Phase Transition and Ring-Puckering Motion in a MetalâOrganic

Article pubs.acs.org/JPCA

Phase Transition and Ring-Puckering Motion in a Metal−Organic Perovskite [(CH2)3NH2][Zn(HCOO)3]

Tetsuo Asaji,*,†,‡ Yoshiharu Ito,‡ Janez Seliger,§,∥ Veselko Ž agar,§ Anton Gradišek,§ and Tomaž Apih§ †

Department of Chemistry, College of Humanities and Sciences, Nihon University, Sakurajosui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan ‡ Department of Chemistry, Graduate School of Integrated Basic Sciences, Nihon University, Sakurajosui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan § Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia ∥ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia ABSTRACT: Phase transitions in a metal−organic perovskite with an azetidinium cation, which exhibits giant polarizability, were investigated using differential scanning calorimetry (DSC) and 1H nuclear magnetic resonance (NMR) measurements. The DSC results indicated successive phase transitions at 254 and 299 K. The temperature dependence of the spin−lattice relaxation time T1 determined by NMR indicated that the activation energy for cation ring-puckering motion was 25 kJ mol−1 in phase I (T > 299 K). The potential energy at the transition state of puckering is expected to decrease when the potential for the motion becomes asymmetric with decreasing temperature in phases II and III. A possible mechanism for the onset of an extraordinarily large dielectric anomaly is discussed. [(CH2)3NH2][M(HCOO)3] (M = Mn, Zn).7 No clear hysteresis has been observed in these metal−organic perovskites; therefore, the observed behavior is reminiscent of a relaxor ferroelectric. To elucidate the role of molecular motion of the azetidinium cation with respect to the relaxor-like behavior, we investigated this motion using 1H nuclear magnetic resonance (NMR) spin−lattice relaxation measurements of a diamagnetic compound [(CH 2 ) 3 NH 2 ][Zn(HCOO)3]. Such an investigation of the molecular motion of an azetidinium cation that is relatively isolated in a crystal is of special interest because cyclobutane is the only four-membered ring molecule, to the best of the authors’ knowledge, for which the motion has been investigated by NMR spectroscopy.8 The present work is the first NMR study of the azetidinium cation in a solid.

1. INTRODUCTION Perovskite compounds have been one of the most important families of ferroelectrics since the discovery of ferroelectricity in BaTiO3, in which the displacement of ions plays a crucial role.1,2 Recently, a new series of perovskite compounds, which consist of an organic cation and a metal ion bridged by formate ions, have been extensively studied.3 The metal ions bridged by formate ions form a cage structure in these compounds with ca. 0.6 nm pore spaces and the organic cation is located inside the cavity. Interesting properties, such as ferro- or anitiferroelectricity, have been reported in these compounds, probably due to the degrees of freedom associated with motion of the organic cation. For example, typical order−disorder type ferroelectric or antiferroelectric phase transitions have been suggested for (NH4)[Zn(HCOO)3] and [(CH3)2NH2][Zn(HCOO)3], respectively.4,5 Very recently, molecular-based materials with perovskite structures that exhibit giant polarizability over a wide temperature range were reported by Zhou et al.;6 in these materials, the ring-puckering motion of a four-membered ring molecule was employed as a form of structural degree of freedom that results in high polarizability. A metal−organic perovskite [(CH2)3NH2][Cu(HCOO)3] with a cyclotrimethyleneammonium cation (or azetidinium cation, (CH2)3NH2+) exhibited an extraordinarily large dielectric anomaly (dielectric constant ε1 > 104 at 1 kHz) in the temperature range of 220−320 K. Figure 1 shows the crystal structure of [(CH2)3NH2][Cu(HCOO)3] at 123 K viewed along the crystal a-axis (left) and the [011] direction (right). Similar dielectric anomalies were also obtained for © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Crystals of [(CH2)3NH2][Zn(HCOO)3] were prepared by the solution diffusion method according to a previously reported procedure.9 A 30 mL methanol solution of 0.5 M HCOOH and 0.5 M (CH2)3NH (azetidine) was placed at the bottom of a glass test tube (30 mm diameter, 40 cm long). Twelve milliliters of methanol was carefully added to the solution, followed by the careful layering of 50 mL of a 0.1 M solution of Zn(ClO4)2·6H2O in methanol. Received: October 13, 2012 Revised: December 4, 2012 Published: December 5, 2012 12422

dx.doi.org/10.1021/jp310132a | J. Phys. Chem. A 2012, 116, 12422−12428

The Journal of Physical Chemistry A

Article

Figure 1. Crystal structure of [(CH2)3NH2][Cu(HCOO)3] at 123 K viewed along the crystal a-axis (left) and [011] direction (right).

