'H, "C and 'Li Nuclear Magnetic Resonance Study of the Lithium Chloride-N,N= Dimethylacetamide System

J. Saint G e d n and M. Vineendon* Laboratoires de Chimie. Groupe MacromolCcules Wgktales, DCpartement de Recherche Fondamentale, Centre &Etudes NuclCaires, 85 X, 38041 Grenoble Cedex. France

The compIex solvent obtained by dissolving 5-10°/. of lithium chloride in NJV-dimethylacetamide @MA) presents a good method for dissolving highly insoluble polymers, such as cellulose. 'H, ''C and 'Li NMR spectroscopy have beem used, together with viscosity and conductivity measurements, for the study of this complex solvent. The 'H and '% chemical shift variations of DMA, on increasing the lithium chloride concentration, are found to be in opposite directions. The TI relmxmtion times show a large decrease in the mobility of DMA in the presence of lithium chloride. Methyl-P-D-gIucopyranopyranoside has been used as a model for cellulose in order to investigate the mechanism of solution of this polymer. It was found that each hydroxy group of the solute interacts with one lithium chloride molecule in solution.

Lithium chloride in N,N-dimethylacetamide (DMA) has a high dissolving potential for highly insoluble polymers. This complex solvent system was first used for synthetic polymers,' especially polyamides? and has also been used for dissolving natural polymers such as chitin' and, more recently, cellulose?

The binding of alkali metal ions to DMA has been widely studied by JR' and 13C NMR spectroscopy." Both methods give results which are in favour of a strong metal ion-carbonyl oxygen interaction. This result can be extcnded to other amides, such as N,N- dimethylformamide or N-methyl-2-pyrrolidone, and to other alkali metals.'

We have been mostly interested in the dissolution of highly insoluble polysaccharides, such as ccllulose and chitin, and the use of NMR as an analytical method for the study of these s~ lu t ions ;~*~ this Icd us to the study of the DMA-LiCl system as a unique solvent for these two hydroxylated polymers. The mechanism for solution probably involves strong interaction of the solvent with the hydroxy groups of these polymers, which implies the destruction of the strong intra- and intermolecular hydrogen bonds in the solid state.

We have studied the influence of the lithium salt concentration, the water content and the solute con- ccntration on the chemical shifts and Tl longitudinal relaxation times of the solvent, by using 'H, "C and 7Li NMR spectroscopy. Methyl-0-D-glucopyranoside (l), which is the monomer of cellulose, has been used as a model to observe the interaction of the solvent with the solute hydroxy groups.

EXPERIMENTAL

Commercial DMA was dried without purification o n 4 ~ 4 molecular sieves. Deuteriated DMA-d, was pro-

* Author to whom correspondence should be addressed.

vided by the Commissariat i I'Energie Atomique, Saclay, France.

Conductivity measurements of the DMA-LiCI com- plex solvent system were carried out by varying the LiCl Concentration between lo-' and 2.46 mol dm-'. A weak electrolyte type of bchaviour was found, with a dissociation cocfficicnt increasing at low concentra- tion. The equivalent conductivity of a molar solution was found to be A = 5 S mol-' dm3, corresponding to an estimated dissociation coefficient of 01 = 0.05.

'H NMR spectra of the solvent were recorded on a Varian XL 100 spectrometer at 100 MHz, using pulse Fourier transform spectroscopy, in a 5 mm tube. A spectral width of 250 Hz was used with a data memory of 8K, giving a digital resolution of 0.07 Hzlpoint. A pulse width of 26ps was used, corresponding to a flip angle of 90". Chemical shift values were taken from external hexamethyldisiloxane (HMDS). No correction for magnetic susceptibility was applied.

Longitudinal relaxation times, T,, were determined by using the inversion recovery method with the sequ- ence (180-t-90-5 T,),, using seven t values. T1 values were obtained by an exponcntial fit in duplicate exper- iments. The solutions were previously flushed with dry nitrogen before TI determination.

H NMR spectra of the sugar solutions were ob- tained by CW spectroscopy on a Cameca 250 spcc- trometer at 250 MHz.

"C NMR spectra were obtained on a Varian XL 100 spectrometer at 25.2 MHz by pulse Fourier trans- form spectroscopy in a 1Omm tube. A spectral width of 5000Hz was used with a data memory of 16K, giving a digital resolution of 0.62Hzlpoint. A pulse value of 16 ps was used, corresponding to a flip angle of 90".

