Hydrogen Bonding in Amorphous Alkaline Earth Carbonates

Article abstract of DOI:10.1021/acs.inorgchem.8b02170

Amorphous intermediates play a crucial role during the crystallization of alkaline earth carbonates. We synthesized amorphous carbonates of magnesium, calcium, strontium, and barium from methanolic solution. The local environment of water and the strength of hydrogen bonding in these hydrated modifications were probed with Fourier transform IR spectroscopy, ¹H NMR spectroscopy, and heteronuclear correlation experiments. Temperature-dependent spin-lattice (T1) relaxation experiments provided information about the water motion in the amorphous materials. The Pearson hardness of the respective divalent metal cation predominantly determines the strength of the internal hydrogen-bonding network. Amorphous magnesium carbonate deviates from the remaining carbonates, as it contains additional hydroxide ions, which act as strong hydrogen-bond acceptors. Amorphous calcium carbonate exhibits the weakest hydrogen bonds of all alkaline earth carbonates. Our study provides a coherent picture of the hydrogen bonding situation in these transient species and thereby contributes to a deeper understanding of the crystallization process of carbonates.

Full text of DOI:10.1021/acs.inorgchem.8b02170

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Hydrogen Bonding in Amorphous Alkaline Earth Carbonates
Sebastian Leukel,†,‡ Mihail Mondeshki,*,† and Wolfgang Tremel*,†
†Institut fu
Germany
̈
̈
r Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universitat, Duesbergweg 10-14, D-55128 Mainz,
‡Graduate School Materials Science in Mainz, Staudingerweg 9, D-55128 Mainz, Germany
S
* Supporting Information
ABSTRACT: Amorphous intermediates play a crucial role
during the crystallization of alkaline earth carbonates. We
synthesized amorphous carbonates of magnesium, calcium,
strontium, and barium from methanolic solution. The local
environment of water and the strength of hydrogen bonding in
these hydrated modifications were probed with Fourier
transform IR spectroscopy, 1H NMR spectroscopy, and
heteronuclear correlation experiments. Temperature-depend-
ent spin−lattice (T1) relaxation experiments provided
information about the water motion in the amorphous
materials. The Pearson hardness of the respective divalent
metal cation predominantly determines the strength of the
internal hydrogen-bonding network. Amorphous magnesium
carbonate deviates from the remaining carbonates, as it
contains additional hydroxide ions, which act as strong hydrogen-bond acceptors. Amorphous calcium carbonate exhibits the
weakest hydrogen bonds of all alkaline earth carbonates. Our study provides a coherent picture of the hydrogen bonding
situation in these transient species and thereby contributes to a deeper understanding of the crystallization process of
carbonates.
carbonate polymorphs.18,19 Several synthetic routes to ACC
INTRODUCTION
■
were developed, ranging from the addition of polymers20 and
Calcium carbonate is the most widespread biomineral on
proteins21 to the confinement in silica shells22 or freeze-
earth.1,2 While biominerals that are purely based on other
drying.23 Numerous studies appeared on the structure and
alkaline earth carbonates have not been reported so far,
composition of ACC, which exhibits short-range but no long-
magnesium andto a lower degreestrontium is incorpo-
rated into calcium carbonate biominerals and interferes
significantly with the crystallization process. Magnesian calcite
was found in the cuticles of crustaceans,3 in sea urchins,4,5 and
echinoderms.6 Certain coccolithophores show a substantial
bioaccumulation of strontium,7−9 which is high compared with
values of precipitated “inorganic” calcite.10 The most
prominent function of magnesium in calcium carbonate-
based biominerals is the stabilization of amorphous calcium
carbonate (ACC),11 which serves as primary building material
for all sorts of hard tissue.12,13 A change of the mechanical
properties by incorporation of magnesium in biominerals has
been discussed as well.14
The crystallization of calcium carbonate via amorphous
intermediates has been studied intensely due to the importance
of ACC for biomineralization. Its formation enables the
accumulation and temporal storage of calcium and carbonate
ions by organisms for subsequent crystallization of complex
biomineral structures. By tailoring the physical and chemical
conditions, polymorph control over the crystallization products
from ACC could be achieved.15 ACC is typically the first phase
to precipitate from solution.16 Since it is metastable,17 it
transforms rapidly to one of the more stable crystalline calcium
range order.24,25 Tobler et al.26 found gradual changes in local
order of ACC on varying the pH during the synthesis, and a
“proto-structuring” of the amorphous phase with regard to the
respective crystalline calcium carbonate polymorphs has been
proposed.27,28
The poorly defined structure of ACC allows the
incorporation of substantial amounts of foreign ions, which is
documented for strontium29 and magnesium.30−33 Therefore,
amorphous magnesium carbonate (AMC) is mainly discussed
in the context of a solid solution in ACC.34 AMC is known to
play a role during the crystallization of magnesium carbonates,
but only one study appeared on the atomic structure of pure
hydrated amorphous magnesium carbonate.35 More often,
amorphous magnesium carbonates are discussed as product of
the thermal decomposition of crystalline magnesium carbo-
nates, for example, nesquehonite.36 Both AMC and ACC are
tested for medical applications. While pH-responsive drug
carrier systems are being developed based on ACC,37
amorphous magnesium carbonate with mesoporous structure38
Received: July 31, 2018
© XXXX American Chemical Society
A
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Figure 1. (a) PXRD of the amorphous carbonates of magnesium (AMC: black), calcium (ACC: blue), strontium (ASC: red), and the sample
background, consisting of two poly(vinyl acetate) films and perfluorinated ether (gray). The amorphous structure is indicated by the absence of X-
ray reflections. (b) TGA of the amorphous carbonates. All samples display a mass loss between room temperature and 250 °C, which is attributed
to a release of structural water. Approximate compositions of the respective samples are calculated based on the mass loss. For AMC, the hydroxide
1
and hydrogen carbonate ions, which were found in H NMR (vide infra), were considered.
