Structural phase transitions induced by molecular motions within an (anilinium)(l-tartrate) ionic molecular crystal

Article abstract of DOI:10.1039/c2ce25945a

Ionic molecular crystals of (anilinium⁺)(l-tartrate^-) were prepared in C₂H₅OH-H₂O via Bronsted acid-base reaction between aniline and l-tartaric acid. Strong intermolecular O-H...O^- hydrogen-bonding interactions between the terminal -COOH and -COO^- groups of the l-tartrate^- monoanion (l-HTart^-) are essential in forming the one-dimensional hydrogen-bonding (l-HTart^-)_∞ chains. Furthermore, N-H⁺...O^- hydrogen-bonding interactions between l-HTart^- and the ammonium moiety of anilinium (Ani⁺) form a π-stacking arrangement of the Ani⁺ cations that are orthogonal to the (l-HTart^-)_∞ chains. DSC analysis revealed a two-step behavior with reversible phase transitions at ～220 and ～270 K, in which three types of crystal phases (I, T < 220 K; II, 220 < T < 270 K; III, 270 K < T) were identified. Phase I possesses the same acentric space group (P1), while its unit cell volume is half of that of phases II and III, in which the π-stacking periodicity of the Ani⁺ cations of phases II and III are double that of phase I. Although the conformation of the hydroxy (-OH) groups of l-HTart^- of phase II (T = 250 K) was identical to that of phase I (T = 100 K), the rotation of the C-OH bond (activation energy of ～70 kJ mol^-1) was observed for phase III. The thermal fluctuation of the polar -OH group in phase III was dominated by a single minimum-type potential energy curve, as evaluated from the calculations. The temperature- and frequency-dependent dielectric constants suggested a low-frequency thermal fluctuation at temperatures above ～250 K. This journal is

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PAPER
Structural phase transitions induced by molecular motions within an
(anilinium)(L-tartrate) ionic molecular crystal{
Yuuya Yoshii,a Norihisa Hoshino,ab Takayoshi Nakamurac and Tomoyuki Akutagawa*ab
Received 13th June 2012, Accepted 23rd August 2012
DOI: 10.1039/c2ce25945a
Ionic molecular crystals of (anilinium+)(L-tartrate2) were prepared in C2H5OH–H2O via Brønsted
2
…
O hydrogen-
acid–base reaction between aniline and L-tartaric acid. Strong intermolecular O–H
bonding interactions between the terminal –COOH and –COO2 groups of the L-tartrate2 monoanion
(L-HTart2) are essential in forming the one-dimensional hydrogen-bonding (L-HTart2)‘ chains.
2
Furthermore, N–H+
O
hydrogen-bonding interactions between L-HTart2 and the ammonium
…
moiety of anilinium (Ani+) form a p-stacking arrangement of the Ani+ cations that are orthogonal to
the (L-HTart2)‘ chains. DSC analysis revealed a two-step behavior with reversible phase transitions
at y220 and y270 K, in which three types of crystal phases (I, T , 220 K; II, 220 , T , 270 K; III,
270 K , T) were identified. Phase I possesses the same acentric space group (P1), while its unit cell
volume is half of that of phases II and III, in which the p-stacking periodicity of the Ani+ cations of
phases II and III are double that of phase I. Although the conformation of the hydroxy (–OH) groups
of L-HTart2 of phase II (T = 250 K) was identical to that of phase I (T = 100 K), the rotation of the
C–OH bond (activation energy of y70 kJ mol21) was observed for phase III. The thermal fluctuation
of the polar –OH group in phase III was dominated by a single minimum-type potential energy curve,
as evaluated from the calculations. The temperature- and frequency-dependent dielectric constants
suggested a low-frequency thermal fluctuation at temperatures above y250 K.
chain of a mono-protonated 1,4-diazabicyclo[2.2.2]octane and
…
Introduction
O–H O hydrogen-bonding network of croconic acid crystals
result in a ferroelectric–paraelectric phase transition.6,7 On the
contrary, binary hydrogen-bonding molecular complexes have
been obtained by mixing a Brønsted acid (HA) with a base (B), in
which the hydrogen-bonding properties were examined in terms
of the acid dissociation constants pKHA and pKHB for the HA
and HB+ species, respectively.8 The ionic ground state of
(HB+)(A2) was obtained under the condition of pKHA > pKHB
via proton transfer from HA to B, whereas the neutral ground
Intermolecular hydrogen-bonding interactions play an impor-
tant role in realizing high-order hierarchical assembly structures
in proteins and complementary base pairs in DNA,1–3 where
various hydrogen-bonding interactions proved to be essential for
the construction of complicated molecular assembly structures.