7Li NMR spectra were recorded on a Bruker WM 250 spectrometer operating at 97.2 MHz. A spectral width of 1000 Hz was used and stored in 4K memory, giving a digital resolution of 0.5 Hzlpoint. A pulse

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0 Wiley IIeyden Ltd, 1983

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ORGANIC MAGNETIC RESONANCE, VOL 21, NO. 6, 1983 371

J. SAINT GERMAIN AND M. VINCENDON

width of 40 ps, corresponding to a flip angle of 90°, was used.

DISCUSSION

NMR of the DMA-LiCI solvent system

'H NMR. Several workers have studied the effect of alkali metal salts on the 'H chemical shifts of organic solutions. The general feature is a downfield shift of the proton signals of the solvent on increasing the salt concentration. This reveals a general deshielding of the solvent protons caused by electron density with- drawal due to the alkali metal cations. This phenome- non has been observed with 1,3-dimethylethylene- urea,'" 3-methyl-2-oxazolidone" and N-methyl-2- pyrrolidone," all of which have the CH,-N-C-R'

fragment in their structures, as in N,N-dimethyla- cetamide.

Figure 1 shows the DMA 'H chemical shift varia- tion with lithium chloride concentration, over the range 0-10% (w/w) (2.45mol of LiCl per dm3 of DMA). There is a linear upfield shift (negative) of the three methyl signals of A 6 = -0.092 ppm for the anti CH,-N, A 6 = -0.116 ppm for the syn CH3-N and A 6 = -0.06ppm €or CH,-C-. The linearity of the

organic solvent shift with increasing alkali metal salt concentration has been ascribed to the fact that there is only one ion for each solvent molecule.'O~'' The upfield shift observed here for lithium chloride has also been observed with sodium and potassium tetra-

I I1 R O

It 0

.i dppm

I

151

+ 1 O/" HZO

* 10% H20

1 , I I % LiCl 1 5 10

Figure 1. 'H chemical shift variation of the DMA methyl pro- tons with LiCl concentration.

phenylborate, and explained by the influence of the large size and the number of phenyl groups of this anion. The negative shift induced by LiCl for the DMA protons must be related to a shielding due to neighbouring chloride, yielding an anisotropy effect such as that for phenyl groups. Conductivity measure- ments show that LiCl is weakly dissociated in the range of concentrations used here (a=0.05 for the molar concentration). Thus, ion pair formation can be assumed. In this case, the chloride atom should be in close proximity to the DMA molecule, inducing a magnetic anisotropy which could explain the sign of the observed shift.

The influence of water on the chemical shifts of DMA protons is also shown in Fig. 1 for DMA containing 10% of LiC1, since water plays an impor- tant part in the dissolving of cellulose in DMA-LiCL4 All the signals are shifted upfield, the shift due to water being more important than that due to LiCI. This upfield shift is produced by an increase in the ionisation of LiCI. The preferential solvation of the lithium salts by water has been demonstrated by 'H and 13C NMRl3,I4 and thermodynamic ~ tud ie s . ' ~ Con- siderable evidence suggests that the chloride ion is strongly associated with the Lit (H,O) DMA com- plex,I4 whereas the presence of water was not neces- sary for strong dissociation in DMA for other an- ions.I6

Table 1 gives the T , longitudinal relaxation time values of the DMA protons under various conditions. The introduction of lithium chloride into the solvent brings about a strong decrease in the TI values, show- ing a specific interaction between the solvent and the solute. The introduction of a third compound, such as water or methyl-P -D-glucopyranoside does not re- move this specific interaction. The small drop (-16%) in the TI relaxation times caused by these components is probably due to the large increase in viscosity; it shows an interaction between the three components of the solution. The addition of a large quantity (18% w/w) of methyl-6-D-glucopyranoside to pure DMA reveals only a weak effect on the relaxation times of the DMA protons, indicating the major role of lithium chloride in the solvation.

13C NMR. The effect of the addition of alkali metal salts to amides is a general downfield shift of the carbon signals, and lithium chloride has the same effect as other alkali metal salt^.".'^ This has been interpreted as being due to a reduced electron density for the carbon atoms in DMA, induced by the interaction of Li with the carbonyl group.6

We have found that the addition of water to a solution of 10% LiCl in DMA gave a small upfield shift for all the carbon signals, except for the carbonyl carbon signal which was shifted slightly downfield. The observation of an upfield shift for the DMA protons with an increase in the LiCl concentration, whereas the carbon nuclei are shifted downfield, can be ex- plained by different interaction mechanisms. This difference originates from the fact that LiCl interacts directly with the carbonyl group, while the interaction with protons is a through space mechanism.

