Raman spectroscopic study of hydrogen bonding of poly(N-vinyl-2-pyrrolidone) in heavy water and dimethyl sulfoxide

Macromol. Chem. Phys. 195,3369-3380 (1994) 3369

Raman spectroscoflc study of hydrogen bonding of poly(N-vinyl-2-pyrrolidone) in heavy water and dimethyl sulfoxide a)

Naoki Tanaka

Department of Polymer Science and Engineering, Faculty of Textile Science, Kyoto Institute of Technology, Kyoto 606, Japan

Kensaku Ito, Hiromi Kitano*

Department of Chemical and Biochemical Engineering, Toyama University, Toyama 930, Japan

(Received: November 11, 1993; revised manuscript of February 16, 1994)

SUMMARY The Raman spectra of solutions of poly(N-vinyl-2-pyrrolidone) (PVP; a, = 1 000, 10000,

40000 and 360000) in D20 and dimethyl sulfoxide (DMSO) were measured at various concentrations. PVP is hydrated by D 2 0 in a manner different from N-ethyl-2-pyrrolidone (NEP) and N-methyl-2-pyrrolidone (NMP), monomer analogues of PVP. The self-association of DMSO molecules in the solution of PVP was found to be different from that in the solution of NMP by using difference spectroscopy. These phenomena were attributed to a net-like structure of concentrated solutions of PVP.

Introduction

In our preceding paper I ) , we reported that the Raman bands of the D,O solution of N-alkyl-2-pyrrolidones shifted markedly by the formation of hydrogen bonds between the nitrogen atom of the pyrrolidones and solvent molecules. Fukushima et al. also found that the Raman bands of various organic compounds (N,W-dimethylpiper- azine ,), N-methylpiperazine 3), N-methylpyrrolidine 4, and N-methyl-2-pyrrolidone 5 ) )

shifted markedly by the formation of hydrogen bonds between nitrogen atoms and water molecules, and attributed the shift to a suppression of the inversion motion at the nitrogen atom by the hydrogen bonding. They obtained an enthalpy change of -2,O kJ/mol for an aqueous solution of N-methyl-2-pyrrolidone (NMP) at a mole fraction of 0,406 from the measurement of the intensity changes). In our previous study'), the enthalpy difference of the inversion motion at the nitrogen atom was estimated for NMP, N-ethyl-2-pyrrolidone (NEP) and N-butyl-2-pyrrolidone, which were the monomer analogues of poly(N-vinyl-2-pyrrolidone) (PVP), using a computer fitting of the shape of the C=O stretching band in Raman spectra, though the concentration range was different from that of Fukushima et al. We found that this inversion motion was affected by a rearrangement of D,O molecules which form a cluster around the N-alkyl-2-pyrrolidones due to hydrophobic hydration.

a) Presented at the 40th Annual Meeting of the Society of Polymer Science, Japan, at Kyoto in May, 1991.

0 1994, Hiithig & Wepf Verlag, Zug CCC 1022-1352/94/$08.00

3370 N. lhnaka, K. Ito, H. Kitano

In the present study, we examined the structure of the D,O and DMSO solutions of PVP (aw = 1000, IOOOO, 40000) at various concentrations. PVP also showed a NMP-like inversion motion at the nitrogen atom because of the hydrogen bonding in D,O, but the degree of hydration of D,O to PVP was lower at the same concentration (in monomeric units per volume). This might be caused by the specific solvent structure of the D,O solution of PVP. To make this point clear, we investigated the Raman spectrum of DMSO solutions of PVP. Applying difference spectroscopy, we could obtain informations on the solvent structure of the DMSO solution of PVP.