Because the energies of such hydrogen-bonding interactions
(y50 kJ mol21), which associate and dissociate between
molecules, are moderate in magnitude, selective molecular
interactions are achievable for constructing specific molecular
assembly structures.4 In addition, the hydrogen-bonding inter-
action plays an important role for structural phase transitions
through intermolecular proton transfer and modulation of
hydrogen-bonding motifs.5 For example, the order–disorder
state of (B)(HA) was obtained under the condition of pKHA
,
pKHB in the absence of proton transfer. In the cases where pKHA
= pKHB, both ionic (HB+)(A2) and neutral (B)(HA) states
contribute to form the molecular complex; very strong hydrogen-
bonding interactions were identified by X-ray crystal structural
analysis, neutron diffraction, and vibration spectra.9,10 The
potential energy curves of the hydrogen-bonding interactions are
dependent on the strength of the hydrogen bonds, in which the
potential energy barrier is gradually lowered with shorter
hydrogen-bonding distances, resulting in a single minimum-type
potential energy curve after a certain critical distance.11 The
smaller potential energy barrier of a double minimum-type can
be readily activated by thermal energy (kBT) to cause an order–
disorder structural phase transition of the proton within the
hydrogen-bonding coordinates. The proton transfer and thermal
fluctuation of hydrogen-bonding systems are dominated by the
transition of protons within the N–H+ N hydrogen-bonding
…
aGraduate School of Engineering, Tohoku University, Sendai 980-8579,
Japan
bInstitute of Multidisciplinary Research for Advanced Materials
(IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku,
Sendai 980-8577, Japan. E-mail: akuta@tagen.tohoku.ac.jp;
Fax: +81-22-217-5655; Tel: +81-22-217-5653
cResearch Institute for Electronic Science, Hokkaido University, Sapporo
001-0020, Japan
{ Electronic supplementary information (ESI) available: TG analysis,
atomic numbering scheme, and structural analyses. CCDC reference
numbers 859666 and 859667. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2ce25945a
7458 | CrystEngComm, 2012, 14, 7458–7465
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shapes of potential energy curve of the proton coordinates and
thermal energy, which also influence the structural phase
transitions of the crystal.
for the preparation of the crystal in C2H5OH–H2O (8 : 2, v/v).
The acid dissociation constants (pKa1 and pKa2) of the two
terminal –COOH groups of L-H2Tart in H2O are 2.99 and 4.40,
respectively, whereas the pKa of anilinium (Ani+) is 4.58.17
Because the value of pKa1 of L-H2Tart (2.99) is lower than that
of Ani+ (4.58), proton transfer from one of the –COOH groups
of L-H2Tart to the –NH2 group of aniline readily occurred
during the formation of the crystals, resulting in single crystals of
proton-transferred (Ani+)(L-HTart2) (1). The stoichiometry of
the crystal was determined using X-ray crystal structural,
elemental, and thermogravimetric analyses. Because the struc-
ture of crystal 1 did not include any crystallization solvent
molecules, weight-loss was not detected in the temperature range
of up to 400 K (Fig. S1{).
As shown in Fig. 1, the temperature-dependent DSC diagrams
of crystal 1 over a temperature range of 180 to 400 K exhibit phase
transitions at 221 and 269 K for the heating process, and at 283
and 269 K for the cooling process. Accordingly, three crystal
phases can be defined as follow: phase I (T , 221 K), phase II (221
K , T , 269 K), and phase III (269 K , T). Hysteresis of roughly
10 K was observed for the phase transitions of both I–II and II–
III, which were reversibly observed for the temperature cycles
from 180 to 350 K. The transition enthalpies (DH) of the I–II and
II–III transitions are 0.697 and 0.289 kJ mol21, respectively,
whereas the transition entropies (DS) are 3.15 and 1.07 J K21
mol21, respectively. The value of DS can be described as DS =
Rln(N2/N1), where R, N1, and N2 are the gas constant and the
numbers of possible orientations of low- and high-temperature
phases, respectively. Values for N2/N1 for phase transitions I–II
and II–III were 1.46 and 1.13, respectively, and are comparable to
N2/N1 = 3/2 and 1, respectively.