Table 2 gives the I3C longitudinal relaxation times,

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Table 1. Proton relaxation times, TI (s), of N,N-dimethylacetamide determined at 60 "C

Solvent N-CH, anti N-CH, syn CO-CH, Viscosity, q (cP)

DMA 8.35 8.4 7.6 0.65 DMA-10% LiCl(w/w) 1.84 1.92 2.1 3.36

DMA-10% LiCI-lO% rnethyl-

DMA-18% methyl-@-

DMA-10% LiCI-10% H,O 1.55 1.60 1.7 4.95

6-D-glucopyranoside 1.55 1.60 1.75 8.02

o-glucopyranoside 5.9 5.95 5.35

Table 2. Carbon-13 relaxation times, TI (s), of N,N-dimethylacetamide at 60°C

Solvent N-CH, anti N-CH, svn C-CI-I, C=O 6

DMA 13.3 13.5 8.7 41.5 DMA-10% LiCl 4.5 4.8 4.3 16.5

DMA-10% H,O 8.9 8.9 6.7 19.4

DMA-10% LiCI-10% methyl-6-o-g Iucopyranoside 3.1 3.3 2.8 10.6

(5.0) (5.1) (4.3) (21.2)'~

DMA-10% LiCI-10% H,O 3.2 3.4 3.2 11

T I , of the DMA molecule under the same conditions as those used for the protons. The relaxation times, T,, of DMA have been determined, in 5.3 M aqueous solution, in Ref. 17. The 13C longitudinal relaxation times TI decrease strongly on addition of lithium chloride, as observed for the proton relaxation times. The constant NOE values of the different methyl carbon atoms (n = 2.2-2.6) show that this decrease is not due to the introduction of impurities on the addi- tion of LiC1. The only possible interpretation of this important TI decrease on addition of lithium chloride is the formation of a large complex corresponding to a slower molecular motion. The addition of an hydroxy- lated component (water or methyl-P-D-glucopyran- oside) reduces the TI values of the solvent carbon atoms significantly. This decrease can be mainly as- signed to the increase in the macroscopic viscosity. The identical values of the different methyl relaxation times can be reasonably explained by an isotropic motion.

'Li NMR. 7Li has a natural abundance of 92.6% and presents a good sensitivity for NMR spectroscopy. Despite the 3/2 value of its spin nucleus the quadrupo- lar effect can often be neglected, and it is possible to obtain a natural line width of a few hertz or less. Among the NMR parameters, the chemical shift varia- tion of 7Li+ with the anion and the nature of the solvent is rather weak compared to the other alkali nuclei.18 The TI relaxation mechanisms involve both a quadrupolar and a dipolar effect, more especially in organic solvents.19 Table 3 gives the T , values for 7Li present as LiCl in organic and aqueous solutions. Comparison of the 7Li TI values in aqueous solution,

Table 3. 'Li relaxation times of LiCl in DMA solution at 30 "C

Solvent T,(s)

1.5 M LiCI-H,O (6.3%) 14.7 8% LiCI-DMA 0.21 8% LiCI-1% H,O-DMA 0.23 8% LiCI-10% H,O-DMA 0.4 8% LiCI-8% methyl-P-o-gluco-

pyranoside 0.18

where the lithium atom is fully ionized and solvated by water molecules, and in a DMA solution where LiCl is only polarized, shows a large drop in this value within the organic medium. One of the possible explanations is an ion-ion contribution to the quadrupolar relaxa- tion mechanism, since ion pairs can be postulated from our previous results.

The introduction of water into the organic medium causes an increase in the relaxation time value, owing to an ionization phenomenon. The relaxation of the lithium atom is only slightly affected by the introduc- tion of the methyl-0-D-glucopyranoside molecule.

NMR of the methyl-fl-D-glucopyranoside solution

DMA is an aprotic solvent and retains this property even when containing LiCl. By means of ' H NMR it is thus possible to observe all the hydroxy proton signals of hydroxylated compounds in a DMA-LiCI system.

