm-' component almost vanishes at a concentration of acetonitrile (-0.47 mole fraction) which is lower than in DMSO-water and acetone-water solutions. The 3450 cm-' component broadens as the concentration of CH3CN is increased, indicating the formation of a high frequency component. The low frequency corn- ponent at 3450 cm-' decreases in intensity and the high frequency component at -3530 cm-' increases in intensity as the concentration of CH3CN is increased.

This aspect, i.e. the increase in intensity of the high frequency component and the decrease in intensity of the low frequency component of the 3450 cm-' band, is clearer in the acetonitrile-water system than in the acetone-water system because the high frequency corn- ponent in the former case appears at -3530cm-' whereas it appears at -3500cm-' in the latter case. Furthermore, in the case of the acetonitrile-water system two bands are observed in the free OH stretching region when the concentration of CH3CN is increased (-3650 cm-' and -3620 cm-'). The approximate positions of these OH components and their intensities (in arbitrary units) are given in Table 3. The relative intensity values given in the last column show the decreasing intensity of .the 3225 cm-I band with increasing concentration of CH3CN; after this band



F

I 1 1 1750 I700 16!

Ramon sh i f t . AT Lcm-'1

Figure 5. Raman C=O stretching bands in aqueous acetone solutions. Mole fractions of acetone are: A, 1.0; B, 0.84; C, 0.74; D, 0.47; E, 0.14; F, 0.02.

~

Table 2. Raman spectral data for the acetone-water system (u and Ad1' in cm-l)

Mole C:O stretching band 0-ti stretching bandsb fraction

of acetone Y Ico = b / 2 V l 4 y2 1, v3 13 Y d /4 "5 15 14/15

- - - 0 0.005 - 1.57 - 0.01 - 0.02 1695 1.65 19.5 0.06 1694 1.67 21 0.14 1697 1.76 21.5 0.27 1699 1.67 20 0.47 1703 1.62 19 0.56 1704 1.69 19 0.64 1704 1.58 18 0.74 1707 1.64 18.5 0.84 1707 1.60 17 1.00 1707 1.63 16

- ?

3642

3629 3647 3650 3654

-

3681 1.1 3680 0.8 3680 0.8

2.7

1.4 1.5 1.2 0.9

-

3501 3503 351 5

3385 3392 3404 3407 3425 3429 3443

3485 15.7 3417 10.6 3413 10.2 3410

3485

- -

11.7 6.1 5.8 7.5 7.7 5.8

13.4 7.6 6.5

10.2 5.5 3.5 -

3220 3227 3224 3237 3230 3249 3263 3275 3251

10.6 1.1 4.9 1.3 4.9 1.2 6.0 1.3 5.7 1.4 3.4 1.7 5.7 2.4 1.0 7.6 0.5 13.0

1.5" 1 .9' 2.9'

- - a Intensity of the C=O band relative to that of the neighbouring SCH3 band at -1450 cm-' (peak heights). blntensities of the OH band components are in arbitrary peak height units for any given spectrum; these values are not strictly comparable from one concentration to another.

131i4.


S . SINGH IND P. J. KRUEGER

3800 3400 3000

R a m o n shift, A;; (cm-') Figure 6. Raman OH stretching bands in aqueous acetone solutions. Mole fractions of acetone are: A, 0.005; B, 0.06; C, 0.27; D, 0.47; E, 0.64; F, 0.74; G, 0.84; H, 1.0.

has vanished the high frequency component of the 3450 cm-' band can be seen to increa7e from the relative intensity values of the 3540 cm- and 3450 cm-' components of this band.

DISCUSSION -~

In the three systems considered above it is found that with increasing concentration of water in the aqueous solutions the intensity of the Raman band arising from the stretching mode of the functional groups (S=O, C=O, or C z N ) decreases, the half bandwidth first increases and then decreases beyond certain concentrations of the aqueous mixtures and band frequencies decrease in the case of and vcz0 bands whereas an increase in the frequency is observed in the case of uCZN. The extents of these perturbations differ, however. The changes in band position and Avl12 values are largest in the case of DMSO and smallest in the case of acetonitrile. The decrease in intensity is again greatest for Iso in the case of DMSO, whereas there is no significant change for Ico in acetone and only a small change in Icw in acetonitrile. These relative intensity changes may depend on the internal standards chosen, and the differences in the behaviour of Ico and IcN may not be a reliable guide to the interaction with water. However, on the basis of the perturbations in the

frequency and AvII2 values, it seems that the interactions with water decrease in the series: DMSO> acetone > acetonitrile.

