Interaction of Alkaline Earth Metal Ions with Acetic and Lactic Acid in Aqueous Solutions Studied by 1 3 C NMR Spectroscopy

Transcription

1 Interaction of Alkaline Earth Metal Ions with Acetic and Lactic Acid in Aqueous Solutions Studied by 1 3 C NMR Spectroscopy A. K o n d o h and T. Oi Department of Chemistry, Sophia University, 7-1 Kioicho, Choyodaku, Tokyo 102, Japan Z. Naturforsch. 52a, (1997); received December 4, 1996 Interaction of alkaline earth metal (magnesium, calcium, strontium and barium) ions with acetic and lactic acid in aqueous media was investigated by C NMR spectroscopy. In the acetate systems, signals whose chemical shifts were the averages of those of the free and bound acetate ions were observed. Downfield shifts of the carboxylate carbon signals with increasing metal ion concentration indicated that the acetate ion acted as a monodentate ligand coordinating to the metal ion using the carboxylate group. The metal ion concentration dependence of the peak positions of the methine and carboxylate carbon signals of the lactate ion in the lactate systems suggested that the lactate ion coordinated to a metal ion using the carboxylate and hydroxyl groups. Unique upfield shifts upon complexation in the magnesium lactate systems suggested that the lactate ion coordinated to the magnesium ion from outside the primary hydration sphere. The present results were consistent with the isotope effects of the alkaline earth metals observed in cation exchange chromatography. Key words: C NMR, Alkaline Earth Metals, Acetic Acid, Lactic Acid, Isotope Effects. 1. Introduction We have carried out a series of experiments on isot o p e s e p a r a t i o n of alkaline e a r t h metals (magnesium [1], calcium [2], strontium [3] a n d barium [4]) by cation exchange c h r o m a t o g r a p h y using the chloride, acetate or lactate ion as the counterion in a q u e o u s solutions, a n d observed isotope separation effects on the order of per unit mass difference. In the chrom a t o g r a p h i c experiments it was indicated t h a t the b e h a v i o r of the magnesium ion upon complexation with the acetate and lactate ions in a q u e o u s media was different f r o m that of the other alkaline earth metal ions studied, and that the acetate a n d lactate ions differed f r o m each other in the c o o r d i n a t i o n m a n ner. To elucidate the observed isotope effects better a n d to ascertain indications given in the c h r o m a t o g r a p h i c experiments, we felt it necessary to c o n d u c t a solution chemical investigation of alkaline e a r t h metal c a r b o x y l a t e s in aqueous media. N M R a n d infrared (IR) spectroscopy is quite often used and seems very effective for such investigations. In this p a p e r we rep o r t the study on interactions of alkaline e a r t h metal ions with acetic a n d lactic acids in a q u e o u s m e d i a by Reprint requests to Dr. T. Oi. C N M R spectroscopy a n d subsidiarily by IR spectroscopy carried out to p u r s u e the a b o v e objectives. 2. Experimental 2.1 Preparation for 1 3 C NMR of Samples Measurements A m a g n e s i u m acetate solution for the 1 3 C N M R m e a s u r e m e n t ( N M R sample solution) was prepared as follows. An a q u e o u s solution c o n t a i n i n g m a g n e s i u m a n d acetic acid at p r e d e t e r m i n e d c o n c e n t r a t i o n s was p r e p a r e d by dissolving required a m o u n t s of magnesium acetate a n d acetic acid in distilled water. S o d i u m h y d r o x i d e solution was then a d d e d so that the ionic strength of the solution was 0.60 M (M = m o l / d m 3 ) [5] a n d the solution p was 7.0, a n d the resultant solution ( N M R sample solution) was subjected to the 1 3 C N M R m e a s u r e m e n t. O t h e r sample solutions were prep a r e d likewise. T h e p value = 7.0 of the sample solutions was chosen since, j u d g i n g f r o m the dissociation c o n s t a n t s of acetic a n d lactic acids a n d the f o r m a t i o n constants of complexes between alkaline e a r t h metal ions a n d acetate a n d lactate ions [6, 7], those acids d o not exist as neutral molecular species in the solutions but practically perfectly as carboxylate ions at that p. T h e / 97 / $ Verlag der Zeitschrift für Naturforschung, D Tübingen