Homonuclear spin decoupling techniques have been used to assign the 'H NMR spectra of methyl-P-D- glucopyranoside (1) in pure 'DMA-d9 and in DMA-

ORGANIC MAGNETIC RESONANCE, VOL. 21, NO. 6, 1983 373

J. SAINT GERMAIN AND M. VINCENDON

-OCH,

H2O

3-OH 4-OH ?-OH 6-OH H-I, H-6a,H-6b,H-3,H-4

- -

DMA-dg

I I I I I I I I

6 5 4 3 8 PPm

Figure 2. 250 MHz 'H NMR spectrum of methyl-0-D-glucopyranoside (1) in DMA-d,-IO% LiCl solution.

d,-10% LiCl solutions. Figure 2 shows the 'H NMR spectrum of 1 in DMA-d,-lO% LiCl solution, where the four hydroxy signals can be observed. The chemi- cal shift variation of these four hydroxy signals on increasing the lithium chloride concentration is shown in Fig. 3. There is a strong downfield shift (positive) of

4.5

u

m- H-1 3.7

Figure 3. 'H chemical shift variation of the hydroxy protons of methyl-0-D-glucopyranoside with lithium chloride concentra- tion.

all these signals, 3-OH being more affected. Figure 3 shows that all the hydroxy protons of 1 are strongly shifted until one lithium chloride molecule is com- plexed with each hydroxy group. The other signals (non-hydroxylated protons) of 1 have the typical be- haviour of H-1, which is also shown in Fig. 3, i.e. a small linear upfield shift (negative), A 8 = -0.044 ppm, of the same order as the shift of the DMA carboxyl methyl protons.

The strong downfield shift of the hydroxy signals of 1 (A8 + 1 ppm), whereas all the other protons of the solution are shifted slightly upfield, indicates a strong interaction between each hydroxy group and one lithium chloride molecule. Lithium chloride behaves, in fact, as a shift reagent. The positive lithium atom interacts with the hydroxy oxygen atom, while the negative chloride atom interacts with the hydroxy proton. The DMA protons have the same behaviour as that shown in Fig. 1. It can be concluded that the DMA-LiCl intermolecular network is reinforced by the introduction of a polyhydroxylated compound. This is in good agreement with the measurements of the relaxation times, and the introduction of a third component in the DMA-LiC1 complex solvent main- taining these low TI values. This means that the 'polymer' structure obtained by the complexation of DMA and LiCl is maintained when a hydroxylated solute is added, as revealed by the viscosity increase.

The specific activity of the LiCl molecule in DMA towards highly hydroxylated polymers, such as cellul- ose and chitin, can be explained by the existence of a DMA-LiC1 complex involving ion pair formation. In this study, we have shown that the DMA proton signals are shifted upfield on increasing the LiCl con- centration (contrary to most other alkali metal salts) while, at the same time, the carbon signals are shifted downfield. This has been ascribed to a difference in the interaction mechanism of the LiCl molecules with the carbon atoms and the methyl protons. 'H and 13C

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longitudinal relaxation times are in favour of the for- mation of a strong DMA-LiCl complex, probably in 'polymer' form, owing to the large decrease in the mobility of DMA and to the high viscosity of the solvent.

The solution of methyl-@ -~-glucopyranoside (1) in DMA-LiC1 shows that there is n o exchange among the hydroxy protons, which are shifted strongly down- field. Each hydroxy group of 1 is solvated by one LiCl molecule. The DMA-LiCl complex interacts with the OH group via the LiCl moiety, with the preservation of the 'polymer' network.

The particular dissolving power of the DMA-LiCl solvent towards cellulose and chitin must be related to the strong interaction with their hydroxy groups,

which destroys the intermolecular hydrogen bonds and causes these polymers to dissolve. We have already given evidence of the existence of intramolecular hyd- rogen bonds for cellulose dissolved in this solvent2" which are therefore not affected on solution.

Steric and electronic factors such as van der Waals radii, bond length and bond polarity can also explain the specificity of the LiC1-DMA complex as opposed to other lithium salts and amides.

Acknowledgements

We thank Le Laboratoire Grenoblois de RCsonance MagnCtique NuclCaire for access to their NMR facilities and Mrs F. Sarrazin for the T, measurements.

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Received 20 October 1982; accepted 20 December 1982

ORGANIC MAGNETIC RESONANCE, VOL. 21, NO. 6, 1983 375