Experimental part

Materials

Poly(N-vinyl-2-pyrrolidone) (aw = 1 OOO, PVPl 000) was kindly donated by Kao Co., Ltd., Toyko, Japan. PVPlO000 (MW = lO000), PVP40000 (aw = 40000) and PVP360000 (aw = 360000) were obtained from Tokyo Kasei, Tokyo, Japan. These samples were used after ultrafiltration (Amicon, Model 202, membrane; Diaflo YC05 for PVPl OOO, YM05 for PVPlO000 and PVP40000) except PVP36OOOO. PVP360000 was purified by dialysis. NMP and NEP from Tokyo Kasei were used after double distillations. The reason for the use of D,O as solvent (instead of H,O) was mentioned previously lo). D,O was obtained from CEA, Gif-sur-Yvette, France. Spectral grade DMSO was purchased from Nacalai Tesque, Inc., Kyoto, Japan.

Spectroscopic measurements

The Raman spectra of solutions of PVP in D20 and DMSO were obtained using an NR-1 100 Raman spectrophotometer (Japan Spectroscopic Co., Tokyo, Japan) with a resolution of 5 cm - I .

The spectra were excited by the 488 nm line of an argon ion laser (GLG 3 200, NEC, Tokyo, Japan) at powers of 300-600 mW. All measurements were carried out in a thermostated chamber con- trolled by a Peltier device (Model RT-IC, Japan Spectroscopic Co.).

Results and discussion

Raman spectra of PVP in various states

Fig. 1 shows the Raman spectra of PVPl OOO solution (10 wt.-Yo) in D,O and of the solid state, and Tab. 1 shows frequencies of the peaks extracted from these spectra along with frequencies of the solid state and those of the monomer analogue NEP. The spectrum of the PVPl OOO solution showed a considerable difference from that of the solid state in the region 1350 to 1750 cm-I. In the solution, the band around 1450 cm-' which was assigned to the CH, bending mode was split, and the band around 1680 cm-' shifted to a lower frequency. These changes reflect the hydrogen bonding of D,O molecules to PVPl 000. We compared the difference of the concentration dependences of PVP and NEP in D,O solution to investigate the interaction between D,O and PVP.

Raman spectroscopic study of hydrogen bonding of poly(N-vinyl-2-pyrrolidone) in . . . 3371

1

m c ar C

c ._

c

+i

Fig. 1. Raman spectra of PVP1 OOO. A: 50 wt.-Yo D,O solution of PVPl OOO, B: neat PVPl OOO (solid state)

9LO

756

i

1800 1500 1200 900 600 Wovenumber in cm-'

Tab. 1. PVPl OOO and D 2 0 solution of PVPl OOOa)

Observed vibrational frequencies (in cm-') and assignment of neat NEP, neat

Assignment NEP neat PVPl OOO

neat D,O solution

v(C=O) v (C'-N) CH, scissor

CH, twist v (C-C) CH, rock v (C-C)

v(C-N)

1674 1496

1449 1427 1229 1035 1 008

929 846 739 653

1674 1498

1450 1430 1238 1032 1016

942 860 760 680

1652 1 498 1472 1456 1430 1238 1032 1006

940 862 756 680

a) Extracted frequencies from the region 600-1 800 cm-'. v = stretch; C' = carbonyl carbon.

Concentration dependence of Raman spectra of PVP and NEP in D,O solution

Fig. 2 shows a series of Raman spectra of D,O solutions of NEP in the region 1350- 1750 cm-' at 25 "C. The scale of each spectrum in this figure was normalized by the intensity of the C-C stretching band at 932 cm-' (the internal standard) to

3372 N. Tanaka, K. Ito, H. Kitano

Fig. 2. Concentration dependence of the Raman spectrum of N- ethyl-2-pyrrolidone-DzO solution at 25 "C. (A:

C: 50 wt.-'70; D: 70 wt.-%; E: neat)

10 wt.-%; B: 30 wt.-%;

1700 1600 1500 ILOO Wavenumber in crn-'

compare the spectra under the same conditions. For the assignment of this peak, the previous spectroscopic work must be consulted6). The use of an internal standard eliminates effects of variation in refractive index and background scattering as well as instrumental factors.