2 A. Kondoh and T. Oi 352 concentrations of alkaline e a r t h metal ions were 0.0, 0.05, 0.10, 0.15 and 0.20 M. 2.2 C NMR Measurements The N M R p r o b e unit used consisted of a 10 m m and a 5 m m o.d. glass probe. In the 5 m m o.d. probe, 1,4-dioxane, used as external reference (chemical shift (5 = 67.8 ppm), was dissolved in a b o u t 1 c m 3 of D 2 0 used as a lock. The tube was capped, inserted into the 10 m m o.d. p r o b e and fixed to it with Teflon fixing kits. 3-4 c m 3 of an N M R sample solution was placed into the 10 m m o.d. probe. The F T - N M R spectrometer was a J O E L J N M G X operated at 67.8 M z a n d 300 ± 0.5 K. T h e n u m b e r of pulses a c c u m u l a t e d for a m e a s u r e m e n t was 500 ~ 20,000, a n d the repetition time was 3.9 JIS. T h e measurements were m a d e in the p r o t o n n o n - d e c o u pling m a n n e r to avoid t e m p e r a t u r e increase of the sample solutions. A reason for this was t h a t interactions of alkaline earth metal ions with acetate and lactate ions in a q u e o u s m e d i a were expected to be sensitive to t e m p e r a t u r e change. In addition, preliminary 1 3 C N M R experiments showed that a change in t e m p e r a t u r e by one degree yielded 0.01 ~ 0.02 p p m shifts in 1 3 C N M R signal positions, which were not negligible in the present m e a s u r e m e n t s. D u e to the present measuring m a n n e r, 1 3 C N M R signals were usually observed as multiplets. T h e p e a k position of a multiplet was determined by algebraically averaging the positions of the individual lines of the multiplet. 2.3 Measurements of IR Spectra The IR spectrum of each N M R sample solution was taken by the a t t e n u a t e d total reflection m e t h o d. The used Perkin Elmer F T I R spectrometer was equipped with a cell for solution samples whose windows were m a d e of ZnSe (Spectra Tech Inc. A T R ). 3. Results and Discussion 3.1 Alkaline Earth Metal-Acetic Acid Systems In any c o m b i n a t i o n of alkaline e a r t h metal and acetic acid only two signals were observed. As an example the spectrum of the calcium acetate solution with the calcium ion c o n c e n t r a t i o n of 0.2 M is shown in Figure 1. The 1 3 C N M R signal at a r o u n d ö = 182 p p m, observed as a quartet, is assigned to the carboxylate c a r b o n ( C O O - ) of the acetate ion, a n d the quartet at a b o u t <5 = 24 p p m to the methyl c a r b o n (C 3 ). Based o n the values of the dissociation constant of acetic acid and the complex f o r m a t i o n c o n stant of the calcium ion with the acetate ion [6, 7], the chemical f o r m s of acetic acid expected for the solution are; 2 5 % as the acetate ion coordinating to the calcium ion ( b o u n d acetate ion), 7 5 % as the free n o n - c o o r d i n a t i n g acetate ion (free acetate ion) a n d 0 % as the neutral molecular acetic acid. Considering the very weak interaction between the calcium a n d acetate ions, observation of only two signals means t h a t the interchange between the b o u n d a n d free acetate ions is fast on the 1 3 C N M R time scale, and hence single signals of the methyl and carboxylate c a r b o n s are observed whose chemical shifts are the averages of those of the b o u n d and free acetate ions weighted by their respective amounts. This is in contrast with the s t r o n g interaction of a metal ion with the acetate ion; for instance, distinct signals corresponding to the b o u n d a n d free acetate ions were observed for the uranyl acetate system [8]. T h e weak c o o r d i n a t i o n of the acetate ion to the calcium ion is also evidenced in the IRspectra. It is well established that the c o o r d i n a t i o n m o d e of a carboxylate ion to a metal ion can be understood by the difference in the vibrational frequency of the asymmetric and symmetric C O O - vibrational m o d e s of the b o u n d carboxylate ion, relative to that of the free carboxylate ion [9]. Figure 2 shows the IR spectra of the calcium acetate solutions with varying ion concentrations in the frequency range of c m - 1 (asymmetric