methanol mixed solvents

I. inorg nucl. Chem. Vol. 42, pp. 1185-1188 © Pergamon Press Ltd., 19~0. Printed in Great Britain

0022-1902/80/0801-1185/$02.00/0

NMR STUDY OF LaC13 SOLUTIONS IN WATER/METHANOL MIXED SOLVENTS DOUGLAS C. Mc CAIN Department of Chemistry, University of Southern Mississippi, Southern Station Box 9281, Hattiesburg, MS 39401, U.S.A.

(First received 17 September 1979; received[or publication 22 October 1979) Abstract-Concentrated LaCI3 solutions in water, methanol and mixed water/methanol solvents were studied by ~39Laand 35CI NMR. As methanol replaces water, an increasing degree of La-CI inner sphere coordination was found. Results indicate rapid chloride exchange in the inner sphere occurs via displacement reactions. INTRODUCTION Previous studies of La(III) salts in water and methanol using NMR[1-5] and Raman[6] spectroscopy, liquid phase X-ray diffraction/l,7], ultrasonic absorption/8] and thermodynamic properties/9] have given sometimes conflicting results, but, from the best available evidence, the La(H20)fl ÷ ion apparently predominates in concentrated aqueous solutions where no inner sphere chlorides can be detected [1,2, 10]. La(H20)93+ and ions with even higher coordination numbers may be more important in dilute solutions/2-8, 10]. X-Ray data show that concentrated LaC13 solutions in methanol contain a dimer ion/l], (La2Ch(CH3OHho) 2+, with two bridging chloride ligands, two non-bridging inner sphere chlorides and two outer sphere chloride ions per dimer unit. The introduction of chloride ligands may be related to the lower dielectric constant of methanol/11, 12]. This paper describes an NMR study of concentrated LaC13 solutions in methanol/water mixtures. The object is to study the effect of solvent on solute structure, and especially to follow the gradual increase in chloride inner sphere coordination as methanol replaces water. EXPERIMENTAL LaC13 solutions for this project were prepared by mixing the identical water and methanol solutions used in previous X-ray and NMR work/l, 7]. The two original solutions had been made from weighed amounts of anhydrous LaC13dissolved in water or anhydrous methanol; no decomposition was evident after more than one year at room temperature. Table l presents results from the original solutions and a series of mixtures defined by their methanol content (as volume per cent). Samples were studied between two and nine days after mixing; this detail is important because the viscosity of some mixtures slowly increases over a period of many months. Viscosities in Table 1, measured with an Ostwald viscometer, are correct for each solution at the time of study by NMR. Density measurements were used to calculate molality. All data reported in this paper were obtained at 32 -+l°C, the ambient temperature in the NMR probe. A conventional wide-line NMR spectrometer (a modified Varian HR-60)operating at 4.2 MHz was used to detect signalsof t39La and 35C1.Suitable precautions were taken to prevent distortion due to power saturation and overmodulation. Chemical shifts, 8, were measured by comparison with the water solution which serves as an external standard for both m39Laand 3~C1in the other solutions. Line widths, AH, were measured as peak-topeak distances on the first derivative spectrum; each value reported is the average of at least fifteen sweeps.

normally displays a number of lines of different heights and widths and various chemical shifts. All the spectra obtained in this study appear to show a single, symmetrical line, although its width and chemical shift do vary with the solvent (Table 1). Nevertheless, since solute structures in water and methanol are distinctly different, two or more chemical species with different NMR properties must be present in at least some of the intermediate mixtures. Two explanations for the single line are possible: (1) All nuclei are chemically equivalent due to rapid exchange. This is possible even if there are a multitude of chemical species. Such a system will present a pure Lorentzian lineshape; (2) The nuclei are not chemically equivalent, but signals overlap to such an extent that only one composite line is seen. The resulting lineshape is a weighted sum of individual contributions. A sum of Lorentzian functions does not generate a Lorentzian curve unless all components possess identical widths and centers. On the first derivative of a true Lorentzian curve, the signal amplitude at twice the linewidth (2AH) or three times the linewidth (3AH) compares to the maximum amplitude (at AH) in ratios of 0.653 or 0.333 respectively/13]. These amplitudes were measured for "9La and 35C1 in all solutions. In only one case did the measured signal deviate by more than two standard deviations (--6% for La, --12% of theoretical amplitude for CI) from a Lorentzian lineshape. The "C1 spectrum in pure methanol solution is slightly non-Lorentzian; at 3AH, amplitude is 64% of the expected value. Chlorine must be present in this solution in two or more chemically inequivalent forms.