A marked spectral change was observed in the region of 1400- 1750 cm-'. Fukushima and Yamaji previously reported the same phenomenon for aqueous solutions of NMP5). They found that the spectra in the region of 1400- 1550 cm-', which were assigned to the CH, bending vibrations, changed with the concentration. They claimed that this was because the shape of the five-membered ring in the NMP molecule changed with the formation of hydrogen bonding between nitrogen atom and water molecules and thus due to the variance of the electronic state of the carbon atom of the CH, groups.

The C=O stretching mode in the region of 1550- 1750 cm-' shifted to lower frequencies as the concentration of NEP decreased (Fig. 2). As we reported previously'), this was caused by two factors, namely (1) the hydrogen bonding between D,O molecule and carbonyl group of NEP, and (2) the change of the electronic state of the carbon atom of the carbonyl group of NEP caused by the distortion of the ring by the formation of hydrogen bonds between the nitrogen atom and D,O molecules. Above 70 wt.-%, some carboxyl groups of NEP were probably not hydrogen-bonded with D,O molecules, because the peak of the C=O stretching band overlapped with that of neat NEP. Below 50 wt.-Vo, all carbonyl groups were hydrogen-bonded with DzO molecules, because n o trace of the peak of neat NEP was found in the spectra of the solution. The transformation of the C=O stretching mode in the concentration range


Fig. 3. Concentration dependence of the Raman spectrum of PVPl 000-D20 solution at 25 "C. (A: 10 wt.-%; B: 30 wt.-%; C: 50 wt.-%; D: 7 0 wt.-%; E: neat) I I I I I

Wavenumber in cm-' 1700 1600 1500 lL00

of 10-50 wt.-Yo is, therefore, due to the hydrogen bonding of D,O to the nitrogen atom of NEP. In the case of N-alkyl-2-pyrrolidones, the spectrum of the C=O stretching mode of the D,O solutions in the concentration range below 50 wt.-Yo, where all carbonyl groups are hydrogen-bonded with D,O, was composed of two elements, namely the C=O stretching mode of 2-pyrrolidone with and without a hydrogen-bonded nitrogen atom.

Fig. 3. shows the spectra of D20 solutions of PVPl OOO, which were normalized by the peak intensity of the C-C stretching band at 940 cm-'. As shown in the region of 1550- 1750 cm-' of this figure, hydration of the carbonyl group of PVP with D,O was enhanced with decreasing concentration of PVP. Deformation of the five- membered ring of PVP was also seen in this figure as the transformation of the shape of the band of CH, bending mode. By comparison of Figs. 2 and 3, we found that the spectra of D20 solutions of NEP changed with concentration more dramatically than those of PVP though both of these solutions were in the same concentration range. The frequency difference of the C=O stretching band between neat NEP and a 10 wt.-Yo D,O solution of NEP was greater than that between neat PVPl OOO and a 10 wt.-Yo D,O solution of PVPlOOO. In addition, though the spectrum of the CH, bending mode of NEP distinctly split into four peaks, the shape of the CH, bending mode of the 10 wt.-Yo D,O solution of PVP was not so much different from that of neat PVP. These spectral features indicate that the carbonyl groups and nitrogen atoms of PVP were less hydrated with D,O molecules than NEP at the same concentration; in the 10 wt.-Yo D,O solution of NEP, most of the oxygen and nitrogen atoms of NEP

3314 N. Xinaka, K. Ito, H. Kitano

molecules were hydrogen-bonded with D,O molecules'), but in the 10 wt.-% D,O solution of PVP, a large amount of oxygen and nitrogen atoms remained unhydrated. This is suspected to be caused by the difference of the solvent structures in PVP and NEP solutions.