Interesting results are obtained for the shapes of the OH band contours as the concentration of the non- aqueous component of the aqueous solutions is increased. The 3225 cm-' component of the OH band contour observed in pure water decreases in intensity relative to that of the 3450cm-' component as the DMSO concentration increases. A similar effect is observed with acetone and acetonitrile. However, in addition to these perturbations, in the case of aqueous solutions of acetone and acetonitrile additional perturbations are observed for the 3450 cm-' band. With increasing concentration of acetone or acetonitrile a new band appears in the high frequency region, so that at higher concentrations the low frequency component of the 3450 cm-' band is almost submerged in the sharp high frequency component. In addition to these bands, in the region of free OH bands (-3540 cm-I), one band is observed in the case of DMSO and acetone and two bands are observed in the case of acetonitrile solutions. These observations are similar to the results reported for two groups of electrolytes by Walrafea3 In his studies it was observed that for aqueous solutions of monatomic ions like C1-, Br- and I- effects similar to the present DMSO results were obtained and for polyatomic ions like ClOi effects similar to the present results for acetone and acetonitrile were obtained. Walrafen categorized these effects as being due to the formation of linear partially covalent hydrogen bonds (for the interaction of water with C1- and Br-) and electrostatic interactions (between water and ClO,). It was noted that even though both X- or ClO, cause structure-breaking of water which is res onsible for a decrease in the intensity of the 3225 cm band in both cases, when ClO; is added the 3450cm-' band for hydrated water decreases in intensity and another band appears in the non-hydrogen bonded region. This is assigned to the OH bond involved in the purely electrostatic complexes formed between ClO, and H20, and its intensity increases with increasing concentration of NaC104. In the case of aqueous solutions of CI-, Br- or DMSO, the formation of linear partially covalent hydrogen bonds of the type X-...H-O and S=O...H -0 increases the concentration of hydrated water, which is responsible for the increase in the intensity of the 3450cm-I band. Initially one might surmise that the three solutes considered in this work should all behave similarly and that if DMSO forms linear partially covalent bonds with water, so should acetone and acetonitrile. On the basis of the Raman spectra of aqueous solutions of these organic molecules and their comparison with Walrafen's results on aqueous electrolytes it seems that DMSO belongs to the class of solutes to which C1- and Br- belong, and acetone and acetonitrile belong to a second class of solutes to which ClO, belongs. Alternatively, we may ignore the similarity of our results with those obtained on electrolytes by Walrafen and propose that a band similar to that obtained in the cases of aqueous solutions of acetone and acetonitrile at -3500 and -3540 cm-', respectively, (because of the presence of hydrogen bonded complexes between water and acetone or acetonitrile molecules) might also be present in the case

-P



I 2300 2256 Z z m

Roman sh i f t , A; (cm")

Figure 7. Raman C r N stretching bands in aqueous acetonitrile solutions. Mole fractions of acetonitrile are: A, 1.0: B, 0.88; C, 0.71; D, 0.19; E, 0.01.

of aqueous solutions of DMSO with a frequency so close to the 3450 cm-' band of pure water that its growth with increasing concentration of DMSO cannot be observed as clearly as for aqueous solutions of acetone and acetonitrile. The frequency shift of the OH band between the free OH and the OH hydrogen bonded to solute molecules would then decrease in the

series DMSO >acetone > acetonitrile, showing that the interaction of water with DMSO is stronger than that with acetone, which in turn is stronger than that with acetonitrile. This conclusion is similar to that drawn form the observed perturbations on the stretching vibrations of the S=O, C=O and CZN bands. Since the interaction is strongest with DMSO, the 3225 cm-'

Table 3. Raman spectral data for the acetonitrile-water system (u and in cm-"3)

Mole fraction of acetonitrile

0 0.007 0.01 0.03 0.08 0.19 0.34 0.47 0.58 0.71 0.80 0.88 1 .oo

CEN stretching band 0-H stretching bandsb

Y

- 2250 2251 2251 2250 2251 2250 2249 2248 2247 2246 2246 2246

ICN a

- 8.55 9.00 8.77

9.40 9.75

8.94

10.8 12.2 10.9 12.1 13.3 12.6

h V

- 11.5 1 1 12 12 12 13 12 11.5 11 1 1 10.5 10.5

"1

3638

3641 3650 3656 3655 3644 3651 -

1 7

3.4

3.2 2.6 1.8 1.7 4.0 2.6 -

1'7

3642 361 1 3626 3620 3608 3607

3620 361 1 3619 361 1 3609 -

I7

2.7 2.3 2.0 2.3 5.4 4.8

4.0 3.4 3.0 6.2 4.4 -

"3 h "4

3385 3400 3398 3395 341 3 3433 3461

3515 8.4 3530 7.0 3446 3529 6.8 3533 15.0 3540 11.3 - - -

14

11.7 7.7 7.8 8.0 16.0 13.5 10.2

5.5

-

"5

3220 3254 3242 3230 3233 3232 3224 3245

-

15 { b l k

10.6 1.1 6.3 1.2 6.6 1.2 6.4 1.3 11.7 1.4 7.7 1.8 3.9 2.7 1.6

- -

a /C=_N is the intensity of the CZN band relative to that of the neighbouring combination band at -2300 cm-' (peak heights).

from one concentration to another. Intensities of the OH band components are in arbitrary peak height for any given spectrum; these values are not strictly comparable



3800 3400 3 00

Roman shift , A, (cm-')

Figure 8. Raman OH stretching bands in aqueous acetronitrile solutions. Mole fractions of acetonitrile are: A, 0.007; B, 0.03; C, 0.34; D. 0.47; E, 0.58; F, 0.71; G, 0.80; H, 0.88; 1, 1.0. Solute bands are marked with an asterisk.

band is probably also shifted to higher frequency with increasing DMSO concentration, as is observed very clearly with DMSO-d6. This behaviour is not shown by the other systems.