has been employed to improve the uptake of poorly soluble
drugs.39 Moreover, hydrated amorphous magnesium carbonate
has been tested as carbon dioxide storage material.40 Even
fewer reports exist on amorphous strontium carbonate (ASC),
which possibly is due to its extremely fast transformation to
crystalline strontiante.41
mobility of proteins,62 and the molecular mobility of water
adsorbed on polymers.63
We probed the local water environment and the strength of
the hydrogen-bonding network in amorphous alkaline earth
carbonates. To this end, we synthesized hydrated amorphous
carbonates of magnesium, calcium, strontium, and barium. By
careful drying, yet without completely removing the structural
water, we stabilized these transient species for sufficiently long
periods of time. Amorphous barium carbonate still crystallized
very rapidly, which prevented prolonged experiments. We
probed the strength of the hydrogen bonds by FTIR
spectroscopy. 1H and 1H−13C heteronuclear correlation
(HETCOR) magic-angle spinning nuclear magnetic resonance
(MAS NMR) techniques were used to identify different proton
species. Also, we probed the carbonate environment through
13C{1H} cross-polarization (CP) MAS NMR. Measuring the
1H spin−lattice relaxation (T1) indicated a deviating mobility
of the water molecules in the respective carbonates, and
especially experiments at variable temperature (VT) allowed to
determine the respective activation energy for the water
motion. Thus, this study provides a complete picture of the
structural water in amorphous intermediate species and
thereby expands our understanding of the crystallization
process of the ubiquitous and eminently important mineral
class of carbonates.
Amorphous carbonates are generally hydrated materials.
While most reports agree that ACC prepared from aqueous
solution contains structural water, the amount of water
depends on the synthesis conditions and the drying
processes.42,43 Studies on amorphous zinc phosphate revealed
that the water network stabilizes the amorphous modification
with regard to crystallization.44 The water molecules form
complex networks, connected through hydrogen bridges. In
amorphous carbonates, water molecules or carbonate groups
serve as hydrogen-bond acceptors. The importance of
hydrogen bonds for the structure,45 stability,46 molecular
conformation,47 and dynamics48,49 in a vast number of
chemical systems cannot be underestimated. The dissociation
energies of hydrogen bonds cover more than 2 orders of
magnitude (0.2−40 kcal/mol).50
1
Fourier transform infrared (FTIR) spectroscopy and H
nuclear magnetic resonance (1H NMR) spectroscopy are
powerful tools to probe the strength of hydrogen bonding. In
FTIR, a broadening of the band corresponding to the O−H
stretching vibration indicates the presence of hydrogen
bonds.51 The O−H bond is weakened, and the band position
shifts to lower wavenumbers, when a hydrogen acceptor is in
its vicinity.52,53 Moreover, the interaction with a hydrogen-
bond acceptor decreases the electron density at the hydrogen
atom,54 resulting in a downfield shift in the 1H NMR
spectrum.55 Solid-state NMR is a particularly powerful
technique to probe the local environment, especially in
amorphous materials. ACC and magnesium-bearing ACC
were studied using 43Ca NMR56 and 25Mg NMR spectrosco-
py.57 A solid-state NMR spectroscopy study of ACC
employing 1H and 13C was performed by Epple and co-
workers.24 The mobility of the nuclei can be determined from
the correlation time τc, which is related to the spin−lattice
relaxation time (T1).58,59 This allowed to determine the
molecular mobility of amorphous organic compounds,60,61 the
RESULTS AND DISCUSSION
■
Composition and Stability of Amorphous Alkaline
Earth Carbonates. Amorphous carbonates were precipitated
without additional stabilization by additives or ligands from
methanolic solution according to a recent study.64 A drying
step at 70 °C proved to be necessary to prevent fast
crystallization, especially of ASC. The noncrystallinity of the
products was confirmed by the absence of reflections in the
powder X-ray diffractograms (PXRD; Figure 1a). To minimize
the background, the samples were prepared on poly(vinyl
acetate) films with perfluorinated ether (Figure 1a, gray line).