In the concentration range beyond 10 wt.-Vo, the polymer chains of PVP must be entangled, and a network structure of polymer chains is formed in the solution, which would be accompanied by changes in the solvent structure. PVP molecules are reported to interact with each other inter- and intra-molecularly through solvent molecules in aqueous '1 and alcohol 8, solutions. (This is different from molecular interactions in aqueous or D,O solutions of poly(acry1ic acid) where direct hydrogen bonds are formed between pendant carboxyl groups of the polymer 9, lo). ) These are examples of a polymer network through solvent molecules, and a similar situation was reported for a gel of polystyrene"). PVP was reported to be a structure maker for but chemically cross-linked PVP behaved as a structure-breaker 14). The concentrated D,O solution of PVP might have the same property as the chemically cross-linked PVP because of the cross-linking through the solvent molecules and, consequently, the structure of water was destroyed in the concentrated PVP solution.

We measured Raman spectra of the 0-D stretching mode of D,O in solutions of PVP and of NMP at 10 wt.-% (Fig. 4). The figure shows a slight difference among the Raman spectra of these solutions and neat heavy water. This indicates that the solvent structures in the solution of PVP differ from those in solution of NMP and neat heavy water. Light water and heavy water molecules form clusters through the hydrogen bonds in the liquid state. This is the origin of the peculiarities of and of hydrophobic interaction. The cluster of D,O must be disturbed by the network of PVP, but we cannot obtain detailed informations from these spectra because the difference in these spectra are not distinct. Further, the assignments of Raman bands in the region of the vibrational stretching frequencies of H,O and D,O have not been established although many works have been reported ''-").

Fig. 4. Raman spectra of the 0-D stretching mode of heavy water at 25 "C. A: 10 wt.-Vo D,O solution rf poly(N-vinyl-2-pyrrolidone) (M, = 1 OOO); B: 10 wt.-Vo D,O solution of N- methyl-2-pyrrolidone; C neat heavy water

3000 2700 2100 2100 Wavenumber in cm-'


Raman spectra of DMSO in solutions of PVP and NMP by difference spectroscopy

DMSO is a dipolar aprotic solvent with a high dielectric constant, and its large dipole moment and polarizability can stabilize molecules and ions through a dipole and induced-dipole moment zo,zl). The physicochemical properties of DMSO are strongly influenced by the S=O group. Pure liquid DMSO has a cluster-like structure caused by strong dipole-dipole interactionsz2). In this respect, the structure of DMSO is analogous to that of water, and the investigation of the cluster of DMSO would be helpful in understanding the solvent structure in D 2 0 solutions of PVP.

Fig. 5 shows the Raman spectra of DMSO solutions of NMP and PVP at various concentrations. The reason for the use of NMP instead of NEP as a monomer analogue will be mentioned later. In these figures, the intensity scale of each spectrum was normalized by the peak intensity of the CH, bending band at 1238 cm-I. Upon addition of NMP or PVP, the spectrum changed as shown. In contrast to the results shown in Fig. 2, the C=O stretching mode of NMP in Fig. 5 (A) scarcely changed. This means that the carbonyl group of NMP does not strongly interact with DMSO molecules. Consequently, the distortion of the five-membered ring does not occur. In the case of PVP in Fig. 5(B), there is a distinct difference between the peaks of the carbonyl group of neat PVP and that of PVP in DMSO solution. This means that the interaction between PVP and DMSO molecules is different from that of NMP.

DifSerence spectroscopy of DMSO solutions of NMP and PVP

To investigate the spectrum of DMSO more closely, we applied difference spectroscopy in this system. This is the method of spectral stripping, and is used to subtract the spectrum of a component from the spectrum of composite systems23). The method requires the characteristic band of the component in the spectrum of the composite system in a spectral range where vibrational modes of other components do not appear. In the present case, the CH, bending band was used as the characteristic band. NMP is a more suitable solute than NEP in this case because the characteristic band appears clearly in the spectrum.

