Download Interaction of Alkaline Earth Metal Ions with Acetic and Lactic Acid in

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, 351-357 (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 13 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:
13
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 1 0 - 5 - 1 0 - 6 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.
13
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 H 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 H 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 H . T h e
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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
13
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 2 7 0 operated at 67.8 M H 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 1 6 5 0 spectrometer was
equipped with a cell for solution samples whose windows were m a d e of ZnSe (Spectra Tech Inc. A T R - H 1403).
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
13
C NMR Study of Acetic and Lactic Acids
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
(CH 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 1 3 0 0 - 1 7 0 0 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-
Ca* concentration
VvMJ^1
OM
O
z
70
CQ
C
D.
><
o
>
IIIIIM|IIIIIIMI|HMinil|IIIIIIMI|
2 0 0 190 180 170 160 150 1 4 0 1 3 0 120 110 100 90
40
30
20
10
0
1600
1700
1500
Chemical shift/ppm
Fig. 1. The 13 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.
pH = 7.0, ionic strength 0.6 mol/dm 3 . 1,4-dioxane was used as the external
reference.
CH
H
H
-o/
H
-<M2+)0
CH3COO"
Q.
-
183.0 -
pH7.0
(a)
CH3COO"
pH7.0
(b)
24.6
Sr
^cTr—
U
,
H
Ca
182.8
S r ^
>
CH,
/
^
Ba "
HC
—
O
H
O" —
H
,H
O (Mg 2+) O
' Ttki x h
H
H
Structure II
0.1
0.2
O"
CH3
H
182.(
ÜT
Metal ion concentration/moldm"'
C
II
o
H
-
0.1
H
Hx
"
^ M g
0
H
Structure I
Mg
24.4
1300
Fig. 2. The IR spectra in the symmetric and asymmetric C O O " frequency
region of aqueous solutions of the calcium acetate system with varying calcium
ion concentration.
CHU
s=
1400
Frequency/cm" 1
'
(M2+)
.-'^O-H
H
Metal ion concentration/moldm"3
Structure in
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.
HC
0
H
H
h, ^ a r /H
O (Mg2+) O
H'
S f ^
o—
Structure IV
Fig. 4. Schematic drawings of possible structures of alkaline earth metal acetate and lactate complexes in aqueous solutions.
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 " >Mg2+ -
+
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
13
C NMR Study of Acetic and Lactic Acids
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
13
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 (CH 3 ), the methine c a r b o n (CH(OH)) 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
355 A. Kondoh and T. Oi •
-
CH3CH(OH)COO-
13
C NMR Study of Acetic and Lactic Acids
CH3CH(OH)COO"
(a)
C a ^
'
(b)
Ca/"^
Sr
Mg
Metal ion concentration/moldm
183.8
Metal ion concentration/moldm"
CH3CH(OH)COCT
183.6
(C)
\ M g
183.4
(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 H 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.
356
3.3 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
13
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 [13], 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
13
C NMR Study of Acetic and Lactic Acids
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:
13
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
13
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 .
357 A. Kondoh and T. Oi •
13
C NMR Study of Acetic and Lactic Acids
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