The coexistence of hydrogen-bonding and charge-transfer
interactions results in interesting phase transition phenomena
associated with coupled proton and electron transfer interac-
tions.6,14 For instance, the binary molecular complex between
electron-donating anilines and electron-accepting polynitrophe-
nols can form a neutral charge-transfer complex or an ionic
proton-transferred salt depending on the differences in the
acidities. Charge-transfer interactions are dominant in the neutral
complex, whereas Brønsted acid–base interactions dominate in the
proton-transferred ionic salt,12,13 in which increased temperatures
can cause a phase transition from the neutral charge-transfer
complex to the ionic Brønsted acid–base salt.12–15 As an example,
at temperatures above 333 K, the yellow 1 : 1 Brønsted acid–base
salt of (2-iodoanilinium)(picrate) transforms into the green charge-
transfer complex of (2-iodoaniline)(picric acid),12 via a proton
transfer from 2-iodoanilinium to picrate within the crystal. In
another example, at temperatures above 450 K, the red-colored
ionic crystal of (4,49-bipyridinium2+)(squarate22) transforms into
the black-colored neutral charge-transfer complex of (4,49-bipyr-
idine)(squaric acid) via a proton transfer from 4,49-bipyridinium to
squarate within the one-dimensional N–H+ O hydrogen-bonding
…
chain.14 Another notable example involves the switch in the spin
state of the 1 : 1 charge-transfer complex between the electron-
donating hydroquinone (H2Q) and the electron-accepting p-ben-
…
zoquinone (BQ) in which one-dimensional O–H O hydrogen-
bonding and charger-transfer interactions occur simultaneously
within the crystal between the H2Q and BQ molecules.15 By
applying pressures over 30 kbar, a magnetic semiquinone radical
state was formed via proton transfer from the proton-donating
H2Q to the proton-accepting BQ. When the charge-transfer
interaction coexisted in the hydrogen-bonding one, the optical
and/or magnetic properties could be coupled with the proton-
transfer within the crystals. The active controls in the motional
freedom of proton and/or molecules have a potential to achieve an
interesting structural phase transition behavior and responses in
bulk physical properties such as optical, magnetic, and dielectric
functions.16 To investigate such possibilities, we examined the
crystal structure, thermal properties, and dielectric properties of a
simple hydrogen-bonding binary ionic molecular crystal (Ani+)(L-
HTart2) (Scheme 1) that was formed between proton-accepting
aniline and chiral proton-donating L-tartaric acid (L-H2Tart).
Crystal structure of phase I (low temperature, T = 100 K)
The crystal structure of the ionic (Ani+)(L-HTart2) crystal is
predominantly governed by the electrostatic cation–anion and
hydrogen-bonding interactions. The chiral anion of L-HTart2
possesses a C1 asymmetrical point group. At 100 K, the
asymmetric units of phase I consists of two forms of Ani+ (A
and B) and two forms of L-HTart2 (C and D). The unit cell of
phase I viewed along the a-axis is shown in Fig. 2a. Strong
Results and discussion
Crystal preparation and phase transition
Brønsted acid–base reaction between the proton-accepting ani-
line and proton-donating L-tartaric acid (L-H2Tart) was utilized
Fig. 1 DSC diagram over the temperature range from 180 to 400 K.
Three types of crystal phases (I, II, and III) were observed in the thermal
hysterics.
Scheme 1 Molecular structure of anilinium (Ani+) and L-tartrate (L-
HTart2).
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intermolecular O–H
O
2 hydrogen-bonding interactions among
b-axis follows the
…
… … …
A B sequence, in which the antiparallel
the L-HTart2 monoanions forms a one-dimensional infinite
hydrogen-bonding (L-HTart2)‘ chain, whereas the alternating
cation–anion layers are elongated along the c-axis. The two
discrete forms of double anionic hydrogen-bonding (L-HTart2)‘
chains (Ha and Hb) are sandwiched between the cationic layers.
The antiparallel arrangement of the Ani+ cations along the c-axis
forms the p-stacking structure along the b-axis.
Ani+ arrangement was observed. The two-dimensional Ani+
layer within the ab-plane is sandwiched between the double (L-
Tart2)‘ chains of Ha and Hb.
Crystal structures of phases II and III (high temperature)
To evaluate structural differences, the crystal structures of
phases II and III were determined at 250 and 300 K, respectively,
and compared against that of phase I. The unit cell parameters of
The hydrogen-bonding interactions of the L-HTart2 anions
2
…
are shown in Fig. 2b. Strong O–H
O
hydrogen-bonding
phase II (T = 250 K) and phase III (T = 300 K) are nearly
3
identical; the unit cell volume at 250 K, V = 1099.8(2) A , is
interactions between the terminal COOH and COO2 groups of
the L-HTart2 anions along the b-axis was observed. Two types
of hydrogen-bonding (L-HTart2)‘ chains (Ha and Hb) are
˚
3
slightly smaller than that at 300 K, V = 1106.2(2) A (see
˚
Table 3). In contrast, the unit cell volume of phase I [V =
… … …
C
… … …
D D anion
formed within the uniform
C
and
3
˚
˚
543.9(2) A , a = 6.1165(8), b = 7.466(2), c = 12.960(2) A, a =
89.758(5), b = 76.883(4), c = 70.706(5)u] is half of those of phases
II and III. Four discrete types of Ani+ cations (A, B, C, and D)
along with four types of the L-HTart2 anions (E, F, G, and H)
were observed within the structural units of phases II and III.