a n d symmetric C O O - frequency region). As is seen, n o appreciable change in the spectra is observed; no distinct adsorption peaks c o r r e s p o n d i n g to the b o u n d a n d free acetate ions are observed. This IR result is consistent with the 1 3 C N M R results above. Similar features as for the calcium acetate system were also f o u n d for the other alkaline earth metal acetate systems examined. In Figs. 3 a a n d 3 b, we plotted the chemical shifts of the methyl c a r b o n s (Fig. 3 a) a n d the carboxylate carb o n s (Fig. 3 b) against the concentrations of the alkaline earth metal ions examined. A higher c o n c e n t r a tion c o r r e s p o n d s to a larger percentage of the b o u n d acetate ions. T h e change in chemical shift with increasing metal ion concentration is much larger for the carboxylate c a r b o n signal t h a n for the methyl car-

3 Ca* concentration VvMJ^1 OM O z 70 CQ C D. >< o > Chemical shift/ppm IIIIIM IIIIIIMI Minil IIIIIIMI Fig. 1. The C NMR spectrum obtained for an aqueous solution with calcium and acetic acid concentrations of 0.20 mol/dm 3 and 0.40 mol/dm 3, respectively. p = 7.0, ionic strength 0.6 mol/dm 3. 1,4-dioxane was used as the external reference Frequency/cm" 1 00 Fig. 2. The IR spectra in the symmetric and asymmetric COO" frequency region of aqueous solutions of the calcium acetate system with varying calcium ion concentration. CU C s= Q. C3COO" - (a) p C3COO" p7.0, (b) Ca -o/ -<M2+)0 Structure I x, C O" O (Mg 2+) O II ' Ttki x h o Structure II U Sr " ^ctr Mg ÜT Metal ion concentration/moldm"' ( > S r ^ ^ M g ^ Ba " Metal ion concentration/moldm" 3 Fig. 3. Plots of chemical shifts of (a) the methyl carbon signal and (b) the carboxylate carbon signal of the acetate ion in the acetate systems against alkaline earth metal ion concentrations. C, C / O (M2+).-'^O- ' O" Structure in C 3 C 0 h, ^ar / O (Mg2+) O ' S f ^ o Structure IV Fig. 4. Schematic drawings of possible structures of alkaline earth metal acetate and lactate complexes in aqueous solutions.

4 A. Kondoh and T. Oi 354 b o n signal in any alkaline e a r t h metal acetate system examined, suggesting that the acetate ion c o o r d i n a t e s to metal ions using its carboxylate g r o u p, as is usually the case. As is seen in Fig. 3, the chemical shifts of the carboxylate c a r b o n s show m o n o t o n o u s downfteld shifts with increasing metal ion c o n c e n t r a t i o n, irrespective of the kind of metal, showing t h a t the electron densities on the carboxylate c a r b o n s decrease with increasing metal ion concentrations. This is best u n d e r s t o o d by assuming that part of the electron cloud on the carboxylate c a r b o n is a t t r a c t e d by the metal ion t h r o u g h the metal ion-carboxylate g r o u p interaction. It is difficult to infer the c o o r d i n a t i o n m o d e of the acetate ion to the alkaline e a r t h metal ions only f r o m the present 1 3 C N M R and IR results. C o n s i d e r i n g the very weak interactions between the alkaline earth metal ions a n d the acetate ion, however, the most p r o b a b l e m o d e is the one in which the acetate ion acts as a m o n o d e n t a t e ligand c o o r d i n a t i n g to a cation using the single b o n d e d oxygen of the c a r b o x y l a t e g r o u p as s h o w n in Fig. 4 (Structure I). The order of the value of the complex f o r m a t i o n constant between the alkaline e a r t h metal ion a n d the acetate ion is Mg2 + C O O ~ > Ca2 + C O O " > Sr2+ C O O - > Ba2+ C O O " [6, 7], C o n t r a r y to this sequence, the order of the deviation of the chemical shift of the c a r b o x y l a t e c a r b o n signal at a given metal ion c o n c e n t r a t i o n f r o m that at 0 M concentration is Ca2+ - COO" >Sr2+ - COO" >Ba2 - C O O " >Mg COO". M a g n e s i u m is not at the right place expected f r o m the complex f o r m a t i o n constant values. This m a g n e s i u m anomaly is best u n d e r s t o o d by a s s u m i n g t h a t the magnesium ion behaves as if it were larger t h a n the barium ion, p r o b a b l y due to the effect of h y d r a t i o n (cf.: Fig. 4, Structure II). T h a t is, in the c o o r d i n a t i o