The appearance of two bands in the free Oh region for the acetonitrile-water system has been attributed to the presence of 1 : 1 and 2 : 1 interactions by Mohr et aZ.25 in their infrared studies of dilute solutions of organic solutes and water in carbon tetrachloride. They also reported two bands in this region for the acetone- water system, one of which is probably obscured in the present studies by the intense neighbouring band at -3500 cm-'.

In the classification given by Franks15 on the basis of the relative magnitudes of excess mixing functions, DMSO and nitriles are classified as typical non-aqueous solutes and ketones as typical aqueous solutes, although DMSO has also been described as a water 'structure maker'.26 The alternative explanations of the Raman spectra of the aqueous solutions reported here assume that the three solutes considered are water 'structure breakers'. These studies on the Raman spectra of aqueous solutions of non-electrolytes are being con- tinued in order to see if a general solvent classification scheme applicable to a wider range of solvents can be established and understood, in the context of observations on other thermodynamic parameters. It is proposed to investigate solutions in pure water as well as in dilute HDO.

Acknowledgements

One of us (S.S.) is grateful to the National Research Council of Canada for the award of a CIDA/NRC Research Associateship (1975-1977). This work was also supported by an operating grant from the National Research Council of Canada (to P.J.K.).

REFERENCES

1. G. E. Walrafen, in Wafer-A Comprehensive Treatise, Vol. I, ed. by F. Franks, p. 158. Plenum Press, New York (1972).

2. G. E. Walrafen, in Structure of WaterandAqueous Solutions, ed. by W. A. P. Luck, p. 302. Verlag Chemie, Weinheim (1974).

3. G. E. Walrafen, J. Chem. Phys. 47, 114 (1967). 4. G. E. Walrafen, J. Chem. Phys. 48, 244 (1968). 5. G. E. Walrafen and L. A. Blatz, J. Chem. Phys. 56,4216 (1972). 6. G. E. Walrafen, J. Solution Chem. 2, 159 (1973). 7. W. F. Murphy and H. J. Bernstein, J. Phys. Chem. 76, 1147

8. G. E. Walrafen, J. Chem. Phys. 52, 4176 (1970). 9. G. E. Walrafen, J. Chem. Phys. 55,768 (1971).

by F. Franks, Plenum Press, New York (1972).

4, 621 (1970).

(1972).

10. F. Franks, in Water-A Comprehensive Treatise, Vol. 1, ed.

11. M. J. Colles, G. E. Walrafen and K. Wecht, Chem. Phys. Lett.

12. G. E. Walrafen, Adv. Mol. Relation Processes 3, 43 (1972). 13. G. E. Walrafen, J. Chem. Phys. 50, 567 (1969). 14. G. E. Walrafen, J. Chem. Phys. 44, 1546 (1966). 15. F. Franks, in Hydrogen BondedSolvent Systems, ed. by A. K.

Corington and P. Jones, Proceedings of Symposium, Univer- sity of Newcastk-Upon-Tyne (1968).

16. F. Franks and D. S. Reid (Chapt. 5) and F. Franks (Chapt. I), in,Water, A Comprehensive Treatise. ed. by F. Franks, Vol. 2, Plenum Press, New York (1972).

17. M. J. Ashwood-Smith, Ann. N.Y. Acad. Sci. 141, 45 (1967). 18. G. F. Doebbler and A. P. Rinfret, Cryobiology 1, 205 (1965). 19. J. Johan Lindberg and D. Majani, Acta. Chem. Scand. 17,

1477 (1963). 20. G. J. Safford, P. C. Shaffer, P. S. Leung, G. F. Doebbler, G.

W. Brady and E. F. X. Lyden, J. Chem. Phys. 50,2140 (1969). 21. G. Brink and M. Falk, J. Mol. Strucf. 5, 27 (1970). 22. R. H. Figneroa, E. Roig and H. H. Szmant, Spectrochim. Acta

23. D. Waddington, PhD Thesis, University of Leicester (1972). 24. J. Grundnes and P. Klaboe, in The Chemistry of the Cyano

Group, ed. by 2. Rappoport, p. 135. lnterscience Publishers, London (1970).

25. C. C. Mohr, W. D. Wilk and G. M. Barrow, J. Am. Chem. SOC. 80,3048 (1965).

26. F. Ralls, F. Rodante and P. Silvestroni, Thermochim. Acta 1, 311 (1970).

22,287 (1966).

Received 12 March 1982


raman spectral studies of aqueous solutions of non-electrolytes: dimethylsulfoxide, acetone and...

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