arrangements, which are sandwiched between the Ani+ layers
…
along the c-axis (Fig. 2a). The hydrogen bonding O O distance
˚
of the Ha chain, d(C–C) = 2.583 A, is slightly longer than that of
˚
the Hb chain, d(D–D) = 2.542 A. The lack of effective
intermolecular hydrogen-bonding interactions between the Ha
…
The hydrogen-bonding O O distances for the anionic chains
…
˚
and Hb chains for O O distances less than 3.283 A indicates
that the Ha and Hb hydrogen-bonding (L-HTart2)‘ chains are
isolated from each other. The Ani+ arrangement of A and B units
within the ab-plane is shown in Fig. 2c. The p-stack along the
and the mean interplanar distances of the C6-plane of the Ani+
cations for phases I, II, and III are summarized in Table 1.
The cation–anion arrangements for phases II and III are
comparable to that for phase I. The unit cell of phase II at 250 K,
as viewed along the a-axis, is shown in Fig. 3a. Although the
crystal periodicity along the c-axis for phase II is identical to that
for phase I (Fig. 2a), the p-stacking and hydrogen-bonding
interactions along the 2a+b-axis are modulated to reflect the
double periodicity. For the A–B and B–A9 pairs of phase I, the
mean interplanar distances between the C6-planes of the Ani+
˚
anions are 3.68 and 3.77 A, respectively. For phase II, the
… … … … …
D
A
B
C
p-stacking order results in the mean inter-
planar distances of d(A–B) = 3.69, d(B–C) = 3.78, d(C–D) = 3.72,
˚
and d(D–A9) = 3.77 A.
… … …
C
… … …
D D (Hb chain) linear
The
C
(Ha chain) and
hydrogen-bonding interactions for phase I of (L-HTart2)‘ were
… … … … …
…
H
modulated to the alternate
E
F
(Ha chain) and
G
(Hb chain) hydrogen-bonding interactions for phases II and III.
…
˚
The O O distance for phase I, d(C–C) = 2.575(5) A, is shorter
than those for phase II, d(E–F) = 2.599(6) and d(F–E9) = 2.587(5)
˚
˚
A. However, the distance of d(D–D) = 2.543(5) A for phase I is
longer than those of d(G–H) = 2.522(5) and d(H–G9) = 2.530(5)
˚
A for phase II. The hydrogen-bonding periodicity for the double
anionic chains was modulated by increasing the temperature.
The changes in the unit cell periodicity along the p-stacking and
…
Table 1 The hydrogen-bonding O O distances and the mean inter-
planar distances of the Ani+ cations for phases I, II, and III
a
˚
Hydrogen-bonding distance, A
˚
Interplanar distance, A
Phase
I
d(C–C) = 2.575(5),
d(D–D) = 2.543(5)
d(E–F) = 2.599(6),
d(F–E9) = 2.587(5)
d(G–H) = 2.522(5),
d(H–G9) = 2.530(5)
d(E–F) = 2.577(5),
d(F–E9) = 2.586(5)
d(G–H) = 2.537(5),
d(H–G9) = 2.531(5)
d(A–B) = 3.68,
d(B–A9) = 3.77
d(A–B) = 3.69,
d(B–C) = 3.78
d(C–D) = 3.72,
d(D–A9) = 3.77
d(A–B) = 3.63,
d(B–C) = 3.83
d(C–D) = 3.61,
d(D–A9) = 3.84
II
Fig. 2 Crystal structure of phase I at T = 100 K. (a) Unit cell viewed
along the a-axis. (b) Double hydrogen-bonding interactions along the
…
III
b-axis; two types of infinite O–H O hydrogen-bonding chains (Ha and
…
… …
Hb) were observed within the anion arrangements of
C
C and
+
, respectively. (c) Two-dimensional Ani arrangements (A and
… … …
B
B
a
The mean interplanar distance between two C6-planes of Ani+.
B) viewed along the c-axis.