Table 1. X-ray Powder Diffraction Data for [(CH2)3NH2][Zn(HCOO)3] Obtained at Room Temperature Using Cu Kα Radiation (λ = 1.5418 Å); Theoretical Values Were Calculated Assuming the Orthorhombic Space Group Pnma with a = 8.51, b = 11.94, and c = 8.63 Å h

k

l

2θ (deg) calcd

2θ (deg) obsd

intensity

h

k

l

2θ (deg) calcd

2θ (deg) obsd

intensity

0 1 0 1

1 0 2 1

1 1 0 1

12.646 14.607 14.827 16.392

12.65 14.63 14.85 16.41 18.15 19.31 19.59 20.49

3 100 36 3 3 4 4 18

8

4 8

23.289 24.279 24.468 24.612 25.449 25.689 26.778 27.553 27.721 29.458 29.910

24.51

22.23 23.03

1 2 1 1 2 0 1 2 1 2 0

6 9

81

0 1 1 3 2 2 3 2 2 0 4

23.31 24.21

20.87

2 1 2 0 0 2 1 1 2 2 0

25.36 25.73 26.71 27.47 27.71 29.39 29.85

2 5 4 5 5 45 26

0 2 1 2 1

0 0 2 1 0

2 0 1 0 2

20.567 20.860 20.871 22.161 23.092

1

H−14N double resonance techniques used were crossrelaxation spectroscopy,10,11 the solid effect technique,12 and the two-frequency irradiation technique.13 Cross-relaxation spectroscopy was used in the high-temperature region, where the 14N spin−lattice relaxation rates exceed the proton spin− lattice relaxation rate at the proton Larmor frequency νL in the range of the 14N NQR frequencies. At low temperatures, where the 14N spin−lattice relaxation rate is comparable to or lower than the proton spin−lattice relaxation rate in the same Larmor frequency region, the solid effect technique was employed to locate the 14N NQR frequencies, and the two-frequency irradiation technique was used to improve the resolution. 2.3. DSC. Differential scanning calorimetry (DSC; PerkinElmer, DSC8500) measurements were conducted using 5.72 mg of sample placed in an aluminum sample pan in the temperature range of 178−323 K. The heating and cooling rate was set to 10 K min−1.

The glass test tube was sealed with a silicone rubber stopper and kept undisturbed. After several days of crystallization, the colorless block-like crystals were collected. The preparation was repeated until an adequate amount of crystals was obtained. The sample was identified by chemical analysis conducted by Center for Organic Elemental Microanalysis, Kyoto University. Anal. Calcd. for [(CH2)3NH2][Zn(HCOO)3]: C, 27.87; H, 4.29; N, 5.42%. Found: C, 27.33; H, 4.11; N, 5.22%. To obtain crystal structural information, X-ray powder diffraction measurements were conducted using a diffractometer (Rigaku, Rint 2100) with monochromated Cu Kα radiation (λ = 1.5418 Å) 2.2. 1H NMR and 14N NQR. A pulse NMR system consisting of a homemade probe, a flow-cryostat (CRYO Industries, RC 152), a superconducting magnet (Cryomagnetics), and a spectrometer (Thamway, PROT 3100MR) was used at a Larmor frequency of 47.35 MHz. The spin−lattice relaxation time T1 was determined using a comb−τ− 90°(x)−τe−90°(y) pulse sequence. The 90° pulse-width was ca. 4.5 μs, and the pulse delay time τe for solid-echo pulses was set at 5.5 μs. For the second moment measurements of the NMR absorption line, the τe was set at 20 μs, and the solid echo signal after the echo maximum in time domain was Fourier transformed to obtain the absorption line. The 14N nuclear quadrupole resonance (NQR) frequencies were measured using double resonance methods on two spectrometers; a homemade magnetic field cycling double resonance spectrometer based on sample shuttling between two magnets, and a fast field cycling (FFC) proton NMR relaxometer (Stelar, FFC 2000). The

3. RESULTS AND DISCUSSION Table 1 shows X-ray powder diffraction data for [(CH2)3NH2][Zn(HCOO)3] obtained at room temperature. Assuming an isomorphous structure with the high-temperature phase of [(CH2)3NH2][Cu(HCOO)3] that belongs to the space group Pnma,6 the observed pattern can be fairly well interpreted to correspond to lattice constants a = 8.51, b = 11.94, and c = 8.63 Å. Weak peaks observed at 2θ = 18−20° may be due to contamination. Figure 2 shows DSC curves after baseline correction for decreasing (blue curve) and increasing (red curve) temper12423



Article

Figure 2. DSC curves after baseline correction for [(CH2)3NH2][Zn(HCOO)3]. The heating (red curve) and cooling (blue curve) rate was set to 10 K min−1.