The amorphous carbonates differ strongly in their stability with
regard to crystallization (Figure S1, Supporting Information).
In crystallization experiments with 20 vol % water in
acetonitrile, AMC crystallized slowly within a day. ACC
required several hours, until crystallization was complete. ASC
B
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Figure 2. (a) FTIR spectra display the characteristic carbonate vibrational modes ν1−ν4. The O−H stretching vibration around 3300 cm−1
indicates that all samples contain structural water. (b) Normalized FTIR spectra of the O−H vibration of the amorphous carbonates AMC, ACC,
and ASC. Data are modeled by multimodal Gaussian functions (difference lines shown in gray). Mean wavenumbers indicate an increase of the
hydrogen bonding strength when moving from ACC via AMC to ASC.
crystallized immediately upon contact with water. The
amorphous modification of barium carbonate (ABC, vide
infra) transformed to witherite already in air.
Figure 2b shows FTIR spectra of the amorphous carbonates
in the region of the O−H stretching vibration. All samples
exhibit a broad band from ∼3700 to 2300 cm−1, which
indicates that the water molecules are involved in hydrogen
bonds in the amorphous compounds.50 Increasing strength of
hydrogen bonding shifts the water band to smaller wave-
numbers due to a weakening of the O−H bond of the water
molecule.52,53
The amount of water in the amorphous carbonates was
determined by thermogravimetric analysis (TGA; Figure 1b).
All samples showed a gradual mass loss between 50 and 250
°C (AMC: 15%, ACC: 7%, ASC: 4%). Because of the incipient
decomposition of AMC to MgO around 300 °C,36,65,66 we did
not observe a constant mass after the loss of water. On the
basis of the mass loss, the approximate composition of the
amorphous carbonates was calculated. For AMC, we also
considered the evidence of hydroxide and hydrogen carbonate
We evaluated the respective bands by multimodal Gaussian
fits. Three Gaussian functions were needed for modeling the
data reasonably.51 A fourth function, centered around 3550
cm−1, was employed for AMC to model the hump of the band
at high wavenumbers. We attribute this hump to the presence
of hydroxide groups in AMC. The increased electron density
strengthens the O−H bond and shifts the signal to higher
wavenumbers. Fitting the bands with Gaussian functions allows
to extract a mean wavenumber for νO−H and allows to compare
the average hydrogen-bonding strength in all three com-
pounds. Water in ACC forms the weakest hydrogen bonds
(3242 cm−1), while stronger bonds occur in ASC (3159 cm−1).
AMC represents an intermediate case (3176 cm−1). The mean
wavenumber of νO−H = 3108 cm−1 shows the strongest
hydrogen bonding for all amorphous carbonates to occur in
ABC (Figure S3, Supporting Information). No other experi-
ments besides FTIR could be performed because of its
instability. According to the empirical formula found by
Libowitzky69 the wavenumber of the O−H stretch is related to
the O−O distance in a hydrogen bond O−H···O. The mean
O−O distance for the amorphous carbonates is expected in the
range from 2.67 Å (for ABC) to 2.72 Å (for ACC).
Aside from AMC, the strength of hydrogen bonding
increases with the ionic radius of the alkaline earth metals.
We suggest that the hydrogen-bond acceptor strength of the
carbonate group is mainly determined by its coordination to
the metal cation. Large (Pearson soft)70 cations like Sr2+ and
Ba2+ coordinate only weakly to the carbonate group. Thus, the
electron density at the carbonate anion is higher, which leads
to stronger hydrogen bonds. The opposite effect is found for
small (Pearson hard)70 cations, which add a significant
covalent character to the ionic metal oxygen bond. As a result,
electron density is drawn away from the carbonate group, and
the hydrogen acceptor capacity decreases. The decrease of
metal oxygen interaction is also indicated by the decreasing
1
groups, which were observed in H NMR and HETCOR
experiments (vide infra). This resulted in a composition of
Mg(CO3)0.8(HCO3)0.1(OH)0.3 × 0.8 H2O. The water content
of the other samples, namely, CaCO3 × 0.4 H2O and SrCO3 ×
0.3 H2O, was significantly lower. The amount of water
correlates with the Pearson hardness of the respective
cations.67 The small and Pearson-hard Mg2+ cation binds
water ligands more strongly than the larger, softer Sr2+ cation.