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(DE) vs. O–H distance (dOH) of the proton coordinate for the (L-
HTart2)2 dimer was evaluated using the B3LYP/6-31+G(d,p)
basis set (Fig. 4a).19 In our plot (Fig. 4a), dO–H = 1.00 A
˚
corresponds to the atomic coordinate from the X-ray crystal
structural analysis and is defined as zero. The DE 2 dO–H plots
illustrates the localization behavior of the protons within the
one-dimensional (L-HTart2)‘ chains. At 100 K, two types of
hydrogen-bonding chains, Ha and Hb, were constructed by the
… … …
… … … …
A
A
and
B
B
arrangements with O O distances of
˚
d(A–A) = 2.583(3) and d(B–B) =2.542(3) A, respectively. The
potential energy curves for the Ha and Hb chains are similar,
indicating an asymmetrical single minimum-type with a potential
˚
energy minimum at dO–H = 1.00 A. Because the DE value at dO–H
= 1.50 A for the Ha and Hb chains is about 200 kJ mol21 higher
˚
˚
than that at dO–H = 1.00 A, it can be assumed that strong proton
localization occurs within the hydrogen-bonding (L-HTart2)‘
chains. And, the possibility of a proton transfer between the L-
HTart2 anions leading to structural transition from phase II to
III can be ruled out.
To evaluate the molecular rotation of the Ani+ cation, the
potential energy curve of the potential energy barrier were
calculated for the (Ani+)7-unit within the cationic layer (Fig. 4b).
The p-stacking structure of the Ani+ cations in the two-
dimensional cationic layer would presumably generate
a
significant potential energy barrier for the rotation of Ani+. In
addition to the rotating Ani+ cation, six neighboring cations
were also included in the calculations to evaluate steric
hindrances based on the crystal structure at 100 K (see upper
in Fig. 4b). The 180u flip-flop motion of the Ani+ cation was
restricted by the p-stacking structure of the cations. Relative
energies were determined as a function of rotational angle Q1
Fig. 3 Crystal structure of phase II at 250 K. The periodicity along the
2a+b-axis of phase II was the twice that along the b-axis of phase I. Four
types of Ani+ (A, B, C, and D) and four types of L-HTart2 anions (E, F,
G, and H) were crystallographically discrete structural units within the
unit cell.
hydrogen-bonding interactions leads to the structural phase
+
around the C–NH3 bond of the Ani+ cation, at every 30u of
transition from phase I to II at 221 K.
rotation. For the rotational angle Q1, the initial atomic
coordinates correspond to the first potential energy minimum
at Q1 = 0u, whereas the second potential energy minimum is
Phase transition from phase II to phase III
The transition from phase I to II can be associated with the
doubling of the unit cell upon increasing the temperature above
y220 K. In contrast, the origin of the structural phase transition
from phase II to III remains unclear due to the similarity
between the unit cell parameters. Because the transition entropy
for the transition of phase II to III is less than that of phase I to
II, minor structural changes can be expected for the transition
between phase II to III. Herein, we assumed three possibilities of
motional freedom within the crystal: i) proton transfer within the
hydrogen-bonding (L-HTart2)‘ chain, ii) two-fold flip-flop
+
motion of the Ani+ cation along the C–NH3 axis, and iii)
rotation of the –OH groups of the L-HTart2 anion at phase III.
The hydrogen-bonding protons of –COOH and –OH groups
of the L-HTart2 anions were determined by differential Fourier
synthesis based on the X-ray crystal structural analyses.
According to the ice rule,18 the protons in the (L-HTart2)‘
chains are localized at the one oxygen side, and therefore, neither
the L-Tart22 dianion nor L-tartaric acid was observed in the
crystal. To study the possibility of proton transfer within the
crystals, the potential energy curves for the position of protons
within the one-dimensional (L-HTart2)‘ chain at 100 K were
evaluated. The behavior of the intermolecular proton transfer is
dominated by the symmetry of the potential energy curve and by
the magnitude of the potential energy barrier. The relative energy
Fig. 4 Potential energy curves for the proton transfer for the (L-
HTart2)2 dimer and two-fold flip-flop motion of the Ani+ cation. (a)
Initial dimer structures of (L-HTart2)2 (upper figure) and the DE 2 dO–H
˚
plots (lower figure) over the range from 0.7 to 1.8 A. The O–H distance
for the (L-HTart2)2 dimer was defined as dO–H. Two types of hydrogen-
…
bonding chains, Ha (black) and Hb (red), were calculated for the (A A)
…
and (B B) dimer arrangements. (b) Potential energy curves for the
+
rotation of the Ani+ cation along the C–NH3 axis. The calculated
structure of (Ani+)7 (upper figure) and DE 2 Q1 plots (lower figure). The
solid lines indicate curve fits.