We applied the difference spectroscopy to every spectrum of PVP and NMP solutions in DMSO at various concentrations, and obtained the spectrum of DMSO alone by subtracting the spectrum of neat NMP or PVP from the spectrum of their DMSO solutions. Fig. 6 shows the asymmetric S=O stretching band of the typical difference spectrum of DMSO. This band can be separated into four symmetrical components which are composed of the Gauss-Lorenz hybrid functions.

Many works have been reported on the assignment of these component^^^-^^) (Xib. 2). Most of them were concerned with the spectral change of the S=O stretching mode of DMSO when the cosolvent was added. Previous authors unanimously assigned the band of the highest frequency at ca. 1070 cm-' (A in Fig. 6) to monomeric DMSO. Figueroa et al. suggested the existence of auto-association of DMSO to a closed dimer and highly associated forms (polymer). The findings obtained by Fore1 and Tranquillez6), which were in agreement with those obtained by Horrocks

3376 N. 'knaka, K. Ito, H. Kitano

Wavenumber in cm-'

Fig. 5 . (A) Concentra- tion dependence of the Raman spectra of N-methyl-2-pyrrolidone in DMSO at 25 "C ( A

C: 48 wt.-%; D: neat). (B) Concentration dependence of the Raman spectra of poly(N-vinyl-2-pyrrolidone) (M, = 1 OOO) in DMSO at 25 "C ( A

C: 46 wt.-%; D: neat). In this figure the carbonyl stretching mode of higher frequency and of relatively lower intensity is that of neat PVP, and the others are those of

I DMSO solutions of PVP

10 wt.-%; B: 30 wt.-%;

10 wt.-%; B 30 wt.-%;

1800 1600 I L O O 1200 1000 Wavenumber in cm-'


Fig. 6 . Computer- resolved spectrum of the S=O stretching vibration of neat DMSO at 25 "C

Wavenumber in cm-'

Tab. 2. Various assignments of the Raman spectrum of neat DMSOa)

Ref. Band A Band B Band C Band D

closed dimer closed dimer

cluster cluster

closed dimer closed dimer (asym. stretch)

24) monomer (asym. stretch) (sym. stretch) r(CH3)

22) monomer (out of phase) (in phase) r (CH3)

(sym. stretch) polymer monomer 27)

monomer p o 1 y m e r closed dimer r (CH, 1 28)

a) The signs of the bands refer to Fig. 6 .

and Cottonx), indicated that dimers and higher aggregates existed, but their assignments of the peaks at ca. 1025 cm-' (D in Fig. 6) were different from each other. The former assigned it to a closed dimer, and the latter to methyl-rocking mode. In contrast to these authors, Fini and Mironezz) claimed that liquid DMSO was composed of clusters, in which the molecules were oriented in a partially ordered way.

The assignment by Sastry and Singh2') was based on the assumption that there exist a closed dimer, a linear dimer and higher polymer. In their assignment, the band of D in Fig. 6 corresponds to a linear dimer and polymer. Gill et al.28) reached the conclusion that the band A represents the DMSO monomer, the band B a linear, associated species (polymer), the band C the symmetrical movement of a closed-ring dimer, and the band D the methyl-rocking vibration. The findings obtained by Gill et al. on SO2-DMSO mixtures revealed undoubtedly that the band D corresponds to the methyl-rocking vibration. This means that the findings reported by Figueroa et al. 25)

3378 N. lhnaka, K. Ito, H. Kitano

I I I I I

Wavenumber in cm-' 1100 1050 1000 1100 1050 1000

Wavenumber in cm-'

Fig. I. Fig. 8.