The observed water content is lower than in previous
reports,22,43 which is due to the drying step at 70 °C. Reeder
and co-workers found a comparable water content for ACC,
pretreated at 87 °C.42 Yet, the similarity of the FTIR band
shape before and after drying suggests that the water
environment is not significantly altered by the drying process
(Figure S2, Supporting Information).
Strength of Hydrogen Bonding. Figure 2a shows the
FTIR spectra of AMC, ACC, and ASC. The hydration is
confirmed by the broad band around 3300 cm−1, which
corresponds to the O−H stretching vibration of structural
water. The three samples show the characteristic vibrational
modes of carbonates68 (broadening due to the amorphous
structure), that is, the symmetric stretching (ν1), the out-of-
plane deformation (ν2), and the asymmetric stretching (ν3)
modes. The broadening of the in-plane-deformation mode (ν4)
around 700 cm−1 increases from strontium, via calcium, to
magnesium carbonate; that is, the bands cannot be resolved
properly. The respective band positions in AMC (ν3 = 1406
cm−1, ν1 = 1090 cm−1, ν2 = 854 cm−1), ACC (ν3 = 1393 cm−1,
ν1 = 1074 cm−1, ν2 = 858 cm−1), and ASC (ν3 = 1382 cm−1, ν1
= 1065 cm−1, ν2 = 862 cm−1) agree with reported values38,64
and confirm the amorphous structure.
C
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Figure 3. (a) 13C{1H} CP MAS NMR of the amorphous carbonates AMC, ACC, and ASC. The carbonate signals exhibit a broad fwhm (AMC:
440 Hz, ACC: 385 Hz, ASC, 335 Hz), which accounts for a variety of different local environments in the amorphous state. (b) Buildup curve of the
carbonate signal at different contact times for 1H−13C CP (fits shown). The efficiency of the CP for the respective carbonates correlates with the
water content, as determined by TGA.
1
energy of the symmetric stretch ν1 of the carbonate ion when
moving from Mg2+ to Ba2+ (1090 to 1058 cm−1). Accordingly,
alkaline earth hydroxides show a decreasing vibration
frequency νO−H from Mg2+ to Sr2+. The stronger metal oxygen
interaction weakens the hydrogen acceptor strength of the
hydroxide ion (Figure S4, Supporting Information).
The stronger hydrogen bonds in AMC compared to ACC
appear to contradict this model. AMC is, however, not a pure
carbonate, and hydroxide anions in the structure (vide infra)
serve as strong hydrogen bond acceptors.71 They contribute
significantly to the strength of the internal hydrogen-bond
network.
of the coupled pairs and the separations between H and 13C
dipolar-coupled spins. A distribution in the correlation times τc
related with the water mobility may not be excluded as well.
A higher water content results in more protons to cross-
polarize. This leads to a stronger 13C signal (provided that the
water molecules do not form large phase-separated domains
within the inorganic structure). The water content increases in
the order ASC < ACC < AMC (as determined by TGA, vide
supra). Additionally, there are HCO3 and OH− groups
−
present in the structure for AMC. Thus, the CP buildup curve
for the AMC is steeper compared to ASC and ACC, whereas
the buildup curves for the latter two compounds are
comparable (Figure 3b). Neglecting the relaxation of the 13C
and 1H rotating frame, the buildup curves were fitted using eq
1:72
Local Carbonate and Water Environment. Chemical
Environment of the Carbonate Group. The chemical
environment of the carbonate group was probed with
13C{1H} CP MAS NMR experiments with variable contact
times. The carbonyl region of the 13C spectra of the
amorphous carbonates is shown in Figure 3a (samples
contained 13C in natural abundance). The full width at half-
maximum (fwhm) of the NMR signals provides information
about the homogeneity of the local field. Typically, amorphous
compounds exhibit broadened carbonyl resonances due to the
increased heterogeneity of the chemical environment.24 The
13C signal of AMC is characterized by a chemical shift of 165.8
ppm with a fwhm of 435 Hz. The resonance of ACC is
detected at 168.0 ppm with a fwhm of 385 Hz, and ASC
exhibits the sharpest signal with a fwhm of 335 Hz at 167.5
ppm. The large fwhm in AMC reflects the higher local disorder
due to the presence of hydroxide and hydrogen carbonate
groups and a higher amount of water.
cp/T ))
I(tcp) = I0·(1 − e(−(t
)
IS
(1)
where I(tcp) is the absolute intensitywhich is correlated to
the absolute water content in the amorphous carbonatesfor
the respective cross-polarization (or contact) time tcp. TIS is the
cross-polarization time constant for the coupled 1H−13C pairs.