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Table 2 Dihedral angles of the C–C–O–H group in the L-HTart2 anion
for phases I, II, and IIIa
observed around Q1 = 180u. Based on our calculations, a
symmetrical double-minimum potential energy curve with two
stable molecular orientations was observed for the Ani+ cation.
Because the magnitude of the potential energy barrier (y300 kJ
mol21) was much larger than the thermal energy at room
temperature, the two-fold flip-flop motions of the Ani+ cation
would not occur at temperatures y270 K.
And thirdly, the structural transition from phase II to III
could also be due to the conformational change of the L-HTart2
anion above 270 K. The temperature-dependent conformational
changes for the L-HTart2 anions at 100 K (phase I), 250 K
(phase II), and 300 K (phase III) are shown in Fig. 5, and the
four representative C–C–O–H dihedral angles for the L-HTart2
anion (w1, w2, w3, and w4 in Fig. 5) are summarized in Table 2.
Four variations of the L-HTart2 anions (E, F, G, and H) were
observed for phases II and III, with similar conformations of the
L-HTart2 anions with the same hydrogen-bonding chains
I
100 K
II
250 K
III
300 K
Phase
w1
w2
w3
w4
a
217(3)
24(5)
7(4)
225(4)
26(4)
1(4)
277(4)
79(5)
21(4)
75(4)
69(4)
69(4)
Representative C–C–O–H dihedral angles of w1, w2, w3, and w4 are
shown in Fig. 5. The average angles for the E and F anions correspond
to w1 and w2, whereas those of the G and H anions correspond to w3
and w4.
similar, but the orientations of the –OH groups for anions E and
F anions were different from those of phases I and II. The
dihedral angles of phase III, w1 = 277(4) and w2 = 79(5)u, have
rotated about 60u relative to those of phase II. Therefore, the
transition from phase II to III can be attributed to the thermally
activated rotation of the –OH groups in the L-HTart2 anions (E
… … …
… … …
(
E
F
and
G H ). Therefore, for the structural
evaluations of the L-HTart2 conformations at phases I, II, and
III, the average dihedral angles were assumed as follows: anion C
at phase I was defined using w1 = 217(3) and w2 = 24(5)u,
whereas anion D was defined using w3 = 7(4) and w4 = 69(4)u.
The L-HTart2 conformations at phase II (T = 250 K) were
comparable to those at phase I (T = 100 K); specifically, average
dihedral angles of phase II, of w1 = 225(4) and w2 = 6(4)u for
anions E and F and w3 = 1(4) and w4 = 69(4) for anions G and H,
were similar to those of phase I. In the case of phase III,
however, the L-HTart2 conformations for anions G and H were
and F).
… … …
The uniform hydrogen-bonding interactions of the
… … …
C C
and
the alternating
and III. The conformations of the L-HTart2 anions within the
… …
D
D
chains for phase I (T = 100 K) were modulated to
… … … … …
…
chains for phases II
E
F
and
G
H
…
G
H
chain for phase III were comparable to those within
… … …
the
within the
from those within the
D
D
chain for phase I. In contrast, the conformations
… … …
E
F
chain for phase III were slightly different
… … …
chain at phase I. The relatively
C
C
large magnitude of the conformational change of the L-HTart2
… … …
anion within the
E
F
chain for phase III can be attributed
… … …
to the deformation of the uniform
C
C
chain.
Subsequently, we calculated the potential energy curves for the
–OH rotations of the C anion based on the crystal structure at
100 K, in which two types of –OH groups (a- and b-groups) were
considered. Rotation angle Q2 was defined as the angle of C–OH
along the C–OH bond. Because the potential energy curves
based on the crystal structure at 300 K and at 100 K differed by
only ¡5 kJ mol21, and in the interest of simplicity, our
discussions of the DE 2 Q2 plots was based only the crystal
structure at 100 K. Two types of –OH groups (a- and b-groups)
for the C anion were considered for the independent rotations, in
which the two anti-parallel Ani+ cations were included to
evaluate the rotational potential energies (Fig. 6a).