n to a magnesium ion, the acetate ion does not replace a water molecule hydrating the m a g n e s i u m ion but interacts with it f r o m outside the p r i m a r y h y d r a t i o n sphere a r o u n d it. As for the c o o r d i n a t i o n m a n n e r of the acetate ion to the alkaline earth metal ions o t h e r t h a n the m a g n e sium ion, two m a n n e r s seem possible. Either the carboxylate g r o u p interacts with a n alkaline e a r t h metal ion from outside the primary h y d r a t i o n sphere as in the case of the magnesium ion, or the acetate ion directly coordinates to a cation in the primary h y d r a tion sphere without the liberation of hydrating water molecule. T h e h y d r a t i o n n u m b e r in the primary hyd r a t i o n sphere is six for the magnesium and calcium ions. Slightly larger n u m b e r s are reported for the s t r o n t i u m a n d b a r i u m ions [10]. A simple solid geometric calculation using the radii of the alkaline e a r t h metal ions a n d the oxide ion suggests that, while the p r i m a r y h y d r a t i o n sphere of the magnesium ion acc o m m o d a t e s only six water molecules, those of the calcium, s t r o n t i u m a n d barium ions a c c o m m o d a t e at least eight. T h a t is, it seems possible to a d d a ligand in the p r i m a r y h y d r a t i o n spheres of calcium, s t r o n t i u m a n d b a r i u m ions without the liberation of a h y d r a t i n g water molecule. A molecular orbital calculation [11] also s u p p o r t s this. In any case, it seems quite unlikely t h a t the acetate ion directly coordinates to an alkaline earth metal ion with the liberation of a h y d r a t i n g water molecule f r o m the primary h y d r a t i o n sphere. 3.2 Alkaline Earth Metal-Lactic Acid Systems In every C N M R spectrum taken, three signals c o r r e s p o n d i n g to the methyl c a r b o n (C 3 ), the methine c a r b o n (C(O)) and the carboxylate c a r b o n ( C O O ") were observed at a r o u n d Ö = 21 p p m as q u a r tet-doublets, at a b o u t 70 p p m as d o u b l e t - q u a r t e t s a n d at a r o u n d 183 p p m as triplets, respectively. As in the case of the acetate systems, no distinct signals corres p o n d i n g to b o u n d and free lactate ions were observed for any of the methyl, methine a n d carboxylate carbons, due to the fast interchange between the t w o states of the lactate ion. Also, no appreciable c h a n g e in the IR spectra in the asymmetric a n d symmetric C O O " frequency region with the change in the metal ion c o n c e n t r a t i o n was observed in any system. In Fig. 5 a, 5 b, a n d 5 c, we plotted the chemical shifts of the methyl c a r b o n signal (Fig. 5 a), the methine c a r b o n signal (Fig. 5 b), a n d the carboxylate carb o n signal (Fig. 5 c) of the lactate ion against the conc e n t r a t i o n s of alkaline earth metal ions. As is seen, all the chemical shifts except that of the methine c a r b o n signal in the magnesium lactate system show m o n o t o n o u s downfield or upfield shifts with increasing metal ion concentrations, and the behavior of the m a g n e sium lactate system differs from those of the o t h e r alkaline e a r t h metal lactate systems. In the calcium, strontium and b a r i u m lactate systems, all the signals shift downfield with increasing

5 355 A. Kondoh and T. Oi - C3C(O)COO- C3C(O)COO" (a) C a ^ ' (b) Ca/"^ Sr Mg Metal ion concentration/moldm Metal ion concentration/moldm" C3C(O)COCT (C) \ M g (U M e t a l ion concentration/moldm" 3 Fig. 5. Plots of chemical shifts of (a) the methyl carbon signal, (b) the methine carbon signal and (c) the carboxylate carbon signal of the lactate ion in the lactate systems against alkaline earth metal ion concentrations. metal ion c o n c e n t r a t i o n s. T h e trend in the concentration d e p e n d e n c e of the three kinds of c a r b o n signal indicates t h a t the electron densities of the c a r b o n s in the free lactate ion are higher t h a n those of the corres p o n d i n g c a r b o n s in the b o u n d lactate ion, as in the cases of metal acetate systems above. T h e deviation of the chemical shift of the m e t h i n e c a r b o n signal at any