For ASC and ACC, we found comparable time constants of
2.48(47) ms and 2.52(19) ms. For AMC, the cross-
polarization time constant of 0.80(17) ms is much shorter.
Water Environment. The water environment of the
1
amorphous carbonates was analyzed by H nuclear magnetic
resonance (1H NMR) and HETCOR experiments (Figure 4).
1
The H NMR spectra were approximated with pseudo-Voigt
profiles.73 All samples contain negligible (compared to the
water signal) amounts of residual solvent (from the synthesis
with shifts of ∼1 ppm).
Figure 3b shows the development of the 13C intensities with
increasing contact time tc in cross-polarization NMR experi-
ments. The buildup curves provide information about the
efficiency of the cross-polarization, which depends on the
AMC. The 1H NMR spectrum of AMC exhibits three
distinct proton signals detected in both experiments (Figure
4a,b). The most intense resonance at 5.18 ppm with a fwhm of
2890 Hz is associated with structural water. The signal at 0.27
ppm (fwhm = 1140 Hz) is compatible with hydroxide ions,
whose higher electron density causes a highfield shift.74 The
presence of hydroxide corresponds to the multitude of basic
magnesium carbonates, for example, dypingite
(Mg5(CO3)4(OH)2 × 5 H2O)75 and hydromagnesite
(Mg5(CO3)4(OH)2 × 4 H2O).76 The signal at 13.20 ppm
1
strength of the H−13C dipole−dipole couplings: (i) on the
1
distance between the dipolar-coupled H and 13C spin pairs,
(ii) on the mobility of the molecule in general, or (iii) the local
mobility of the functional groups containing the spin pair. For
an isolated pair with dipolar coupling the CP buildup curve is
oscillatory. For the alkaline earth carbonates in this work, a
smooth exponential increase of the signal intensity was
observed, which is due to a distribution of the orientations
1
(fwhm = 2160 Hz) appears as a shoulder in the H NMR
D
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Figure 4. 1H NMR and respective HETCOR spectra of the amorphous alkaline earth carbonates. The spectra are fitted with pseudo-Voigt profiles;
difference lines are shown in gray. AMC (a, b) exhibits three different proton signals. The most intense signal at 5.19 ppm originates from water,
while we attribute the weak signal at 0.25 ppm to stronger shielded hydroxide groups. The coupling to a higher field shifted carbonate and the
downfield shift of the signal at 13.20 ppm hint at the acidic proton of a hydrogen carbonate group. ACC (c, d) and ASC (e, f) show only one signal
at 4.46 ppm, respectively, at 4.92 ppm. The ACC water signal has the smallest chemical shift, which corresponds to weaker hydrogen bonding.
1
spectrum and is much better resolved in the HETCOR
spectrum. It is clearly related with the presence of acidic
protons in AMC. The coupling with a carbonate group shifted
to higher field (154.8 ppm) in the HETCOR experiment
suggests the presence of hydrogen carbonate groups.24 For
nesquehonite (MgCO3 × 3 H2O), Moore and co-workers
proposed a description as hydrogen carbonate (Mg(OH)-
(HCO3) × 3 H2O) to be more accurate.77 We assume to have
a comparable situation in AMC, even though the intensity
ratios of the three signals (80% to 16% to 4%) show that
hydrogen carbonate can only be a minority species.
ACC. The H NMR spectrum of ACC exhibits only one
proton signal at 4.46 ppm with an fwhm of 1170 Hz (Figure
4c,d). This chemical shift is associated with the weak hydrogen
bonds that we already observed in the FTIR spectra (Figure
2b). In comparison, for monohydrocalcite (Figure S5,
Supporting Information), the hydrated crystalline modification
1
of calcium carbonate, both FTIR and H NMR reveal better
order and stronger hydrogen bonding than in ACC.
Monohydrocalcite exhibits hydrogen bonds with a nearly
optimal O−H−O angle of 180° (Figure S5a, inset). In the
amorphous state, it is unlikely that this angle occurs frequently
E
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Figure 5. (a) Temperature dependence of the 1H chemical shift of the water signal in the amorphous carbonates. The signal in ACC is less sensitive
to temperature, as it is primarily less affected by hydrogen bonding. (b) Temperature dependence of the spin−lattice relaxation time (T1) allows
determination of the activation energy for the water motion (values given in the figure). For AMC and ASC, these values are in a comparable range
of 6 kJ/mol. The activation energy for the water motion in ACC is significantly lower due to the weaker hydrogen bonding.