The DE 2 Q2 plots for the –OH rotations of the a- and
b-groups in the (Ani+)2(L-HTart2) structures are shown in
Fig. 6b. The potential energy barrier for the rotation of the a-
and b-groups occurred at Q2 = 270u and 180u, respectively. At
these positions, the –OH groups of L-HTart2 are directed
+
towards the –NH3 group of Ani+, where electrostatic repulsion
between the –NH3+ and –OH groups causes the potential energy
barriers of y80 kJmol21 and y150 kJmol21 at Q2 = 270u and
180u, respectively. The potential energy for a-group’s –OH was
around 0 kJ mol21 over the range of Q2 = 0 to 120u, where the
–OH rotation along the C–OH axis was thermally activated. The
magnitude of potential energy barrier has been reported as
y80 kJ mol21 for the rotation of a-group’s –OH was
comparable to that of the thermally-activated twofold flip-flop
motion of an Ani+ cation within (Ani+)(18crown-6)[Ni(dmit)2]
crystals,20 which has been reported as y80 kJmol21. The
Fig. 5 Temperature-dependent conformation changes of the L-HTart2
anions at 100 K (phase I), 250 K (phase II), and 300 K (phase III).
Because the E–F and the G–H anion pairs at 250 and 300 K adopted the
same conformations, the conformations of the E and G anions are used
in the figures. The anion conformations are represented by the four C–C–
O–H dihedral angles w1, w2, w3, and w4.
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Fig. 7 Temperature- and frequency-dependent dielectric constants (e1)
of a compressed pellet of (Ani+)(L-HTart2) during the heating process.
molecular motions around y220 K. Above y220 K, the
periodicities of p-stacking and hydrogen-bonding doubled, while
at temperatures above 270 K, the –OH rotations of the L-
HTart2 anion were thermally activated (Fig. 6). Despite the
influence of the rotation of the –OH group on the direction of
dipole moment, frequency-dependent e1 enhancements were
observed at temperatures above 270 K.
Fig. 6 Potential energy curves for two types of –OH rotations (a- and
b-groups) along the C–OH axis of L-THart2 at 100 K. (a) The calculated
structure of (Ani+)2(L-HTart2), where two types of –OH rotational
modes (rotations a and b in the left and right figures, respectively) were
evaluated at every 30u. (b) DE 2 Q2 plots for the rotation a (red) and
rotation b (blue). The solid lines indicate curve fits.
Conclusions
Brønsted acid–base reaction between proton-accepting aniline
and proton-donating L-tartaric acid in C2H5OH–H2O results in
an ionic molecular crystal of (anilinium+)(L-tartrate2). The
crystal exhibited two-step reversible phase transitions at y220
and y270 K, that were characterized using DSC measurements,
temperature-dependent X-ray crystal structural analyses, and ab
initio calculations of the potential energy curves from the
viewpoint of motional freedom within the crystal. Three phases
were identified: phase I (T , 220 K), phase II (220 , T , 270
K), and phase III (270 K , T), all possessing an acentric space
group of P1, but in which the unit cell of phase I was half of
those of phases II and III. The crystal structures consisted of
alternate arrangements of the one-dimensional hydrogen-bond-
ing of the (L-tartrate2)‘ chains and the p-stack of the anilinium
ions. Intermolecular hydrogen-bonding interactions between the
–COOH and –COO groups of the L-tartartae2 anions formed
the one-dimensional (L-tartrate2)‘ chains, where the protons of
the hydrogen bonds were localized at one oxygen due to the
asymmetrical potential energy curve. By increasing the tempera-
ture above 220 K, the hydrogen-bonding and p-stacking
interactions of phase I were modulated to the double periodicity
for phases II and III. At high temperatures (T > 270 K), the
rotation of the –OH groups in the L-tartrate anion of phase
III was thermally activated with a potential energy barrier of
y70 kJ mol21. Thermal fluctuation of the –OH group in the
single minimum-type potential energy curve were consistent with
the frequency- and temperature-dependent dielectric constants
above y270 K. Several types of motions of the hydrogen-
bonding protons, rotations of the anilinium ion, and rotation of
thermally-activated –OH rotations are activated around the
phase transition temperature of y270 K, which causes the
transition from phase II to III. It should be noted that the shape
of potential energy curves for the –OH rotations (a- and
b-group) was the single minimum-type. Because a double
minimum-type potential energy curve involves both bistability
and dipole inversion, the transition from phase II to III was not
that of the ferroelectric–paraelectric type.
Dielectric properties
The temperature- (T) and frequency- (f) dependent dielectric
constants (e1) of the compressed pellet of (Ani+)(L-HTart2)
crystal are shown in Fig. 7. Because the dielectric properties of
the needle-shaped crystal were very difficult to determine in
the single crystal state, a compressed pellet with a diameter of
13 mmQ was used for the dielectric measurements. In the DSC
diagram, two phase transitions were observed at 221 and 269 K
during the heating process. Although the dielectric peaks were
not detected in the e1 2 T plots, the frequency-dependent e1
enhancements were observed at temperatures above y220 K.