RESULTS AND DISCUSSION

Lineshapes The NMR spectrum of a complex chemical system 1185

Areas The area under an NMR absorption curve is proportional to the number of contributing nuclei. If lines are strictly Lorentzian and are unaffected by power saturation or modulation distortion, the area per nucleus, A may be estimated by use of the expression/13]: A = KS(AH)2/P 1Z2GMB where S is the peak-to-peak amplitude on the first derivative spectrum. G is the amplifier gain. M is the concentration of LaC13 in moles per liter. P is the radio frequency power input to the probe, and B is the magnetic field modulation amplitude. K is a normalization constant which may be chosen so that A = 1.00 for the water solution which does have a Lorentzian lineshape. X-Ray measurements/l] show that water contains only the L a ( H 2 0 ) s 3+ arid free Cl- ions. Defined in this way, A

1186

DOUGLAS.C. Mc CAIN Table 1. Methanol/water solutions and measured NMR parameters/l] SOLUTIONS

~. I,l e t h a n o l 2

X 3 H20

lq4

La-139

C1-35

q5

d6

~H±3%7

6±48

A±.19,10

AII_+5~7

5_+108

A±.29,1()

0.0

1.00

1.62

1.80

1.33

200

(0)

(1.00)

70

(0)

16.7

.92

1.60

2.70

1.50

300

0

1.00

160

0

(1.00) 1.00

33.3

.82

1.59

3.74

1.28

470

-3

.99

270

0

1.02 1.01

50.0

.69

1.58

4.59

1.25

790

0

1.01

520

2

66.7

.53

1.56

6.20

1.22

1310

16

.98

1100

24

1.02

75.0

.43

1.54

5.38

1.20

1750

43

.90

1670

30

1.09

83.3

.31

1.52

5.14

1.17

2500

85

.83

2530

58

.95

87.5

.24

1.51

4.66

1.15

2860

123

.84

5240

67

1.05

93.8

.13

1.47

3.78

1.12

3640

204

.88

4130

118

1.03

96.9.

.07

1.46

3.11

1.11

3810

254

.94

4690

182

.97

10o.b

.00

1.45

2.78

1.10

3800

293

.96

4640

167

.90

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

A l l m e a s u r e m e n t s a t 32°C.

~ m e t h a n o l by volume. LaC13 s o l u t i o n s i n p u r e w a t e r ( 0 ~ ) , p u r e methanol (100~) and i n t e r m e d i a t e s o l v e n t s o b t a i n e d by m i x i n g t h e o r i g i n a l 04 and 1004 s o l u t i o n s i n t h e i n d i c a t e d volume r a t i o s . Hole f r a c t i o n o f w a t e r i n t h e s o l v e n t . LaC13 c o n c e n t r a t i o n i n moles p e r l i t e r o f s o l u t i o n . Solution viscosities ( i n c e n t i p o i s e ) measured w i t h an O s t w a l d v i s c o m e t e r . D e n s i t y i n g r a m s / c m 3. L i n e w i d t h ( i n Hz). E r r o r l i m i t s a r e g i v e n as f r a c t i o n s o f t h e measured l i n e w i d t h . Chemical s h i f t ( i n p a r t s p e r m i l l i o n ) v e r s u s t h e 04 s o l u t i o n which s e r v e s as an e x t e r n a l s t a n d a r d . Positive values are downfield shifts. The a r e a f u n c t i o n , n o r m a l i z e d t o 1 . 0 0 f o r t h e 04 s o l u t i o n . Estimated error limits vary with linewidth; t h e l i m i t s shown h e r e a p p l y t o t h e 100~ s o l u t i o n . In 0R m e t h a n o l , e r r o r l i m i t s a r e a b o u t h a l f as l a r g e .