(assuming a distribution of angles), which attributes to a
weakening of the hydrogen bonding.
signal, while the effect of hydrogen bonding on the chemical
shift of the water signal in ACC is less pronounced.
VT inversion recovery spectra (Figure S6, Supporting
Information) were recorded to determine the spin−lattice
relaxation time T1, which correlates with the molecular spatial
structure and dynamics and is related to the strength of the
hydrogen bonding. The spectra were evaluated by a linear fit
using eq 2.81
ASC. ASC exhibits one proton signal at 4.92 ppm with an
fwhm of 810 Hz. This chemical shift corresponds to hydrogen
bonding comparable in strength with that in AMC, in
agreement with the FTIR results. ASC has the sharpest
water signal of the amorphous carbonates (AMC, ACC, and
ASC), which may be associated with better local organization
or higher water mobility.
1
i
j
y
z
−t
T1
i
j
y
A comparison of the results of H and HETCOR NMR
z
j
z
I(t) = I · 1 − expj z + I
j
z
∞
0
j
z
{
experiments reveals that the water resonance in the amorphous
carbonates changes from 4.46 ppm (ACC) to 5.18 ppm
(AMC). The chemical shift of the protons is related to the
local electron density, which is influenced by hydrogen
bonding. Therefore, strong hydrogen bonds lead to a
deshielding of the protons.50,54,78 We assume that the
hydroxide ions in AMC act as strong hydrogen acceptors.
Additionally, an increase of the signal broadening when going
from ASC, via ACC, to AMC is clearly seen. The same trend
appears in the 13C NMR (Figure 3a) and the FTIR spectra
(Figure 2a).
(2)
k
k
{
We measured T1 at 20, 40, and 60 °C for all amorphous
carbonates (as well as for monohydrocalcite as crystalline
reference). The results are displayed in Figure 5b. The T1
relaxation times at ambient temperature were T1 = 953 ms
(AMC), T1 = 798 ms (ACC), and T1 = 441 ms (ASC). All T1
values decrease with increasing temperature. The spin−lattice
relaxation time T1 is described by the Bloembergen−Purcell−
Pound model,58 which attributes for the tumbling motion of
molecules on the local magnetic field disturbance (eq 3):
In essence, the variation in the local structure in the
amorphous alkaline earth carbonates is highest in AMC and
decreases for the heavier alkaline earth homologues. In parallel,
the stability of the amorphous state with regard to
crystallization decreases (e.g., difficulty of stabilizing amor-
phous barium carbonate), which further suggests that the
degree of disorder is highest in AMC.
Ä
É
Å
Ñ
Å
Å
Ñ
Ñ
τ
4τ
1
T1
c
c
Å
Å
Å
Å
Å
Å
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
= C·
+
2
1 + (ωτ )
1 + (2ωτ )2
(3)
c
c
Å
Ç
Ñ
Ö
This tumbling motion is characterized by the rotation
correlation time τc. C is a constant, which is independent of the
temperature and the frequency. If the tumbling rate is
comparable to the Larmor frequency ω, relaxation is most
effective. Faster or slower tumbling increase the relaxation
time. The tumbling rate in solids is much smaller than the
Larmor frequency. Thus, a faster motion, which can be caused
by increased temperature, should lead to a decrease of the
relaxation time T1. The temperature dependence of the
tumbling motion can be approximated by an Arrhenius
approach with the activation energy Ea (eq 4).82
Motional Behavior of Water. The motional behavior of
water in the amorphous carbonates was derived from variable
1
temperature (VT) H NMR spectra (20, 40, and 60 °C). The
water signal shifts to higher field for all amorphous compounds
with increasing temperature. Such a shift is usually attributed
to a weakening/breaking of hydrogen bonds due to the
increase of interatomic distances.79,80
The temperature coefficients Δδ/ΔT for the respective
carbonates were obtained from a linear fit of the chemical shift
versus temperature (Figure 5a). Similar values were found for
AMC (0.0170 ppm/K) and ASC (0.0168 ppm/K), while the
temperature coefficient for ACC is much lower (0.0093 ppm/
K). This underlines the weakness of the hydrogen bonds in
ACC. Weakening/breaking of the strong hydrogen bonds in
AMC and ASC results in a distinct high field shift of the water
E
kT
i
y
a
j
j
j
z
z
z
τ = τ ·exp
c
c0
(4)
k
{
Inserting eq 4 in eq 3 yields the dependence of the relaxation
time T1 from the temperature T. Assuming ωτc ≫ 1 in solids,
we obtain
F
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−E
kT
cite) suggests that the hydrogen bonds are not ideally oriented
in the amorphous solid, which explains their softening. The
minimum hydrogen bonding strength in ACC corresponds to
the facile dehydration of biogenic ACC prior to crystalliza-
tion.84−86 This study provides a complete picture of hydrogen
bonding and the water environment in amorphous alkaline
earth carbonates. Structural water is one of the most profound
differences between amorphous intermediates and the
thermodynamic crystalline modifications in the carbonate
system, which are predominantly anhydrous (some crystalline
hydrated modifications of basic magnesium carbonate exist).