metal ion c o n c e n t r a t i o n f r o m that at 0 M concentration is larger t h a n t h a t of the carboxylate c a r b o n signal for a n y of the three lactate systems. This suggests that the lactate ion c o o r d i n a t e s to the alkaline earth metal ions using b o t h the carboxylate g r o u p and the hydroxyl g r o u p to f o r m a five m e m b e r ring as is s h o w n in Fig. 4 (Structure III). T h e hydroxyl g r o u p is expected to c o o r d i n a t e to a metal ion w i t h o u t dissociation of its p r o t o n u n d e r the present experimental conditions. In fact, n o value of the dissociation constant of the hydroxyl g r o u p of the lactic acid is given in the literature. T h e interaction of the hydroxyl g r o u p with the alkaline e a r t h metal ion is thus the one utilizing a lone electron pair of the hydroxyl oxygen. Then, the m o s t r e a s o n a b l e e x p l a n a t i o n of the results of the calcium, s t r o n t i u m a n d b a r i u m lactate systems shown in Fig. 5 is as follows. T h e alkaline earth metal ion a t t r a c t s electron clouds of the single-bounded carboxylate (C( = 0 ) 0 ~ ) a n d hydroxyl oxygens, which causes a decrease in the electron density of all the c a r b o n s of the lactate ion, resulting in downfield shifts of their signals with increasing metal ion c o n c e n t r a tion. The decrease of the electron density of the carboxylate c a r b o n u p o n the c o m p l e x a t i o n is smaller t h a n that of the m e t h i n e c a r b o n, p r o b a b l y because the decrease of the electron density of the c a r b o x y l a t e c a r b o n is partially c o m p e n s a t e d by the influx of the electron cloud f r o m the d o u b l e - b o n d e d oxygen of the carboxylate g r o u p (C( = 0 ) 0 ~ ). C o n t r a r y to the calcium, s t r o n t i u m a n d b a r i u m lactate systems, in t h e m a g n e s i u m lactate system the signals of the c a r b o n s of the lactate ion showed upfield shifts u p o n c o m p l e x a t i o n. Also, it is characteristic of the magnesium lactate system that, while the metal ion c o n c e n t r a t i o n dependence of the methine c a r b o n signals is larger t h a n t h a t of the c a r b o x y l a t e c a r b o n signals in the o t h e r systems, the metal ion c o n c e n t r a tion dependence of the c a r b o x y l a t e c a r b o n signal is by far the largest a m o n g the three kinds of c a r b o n, a n d t h a t of the m e t h i n e c a r b o n signal is very slight in the m a g n e s i u m lactate system. Generally the 1 3 C N M R signal of the carboxylate c a r b o n (and p r o b a b l y the hydroxyl c a r b o n, too) of a carboxylate ion seems to show a downfield shift u p o n complexation with a metal ion by reason of the attraction of electron clouds of the oxygens of the c a r b o x y late a n d hydroxyl g r o u p s by the metal ion, a n d consequently those of the carboxylate a n d m e t h i n e carbons. This general e x p l a n a t i o n can be applied to the present calcium, s t r o n t i u m a n d b a r i u m acetate a n d lactate systems as s h o w n above. E x p e r i m e n t a l facts t h a t d o n o t obey this generality were f o u n d in the case of downfield shifts of carboxylic c a r b o n s of carboxylic acids u p o n dissociation ( C O O to C O O " ), the decisive explanation for which h a s not yet been established [12]. T h e upfield shift of the carboxylate c a r b o n signal upon c o m p l e x a t i o n in the present m a g n e s i u m lactate system c a n be u n d e r s t o o d by referring to the behavior of carboxylic c a r b o n signals of carboxylic acids upon dissociation. T h e m a g n e s i u m ion does not interact directly with the c a r b o x y l a t e g r o u p (and m a y b e the h y d r o x y l group, either) of the lactate ion but indirectly t h r o u g h a h y d r a t i n g water, as schematically shown in Fig. 4 (Structure IV). T h e carboxylate g r o u p of the b o u n d lactate ion interacts a p p a r e n t l y with the h y d r o g e n of a h y d r a t i n g water. This situation is similar to the situation in which a carboxylate ion interacts with a p r o t o n to f o r m a neutral carboxylic acid.