should equal 1.00 for all samples in which lines are Lorentzian. If a solution contains two or more species with the same chemical shift but different linewidths, A must be less than 1.00; if the species have equal widths but different chemical shifts, A will be greater than 1.00. In the limiting case where a solution contains several species and one has much narrower resonance lines, A equals the fraction of nuclei present in the narrow line species. The shape and area methods are sensitive to different features of a non-Lorentzian line, and one might easily be informative when the other fails to show results. Table 1 presents the area data. Accurate area measurements are not easily achieved in practice because so many variables enter the calculation. The error limits shown in Table 1 are intended to represent one standard deviation; they represent statistical scatter plus a generous estimate of possible systematic errors which could be rather large for area measurements. Lanthanum NMR results are better because the signalto-noise ratio for '39La was three or four times that of 3~C1. The J39La area function shows deviations from unity of about two estimated standard deviations in the 83.3 and 87.5% solutions. Evidently, two or more chemically distinct lanthanum species with considerably different linewidths are present in these samples. For 35Ci A = 1.00-+0.2 in all solutions.

Linewidths Figure 1 compares the linewidth/viscosity ratios of I ~ . a and 35C1 in the various solvent mixtures. Line broadening mechanisms which are likely to be most important in these solutions normally produce a line width proportional to viscosity[4, 12]; the function AH/~ compensates for this solvent dependence, providing data

depending primarily on the solute species, rather than its environment. The only important relaxation mechanism for either 139La or 35C1in these solutions involves interaction of the nuclear quadrupole moment with electric field gradients/3, 4]. Formation of an inner sphere La--CI complex should be observable as a strong increase in AH/~. Nakamura and Kawamura[2] studied ~39La NMR in aqueous La(III) salts. Measuring linewidth as a function of concentration, they found a slight broadening trend in LaCi3 and La(CIO4)3as well as much larger effects in the nitrate and sulfate salts where inner sphere coordination is known to be important [ l 1]. At 23°C in a sample which corresponds to the water solution (Table 1) they observed a linewidth of 230 Hz. This result has been critized by Reuben[3] whose pulsed NMR measurements indicate that AH should be about 150Hz at the same temperature and concentration. Neither result is directly comparable with the data in Table 1 because of the temperature difference; however, some rough measurements were made with cooled samples (near 23°C). The water solution gave linewidths of about 140 Hz. This indicates that there is a temperature effect and that Reuben's data is probably correct. Holtz, Weingartner and Hertz/14] examined 3~C1 relaxation in methanol/water mixed solvents containing LiC1. For 1 M LiC1 in water, 50% methanol and pure methanol they measure relaxation times which may be converted to linewidths of 180, 420 and 740 Hz respectively. Comparing these with data from Table 1 (70, 520 and 4640 Hz), it is evident that the dramatic rise in LaCI3 linewidth as the solvent composition approaches pure methanol is not due to chloride-solvent interactions, but must be due to La-CI complex formation. In water solutions, chloride apparently interacts more easily with

NMR Study of LaCI3 solutions in water/methanol mixed solvents

1187

0-50% Methanol

X-Ray data[l] show that LaCI3 in water has no significant degree of La-CI inner sphere coordination. Figure 1 clearly indicates that this condition persists to about 50% methanol.

t

50-97% Methanol All four curves of Fig. 1 follow the same trend, showing that every curve measures nearly the same process, an increasing degree of La-CI inner sphere interaction through this range. Area and lineshape data show that lanthanum is present in several species not rapidly exchanging but that chloride does rapidly exchange. Here is a paradox. If dissociation reactions:

1500

I000 I

II'

/

~//I

AH/,q

I

250

La 3+ + CI- ~ L a C I 2+ 2OO

150

I00

?