With this detailed characterization of their hydration, we
contribute new aspects to illuminate the crystallization process
of carbonates in general.
i
y
1
T1
a
j
j
j
z
z
z
∝ exp
(5)
k
{
The activation energy Ea for the tumbling movement of the
water molecules (Figure 5b) was obtained by fitting our data
using eq 5. AMC and ASC show comparable activation
energies of 6.4 1.2 and 5.6 1.6 kJ/mol. Ea is significantly
lower for ACC (1.5 0.3 kJ/mol). The activation energies for
the water movement correspond to the strength of the
hydrogen bonding that was deduced from FTIR and 1H NMR.
The activation energy for crystalline monohydrocalcite (Ea =
5.4
0.1 kJ/mol) is higher than in ACC (Figure S5d,
Supporting Information). We attribute this difference to the
strong directed hydrogen bonding in monohydrocalcite. This
bonding situation also potentially causes the high absolute
value for the T1 relaxation of 1376 ms at ambient conditions in
monohydrocalcite. Shorter relaxation times in amorphous
compounds compared to the crystalline modification were also
reported for magnesium phosphates.83
EXPERIMENTAL SECTION
■
Materials. CaCl2 (98%, Merck; 99%, Sigma-Aldrich), SrCl2
(99.5%, Alfa Aesar), MgCl2 × 6 H2O (99%, p.a., Carl Roth), BaCl2
(99.5%, Merck), Cs2CO3 (99%, Sigma-Aldrich), K2CO3 (99%,
Merck), Ca(OH)2 (96%, puriss. p.a., Sigma-Aldrich), Sr(OH)2 × 8
H2O (99%, Alfa Aesar), Mg(OH)2 (95%, Sigma-Aldrich), acetone
CONCLUSION
■
̈
(99.5%, Riedel-de Haen), methanol (high-performance liquid
We examined the water content, local environment, and
motional behavior in hydrated amorphous carbonates of
magnesium, calcium, strontium, and barium with TGA,
FTIR, and NMR techniques to gain information about the
hydrogen bonding. The low stability of amorphous barium
carbonate allowed only fast FTIR experiments. The Pearson
hardness of the coordinating metal cations mainly determines
the hydrogen acceptor strength of the carbonate group. At first
glance, AMC seems to contradict this trend. The solution is
revealed in 1H NMR and HETCOR experiments, which
indicate that AMC is not a pure carbonate but contains small
amounts of hydrogen carbonate ions and additional proton
species, which were identified as hydroxide ions. These
hydroxide ions act as strong hydrogen acceptors and stabilize
the water network in AMC. There is no experimental evidence
for hydroxide ions in any other amorphous alkaline earth
carbonate.
chromatography (HPLC) grade, Fisher Chemicals), acetonitrile
(HPLC gradient grade, Fisher Chemicals), milliQ deionized water.
Synthesis of Amorphous Carbonates. CaCl2 (2 mmol) was
dissolved in 20 mL of methanol. Subsequently, Cs2CO3 (2 mmol) was
added, and the solution was sonicated for 1 min. After dissolution of
the Cs2CO3, a colorless solid (ACC) precipitated, which was
separated by centrifugation and washed twice with 10 mL of
methanol to remove residual cesium chloride. Subsequently, the solid
was dispersed in 20 mL of acetone under sonication and centrifuged
again. The washing step with acetone was repeated. Finally, the solid
was dried at 70 °C for several hours. The preparation of AMC and
ASC proceeded analogously with the respective metal chlorides as
reactants. The drying step at elevated temperatures was necessary,
especially for ASC, as it crystallized during slow drying even in vacuo.
Additionally, adsorbed surface water was removed, which increased
the stability of the amorphous samples. To prevent spontaneous
crystallization, the synthesis of amorphous barium carbonate required
drying the product in a vacuum oven at 40 °C.
The IR and NMR spectroscopic studies (Table 1) allow a
classification of the hydrogen bonding strength in the
Synthesis of Monohydrocalcite. According to the optimized
calcium magnesium ratio for the formation of monohydrocalcite,87
CaCl2 (2 mmol) and MgCl2 × 6 H2O (0.5 mmol) were dissolved in
30 mL of deionized water. In the next step, K2CO3 (3.5 mmol) was
dissolved in 15 mL of deionized water, and the solutions were
combined, whereupon a colorless solid precipitated. The reaction
solution was stirred for 8 h. Afterward, the precipitate was separated
by centrifugation and washed with methanol. The product,
monohydrocalcite, was dried in vacuo.