Fig. 7. Difference spectrum: Raman spectra of the S=O stretching band of DMSO obtained from those of DMSO solutions of N-methyl-2-pyrrolidone at 25 "C. (A: neat DMSO; B: 10 wt.-To; C: 24 wt.-%; D: 30 wt.-%; E: 48 wt.-%)

Fig. 8. Difference spectrum: Raman spectra of the S=O stretching band of DMSO obtained from those of DMSO solutions of poly(N-vinyl-2-pyrrolidone) (M,,, = 1 OOO) of various concentrations at 25 "C. ( A neat DMSO; B: 10 wt.-To; C: 30 wt.-Vo; D: 36 wt.-Yo; E: 46 wt.-'To)

and by Sastry and SinghZ7) are not reliable. The most recent work on this subject was reported by Rintoul and Shurvel129), but we cannot agree with their assignment because it was based upon that of Sastry and S i ~ ~ g h ~ ~ ) . Further, according to Gill et al., the intensities of bands A and B varied independently when DMSO was diluted. Therefore, these bands cannot be ascribed to two different vibrations of the same associated species. Accordingly, the finding obtained by Fore1 and Tranquillez6) and by Fini and MironeZ2) would not be correct. Consequently, we consider that the assignment by Gill et al. is the most reliable.

Fig. 7 shows the difference spectra of DMSO extracted from the DMSO solution of NMP at various concentrations. The scale of the intensity of these bands was normalized by that of the CH, bending band at 1421 cm-'. As shown in the figure, the addition of NMP made the intensity of the band at 1042 cm-' lower, and the intensity of the band at 1070 cm-' higher. This is the same phenomenon as found in


+ ._ U C

C a c

c

Fig. 9. Difference spectrum: Raman spectra of DMSO obtained from those of DMSO solutions of poly(N-vinyl-2-pyrrolidone) of various molecular weights at 25 "C. (A: neat DMSO; B: ii?, = 4OOOO;

D: a, = 1 OOO) c: a, = 10ooo;

1100 1050 1000 Wavenumber in cm-'

the case of a DMSO-CCI, mixture, and it was interpreted by Forel and Tranquille as being due to an increase of monomer DMSO and a decrease of dimer- and polymer- DMSO as a result of the dilution of DMSO with CCl,.

On the other hand, a quite opposite phenomenon was observed for the DMSO solution of PVPlOOO as shown in Fig. 8. As the concentration of PVPlOOO in the DMSO solution increased, the intensity of the components at higher frequency decreased and that at lower frequency increased. According to the assignment of Gill et al. 28) the results shown in Fig. 8 can be interpreted as implying that the amount of polymer of DMSO decreased and that of the dimer increased (while the amount of monomer was similar or hardly decreased) as the concentration of PVP increased.

Fig. 9 shows the change in the difference spectroscopy of the asymmetric S=O stretching mode of DMSO in the DMSO solutions of PVP of various molecular weights at 10 wt.-%. In this figure, we found that the spectral change of the asymmetric S=O stretching band is notable as the molecular weight of PVP becomes smaller. The asymmetric S=O stretching band of DMSO extracted from the DMSO solution of PVP36OOOO at 10 wt.-To was almost identical with that of neat DMSO. (This result is not shown in the figure.)

The network structure of PVP in DMSO solution explains well the results shown in Figs. 8 and 9 as in the case of D 2 0 solution. Fig. 5 also supports the hypothesis of the network of PVP in DMSO solution. The C=O stretching band of PVP in DMSO solution was different from that of neat PVP, which indicates the presence of an interaction between carbonyl groups of PVP and DMSO. On the other hand, the C=O stretching band of NMP was almost constant. This indicates that the interaction

3380 N. Tanaka, K. Ito, H. Kitano

between PVP and DMSO is due to the co-operative effect of polymer chains. NMP and CC1, , the low-molecular-weight solutes, dilute the DMSO molecules, but the network of polymer chains, whose mobility is much smaller than that of monomers, produces a different effect o n the structure of DMSO molecules. The slow movement of the network may confine DMSO molecules into small spaces, and prevent them from forming the polymer. The amount of dimer of DMSO was increased by the addition of PVP because the DMSO molecules, which formed the polymer in a neat DMSO liquid, changed to the dimer. The larger the molecular weight was, the looser the network became; thus the polymer structure was more easily formed. The hypothesis of the network made by the crosslinking through the solvent molecules explains well the structural change of the solvent in the concentrated aqueous and DMSO solutions of PVP.