# 40

60

80

were the only mechanism for linewidth averaging we should expect more nearly Lorentzian lines from ~39La than from 3~C1. This is because a statistical factor (a CI:La ratio of 3: 1) requires the chlorides to exchange more slowly than lanthanum. The displacement reaction: LaCI 2+ + CI*~LaC1.2+ + C1-

50

0

20

LaCI 2+ + CI ~LaC12 ~

I00

can explain the data. Here the starred atom, CI*, carries a specific chlorine nucleus which can pass in and out of the coordination sphere while the lanthanum ion retains its inner sphere coordination.

% Methenol

Fig. 1. Linewidth/viscosity ratio, AHI~ (upper curves) and chemical shift, 6 (lower curves) vs per cent methanol for mLa (solid lines) and 35CI(dashed lines). lithium than lanthanum, perhaps because it cannot penetrate the La(HzO)83+ inner sphere. Chemical shifts

Figure 1 and Table 1 display the measured chemical shifts. The downfield trend of chemical shifts as water is replaced by methanol is evidence for a change in the average lanthanum and chloride coordination species. Downfield shifts in other diamagnetic ion-pair systems are often considered to indicate inner sphere coordination [ 12, 15]. Rinaldi, Kahn, Choppin and Levy[5] measured '39La chemical shifts as a function of concentration in aqueous LaCl3. Using 0.5 M La (C104)3 as a reference they found rather small shifts of about 10ppm. A shift of this magnitude could result from formation of an outer sphere complex or from a change in coordination number and thus is not inconsistent with X-ray data[l] which show no inner sphere chlorides in the water solution. According to Rinaldi et al., 8 is ca. 100 ppm for a true inner sphere complex. By this criterion, the methanol solution (6 = 293 ppm) must contain some La-CI inner sphere complex; X-ray shows it to have a bridge bonded dimer with three inner sphere chlorides around each lanthanum. CONCLUSIONS The solutions studied may be conveniently divided into three ranges.

100% Methanol

X-Ray data[l] shows all lanthanum ions to be equivalent and, as expected, their NMR lines are Lorentzian. However, chloride exists in three different chemical environments, bridge bonded or single coordinated to lanthanum and as the free chloride ion. Results from intermediate solvents (above) suggest that free and single coordinated chlorides must rapidly exchange, but bridging chlorides are surely more difficult to replace. Their presence can explain the non-Lorentzian 3~C1 lines in methanol. REFERENCES

1. L. S. Smith, D. C. McCain and D. L. Wertz, J. Am. Chem. Soc. 97, 2365 (1975). 2. K. Nakamura and K. Kawamura, Bull. Chem. Soc. Japan 44. 330 (1971). 3. J. Reuben, J. Phys. Chem. 79, 2154 (1975). 4. J. Reuben and Z. Luz, J. Phys. Chem. 80, 1357 (1976). 5. P. L. Rinaldi, S. A. Kahn, G. R. Choppin and G. C. Levy, J. Am. Chem. Soc. 10l, 1350 (1979). 6. W. C. Mundy and F. H. Spedding, J. Chem. Phys. 59, 2183 (1973). 7. L. S. Smith and D. L. Wertz, J. Am. Chem. Soc. 97, 2365 (1975). 8. B. Jezowska-Trzebiatowska, S. Ernst, J. Legendziewicz and G. Oczko, Bull. Acad. Polon. Sci., Ser. Chim. 24, 997 (1976): 25, 649 (1977), 9. F. H. Spedding, M. J. Pikal and B. O. Ayers, J. Phys. Chem. 70, 244O (1966). 10. D. G. Karraker, lnorg. Chem. 7, 472 0%8). II. H. B. Silber and A. Pezzica, J. lnorg. Nucl. Chem. 38, 2053 (1976). 12. A. 1. Mishaustin and Yu. M. Kessler, J. Solution Chem. 4, 779 (1975).

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DOUGLAS. C. Mc CAIN

13. C. P. Poole, Jr., Electron Spin Resonance, Wiley-Interscience, New York (1967). 14. M. Holtz, H. Weingartner and H. Hertz, J. Chem. Soc.

Faraday I, 73, 71 (1977). 15. C. DevereU, Prog. Nucl. Magn. Res. Spectroscopy 4, Chap. 4 (1969).