Crystallization Experiments. The respective amorphous carbo-
nates were prepared as described above. Instead of dispersing them in
acetone and subsequently drying them for several hours, 20 mL of
acetonitrile was added to yield 0.1 M dispersions of the amorphous
carbonates. For a crystallization experiment, the respective carbonate
dispersion was added to a water/acetone mixture to yield a final water
concentration of 20 vol % (5 mL of the carbonate dispersion to a
mixture of 11 mL of acetone and 4 mL of water). Samples were taken
by transferring 250 μL of the reaction solution to a microcentrifuge
tube and quenching further crystallization by the addition of 1.5 mL
of acetone. The sample was centrifuged for 1 min at 14 000 rpm.
Then, the supernatant was decanted. The sample was washed again
with 1.5 mL of acetone and dried afterward in vacuo. Through FTIR,
the progress of crystallization was monitored (Figure S1, Supporting
Information).
Table 1. Hydrogen Bonding in Amorphous Alkaline Earth
a
Carbonates
νO−H
,
fwhm,
Hz
T1,
ms
sample
AMC
cm−1
3176
δ(1H), ppm
Ea, kJ mol−1
5.18 (H2O)
0.27 (OH−)
2890
1140
2160
1170
810
953
6.4 1.2
−
13.20 (HCO3
4.46 (H2O)
4.92 (H2O)
)
ACC
ASC
3242
3159
798
441
1.5 0.3
5.6 1.6
a
Mean wavenumber of the O−H stretching vibration νO−H,
1H
chemical shift δ, the respective line width, T1 relaxation time, and
activation energy for the water motion Ea are given.
amorphous carbonates: AMC ≈ ASC > ACC. The FTIR
spectrum of amorphous barium carbonate suggests that
hydrogen bonding is even stronger here, but the short lifetime
prevented a more detailed analysis. The activation energy of
the water motion Ea, determined by VT T1 measurements,
scales with the strength of hydrogen bonding and has a
minimum for amorphous calcium carbonate. A comparison
with crystalline hydrated calcium carbonate (monohydrocal-
Characterization. X-ray Powder Diffraction. X-ray diffracto-
grams were recorded with a STOE Stadi P equipped with a Mythen
1k detector using monochromatized Mo Kα radiation. The sample
was attached to poly(vinyl acetate) films with perfluoroether
G
DOI: 10.1021/acs.inorgchem.8b02170
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(Fomblin Y, Aldrich). The sample was measured in transmission in
0.015° steps (continuous scan, 150 s/deg) covering a 2θ range from
1.5° to 43°. Crystalline phases were identified according to the PDF-2
database88 using Bruker AXS EVA.89
FTIR Spectroscopy. ATR-FTIR spectroscopy was performed using
a Nicolet iS10 spectrometer manufactured by Thermo Scientific. The
spectra were recorded in a frequency range from 650 to 4000 cm−1
with a resolution of 1.4 cm−1 per data point.
ACKNOWLEDGMENTS
■
S.L. is recipient of a Carl-Zeiss fellowship and a collegiate of
the MAINZ Graduate School of Excellence (GSC 266). We
thank Prof. H. Frey for access to the IR spectrometer and Prof.
̈
A. Moller for access to the diffractometer. Moreover, we thank
S. Hanif for help with the TGA.
Solid-State NMR Spectroscopy. All solid-state NMR spectra were
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02170.
Stability of amorphous carbonates with regard to
crystallization on water contact; FTIR spectra of ACC
before (black, straight) and after drying at 70 °C; PXRD
and FTIR of amorphous barium carbonate; FTIR
spectra of alkaline earth hydroxides; PXRD, FTIR, H
NMR, and VT T1 relaxation of monohydrocalcite;
1
determination of T1 for ACC at 20 °C (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: mondeshk@uni-mainz.de. (M.M.)
*E-mail: tremel@uni-mainz.de. (W.T.)
ORCID
Wolfgang Tremel: 0000-0002-4536-994X
Author Contributions
S.L. performed the experiments; S.L., M.M., and W.T.
conceived the experiments; M.M. contributed to methods;
S.L. wrote the manuscript with contributions of M.M. and
W.T.
Notes
The authors declare no competing financial interest.
H
DOI: 10.1021/acs.inorgchem.8b02170
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DOI: 10.1021/acs.inorgchem.8b02170
Inorg. Chem. XXXX, XXX, XXX−XXX