6 Correspondence of the Present 1 3 C NMR to the Isotope Effects Observed in Ion Exchange A. Kondoh and T. Oi Results Systems The present 1 3 C N M R spectroscopic results help to understand the isotope effects of alkaline e a r t h metals observed in ion exchange c h r o m a t o g r a p h i c separation systems with chloride, acetate or lactate ion as the counterion [1-4]. The essence of the observed isotope effects is summarized and some relations between the C N M R a n d c h r o m a t o g r a p h i c results are discussed below. 1) F o r any alkaline e a r t h metal studied, the heavier isotopes were fractionated into the solution phase [1-4]. Based o n a theory of isotope d i s t r i b u t i o n between two phases [], this is equivalent to state that the value of the heavier isotope-to-lighter i s o t o p e reduced partition function ratio ( R P F R ) [14] of a chemical species is larger in the solution phase t h a n in the ion exchanger phase. 2) T h e m a g n i t u d e s of isotope separation effects of the calcium, s t r o n t i u m a n d b a r i u m acetate systems [2-4], in which certain percentage of m e t a l ions formed complexes with acetate ions both in the external solution a n d ion exchanger phases, were larger t h a n or equivalent to those of the metal chloride systems in which c o m p l e x a t i o n s between metal a n d chloride ions were negligible. This m e a n s that the sum of the forces acting on the metal ion is larger in the complex form t h a n in the simply hydrated f o r m [14], and seems m o r e compatible with the inference t h a t the acetate ion c o o r d i n a t e s to a metal ion in the p r i m a r y hydration sphere w i t h o u t the liberation of a h y d r a t i n g water molecule t h a n the one t h a t the acetate ion coordinates to the metal ion f r o m outside the p r i m a r y h y d r a t i o n sphere (cf.: Fig. 4, Structure I). 3) C o n t r a r y to the other acetate systems examined, the isotope separation effect of the m a g n e s i u m acetate system was smaller t h a n that of the m a g n e s i u m chloride system [1], showing t h a t the sum of the forces acting on the simply h y d r a t e d m a g n e s i u m ion is larger t h a n that acting o n the ligand c o o r d i n a t e d m a g n e sium ion. This situation can be realized if the ligand c o o r d i n a t i o n f r o m outside the primary h y d r a t i o n sphere a r o u n d the m a g n e s i u m ion disturbs the interaction between the m a g n e s i u m ion and h y d r a t i n g waters in the p r i m a r y h y d r a t i o n sphere, which results in weakening the b o n d i n g between the m a g n e s i u m ion a n d hydrating waters. 4) T h e isotope separation effect of any of the alkaline earth metal lactate systems examined is larger t h a n or equivalent to that of the metal chloride system, which means that sum of the forces acting o n a metal ion is larger in the complex f o r m t h a n in the simply hydrated form as in the cases of the calcium, s t r o n t i u m and b a r i u m acetate systems. In any lactate system, the c o o r d i n a t i o n of the lactate ion, using b o t h the carboxylate a n d hydroxyl g r o u p s a n d the f o r m a tion of a ring structure is expected to increase the s u m of the forces acting on the metal ion, which results in a larger isotope effect t h a n that in the metal chloride systems. 4. Conclusion T h e m a j o r findings of the present troscopic study are as follows: C N M R spec- 1) N o distinct 1 3 C N M R signals c o r r e s p o n d i n g to free a n d b o u n d acetate ions were observed for the methyl and carboxylate c a r b o n s in the a q u e o u s m a g nesium, calcium, s t r o n t i u m and b a r i u m acetate systems. The metal ion concentration d e p e n d e n c e of the C N M R signal positions of the methyl a n d c a r b o x y late carbons of the acetate ion a n d IR spectra of the sample solutions in any system suggested that the acetate ion worked as a m o n o d e n t a t e ligand (Fig. 4, Structure I). It was also indicated t h a t the acetate ion c o o r d i n a t e d to the magnesium ion f r o m outside the p r i m a r y hydration sphere a r o u n d the m a g n e s i u m ion (Fig. 4, Structure II). 2) Similarly to the acetate systems, n o distinct 1 3 C N M R signals corresponding to free a n d b o u n d lactate ions were observed for either of the methyl, m e t h i n e a n d carboxylate c a r b o n s in any of the a q u e o u s alkaline earth metal lactate systems. T h e metal ion concent r a t i o n dependence of the 1 3 C N M R signal positions of the methine a n d carboxylate c a r b o n s of the lactate ion in any system suggested that the lactate ion c o o r dinated to the metal ion using b o t h the c a r b o x y l a t e g r o u p and the hydroxyl g r o u p w i t h o u t dissociation of the hydroxyl p r o t o n (Fig. 4, Structure III). T h e u n i q u e upfield shift of the carboxylate c a r b o n of the lactate ion u p o n compelxation with the m a g n e s i u m ion indicated that the lactate ion c o o r d i n a t e d to the m a g n e sium ion from outside the primary h y d r a t i o n sphere of the magnesium ion (Fig. 4, Structure IV). 3) T h e present 1 3 C N M R spectroscopic results are consistent with the isotope effects of the alkaline e a r t h metals observed in cation exchange c h r o m a t o g r a p h y.

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