We thank Professor N. Zse for his continuing interest, encouragement and help. We are grateful to Kao Co. for the kind donation of PVPl OOO. This work was supported by the Ministry of Education, Science and Culture (Grants-in-Aid 034531 12, and that for Specially Promoted Research 63060003)

’) N. Tanaka, K. Ito, H. Kitano, N. Ise, Spectrochim. Acta Part A: 48A, 237 (1992) 2, K. Fukushima, Bull. Chem. SOC. Jpn. 52, 2871 (1979) ’) K. Fukushima, H. Takahashi, Bull. Chem. SOC. Jpn. 54, 2603 (1981) 4, K. Fukushima, H. Sasaki, Bull. Chem. SOC. Jpn. 56, 1740 (1983)

K. Fukushima, M. Yamaji, Bull. Chem. SOC. Jpn. 85, 1495 (1985) 6, P. McDermott, J. Phys. Chem. 90, 2569 (1986) ’) 0. Giiven, E. Eltan, Makromol. Chem. 182, 3129 (1981) 8, 0. Giiven, F. Yigit, Colloid Polym. Sci. 262, 892 (1984) 9, T. Miyamoto, H.J. Cantow, Makromol. Chem. 169, 211 (1973)

lo) N. Tanaka, H. Kitano, N. Ise, Macromolecules 24, 3017 (1991) ‘ I ) M. Kobayashi, T. Nakaoki, N. Ishihara, Macromolecules 23, 78 (1990) 12) P. Molyneux, in “Water. A Comprehensive Treatise”, F. Franks, Ed., vol. 4, Plenum Press,

13) B. Y. Zaslavsky et al., Colloid Polym. Sci. 265, 548 (1987) ’‘) R. Huttenruch, S. Fricke, P. Zielke, Pharmazie 44, 354 (1989) ‘ 5 ) H. S. Frank, M. W. Evans, J. Chem. Phys. 13, 507 (1945) 16) A. Ben-Naim, .I Phys. Chem. 77, 95 (1973) ”) A. Sokolowska, Z. Kecki, J. Raman Spectrosc. 17, 29 (1986) “) F. Rull, J. A. de Saja, J. Raman Spectrosc. 17, 167 (1986) 19) M. H. Brooker, G. Hancock, B. C. Rice, J. Shapter, J. Raman Spectrosc. 20, 683 (1989) 20) D. Martin, A. Weise, N. Niclas, Angew. Chem., Znt. Ed. Engl. 6, 318 (1967) 2’) A. Bertoluzza, S. Bonora, G. Fini, M. A. Battaglia, P. Monti, J. Raman. Spectrosc. 11, 430

22) G. Fini, P. Mirone, Spectrochim. Acta 32, 625 (1976) 23) J. L. Koenig, M. K. Antoon, Appl. Opt. 17, 1374 (1978) 24) W. D. Horrocks, A. F. Cotton, Spectrochim. Acta 17, 134 (1961)

26) M.-T. Forel, M. Tranquille, Spectrochim. Acta 26, 1023 (1970) 27) M. I. S. Sastry, S. Singh, J. Raman Spectrosc. 15, 80 (1984) 28) J. B. Gill, D. C. Goodall, B. Jeffreys, P. Cans, J. Chem. Soc., Dalton Trans. 2597 (1986) 29) L. Rintoul, H. F. Shurvell, .l Raman Spectrosc. 21, 501 (1990)

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Raman spectroscopic study of hydrogen bonding of poly(N-vinyl-2-pyrrolidone) in heavy water and dimethyl sulfoxide