The temperature-independent e1 values (e1y8.5) over the
temperature range from y50 to y200 K indicates complete
suppression of thermally-activated molecular motions. The low
frequency e1 values (f = 1 and 10 kHz) above 220 K were larger
than those at high frequency (f = 100 and 1000 kHz), suggesting
that the dielectric behaviors were dominated by low frequency
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the –OH group in the L-tartrate anion were evaluated in the
hydrogen-bonding ionic molecular (anilinium+)(L-tartrate2)
crystal. Chemical modifications of the anilinium species would
allow the control and design of the structural phase transitions of
the hydrogen-bonding crystals.
energies of the (Ani+)2(L-HTart2) structure were calculated for
the rigid –OH rotation along the C–OH bond of L-HTart2 at
every 30u. The rotational potential energies of an Ani+ cation
+
along the C–NH3 bond were calculated using the RHF/6-31(d)
basis set (ii).20 The atomic coordinates based on the X-ray crystal
structural analysis were used for the calculations, and the
neighboring six Ani+ cations were included in the calculations to
evaluate steric hindrance. The relative energies of the structures
were obtained by evaluating the rigid rotation of the Ani+ cation
Experimental
Commercially available aniline and L-tartaric acid were used
without further purification. Single crystals of colorless needles
were grown by the slow evaporation of a 1 : 1 mixture of aniline
and L-tartaric acid in C2H5OH–H2O (8 : 2) with a yield of
y20%. Stoichiometries of the crystals were determined by X-ray
crystal structural analyses, thermogravimetric measurements,
and elemental analyses (see ESI{). Elemental analysis:
C10H13NO6: C 49.38, H 5.39, N 5.76; found C 49.20, H 5.34,
N 5.74. Temperature-dependent crystallographic data (Table 3)
were collected using a Rigaku RAXIS-RAPID diffractometer
+
along the C–NH3 bond.
Temperature-dependent dielectric constants were measured
using the two-probe AC-impedance method over a frequency
range from 1 6 103 to 1000 6 103 Hz (HP4194A). The
compressed pellet (13-mm Q) was placed in
a cryogenic
refrigerating system (Daikin PS24SS). The electrical contacts
were prepared using carbon paste to attach the 25 mm Q gold
wires. TG-DTA and DSC analyses were carried out using a
Rigaku Thermo plus TG8120 thermal analysis station with an
Al2O3 reference over a temperature range of 293 to 500 K for
TG-DTA and 180 to 400 K for DSC analyses, with a heating rate
of 5 K min21 under a nitrogen atmosphere.
˚
with Mo-Ka (l = 0.71073 A) radiation from a graphite
monochromator. Structure refinements were carried out using
the full-matrix least-squares method on F2. Calculations were
performed using Crystal Structure software packages.21
Parameters were refined using anisotropic temperature factors
except for the hydrogen atom.
Acknowledgements
This work was supported by a Grant-in-Aid for Science
Research from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan, Management Expenses
Grants for National Universities of Japan.
The motion within the crystal was evaluated using three types
of calculations: i) proton transfer in the hydrogen-bonding L-
HTart2 chain, ii) two-fold flip-flop motion of an Ani+ cation
along the C–NH3+ axis, and iii) rotation of an –OH group in the
L-HTart2 anion. Potential energies of the proton transfer in the
2 hydrogen-bonding between the (L-HTart2)2 dimer (i)
…
O
O–H
References
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coordinates based on the X-ray crystal structural analysis were
used for the calculations, resulting in a single point energy with
fixed atomic coordinates. The relative energies of the (L-
HTart2)2 dimer were calculated for each proton coordinate of
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Phase
I
II
III
T/K
Formula
Formula weight
Space group
100
250
C10H13NO6
243.22
300
C10H13NO6
243.22
´
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243.22
P1 (#1)
6.1165(8)
7.466(2)
12.960(2)
89.758(5)
76.883(4)
70.706(5)
543.9(2)
2
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8.0038(6)
11.0971(8)
13.024(2)
101.247(3)
95.778(4)
101.541(3)
1099.8(2)
4
P1 (#1)
8.0322(6)
11.0953(8)
13.0247(9)
100.127(2)
96.793(2)
101.350(2)
1106.2(2)
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a/A
˚
b/A
˚
c/A
a (u)
b (u)
c (u)
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˚
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Dcalc, g cm21
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0.0487
0.2096
1.057
1.460
1.219
10927
8224
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1.1777
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