Figure 4. Temperature dependence of the 1H NMR spin−lattice relaxation time T1 for [(CH2)3NH2][Zn(HCOO)3] in the phase transition region.

ature. An exothermic peak was observed at 297 K with decreasing temperature, while two endothermic peaks were located at 254 and 299 K with increasing temperature, and these were associated with a broad peak between them. Integration of the heat flow with respect to time gave the enthalpy change through the transition region (200−320 K) as 28 and 27 mJ for decreasing and increasing temperature, respectively. These DSC results show occurrence of successive phase transitions of [(CH2)3NH2][Zn(HCOO)3] at Tc (I−II) = 299 K and Tc (II−III) = 254 K, instead of a single phase transition as observed for [(CH2)3NH2][Cu(HCOO)3].6 These observations are in reasonable agreement with those reported by Imai et al.7 Hereafter, the three crystal phases are denoted as I, II, and III from the high-temperature side. The transition entropy ΔS for the phase change from I to III was estimated to be ca. 4.3 J K−1 mol−1 by dividing the enthalpy change by the average transition temperature (ca. 280 K). This value is comparable with R ln 2 = 5.8 J K−1 mol−1, which suggests that the successive transition is related to the order− disorder of the azetidinium cation.6 The overall temperature dependence of the 1H NMR T1 is depicted in Figure 3, and Figure 4 shows the detail near the phase transition region. The overall dependence can be

described in terms of the superposition of a relaxation due to molecular motion of the azetidinium cation and another relaxation due to lattice vibration and/or paramagnetic impurities that occur even at very low temperatures. Proton relaxation above 200 K is expected to be dominated by the cationic motion. To determine the possibility of ring-puckering motion as the origin of the 1H T1 minimum (0.12 s) observed around 340 K, the T1 minimum due to the ring-puckering motion of azetidinium cation was theoretically estimated using the Bloembergen−Purcell−Pound (BPP) formalism.14 The proton−proton dipole interaction is modulated by molecular motion; therefore, molecular motion can be an effective relaxation mechanism for NMR. The spin−lattice relaxation rate T1−1 due to molecular motion can be expressed as T1−1 =

⎛ ⎞ τ 2 4τ ⎟ ΔM 2⎜ + 2 2 2 2 3 1 + 4ω0 τ ⎠ ⎝ 1 + ω0 τ

(1)

where ω0, τ, and ΔM2 are the NMR angular frequency, the correlation time of molecular motion, and the reduction of the second moment of the NMR absorption line, respectively. According to eq 1, T1 takes a minimum value when ω0τ = 0.616, as given by 2 1.425 −1 = ΔM 2 T1,min ω0 3 (2) Therefore, the T1 minimum can be estimated for the molecular motion by calculating the second moment reduction ΔM2 due to the motion. The ΔM2 is defined as the difference between the rigid lattice and motionally averaged second moments. The second moment calculation was carried out based on the crystal structure reported for [(CH2)3NH2][Cu(HCOO)3] at 123 K,6 using the Van Vleck formula15 and the second-order spherical harmonic functions representation.16 In the calculation, only the proton−proton intramolecular contribution within the azetidinium cation was taken into account. For the rigid azetidinium cation, we obtained M2rigid = 32.5 G2, which is quite large compared to the value of 13.9 G2 reported for the cyclobutane ring by Hoch and Rushworth.8 This may be due to the rather short interproton distance in the azetidinium cation, as determined by single-crystal X-ray structural analysis. According to the crystal structure reported by Zhou et al.,6

Figure 3. Temperature dependence of the 1H NMR spin−lattice relaxation time T1 for [(CH2)3NH2][Zn(HCOO)3]. 12424



Article

crystal analysis, then the situation will be improved. On the basis of this discussion, we concluded that the ring-puckering motion of the azetidinium cation dominates spin−lattice relaxation in the high-temperature phase of [(CH2)3NH2][Zn(HCOO)3] above 299 K, considering the planar structure reported for the cation in the high-temperature phase of [(CH2)3NH2][Cu(HCOO)3].6 Assuming the Arrhenius relation 3 for the temperature dependence of the correlation time for ring-puckering motion of the azetidinium cation, the temperature dependence of 1H T1 in the phase transition region was analyzed using eq 1.

the range of interproton distance in the CH2 (or NH2) group is 1.49−1.61 Å, which is significantly shorter than the value of 1.84 Å for the CH2 group in cyclobutane, determined using electron diffraction.8,17 Therefore, a large calculated value is obtained. Considering the uncertainty of the hydrogen position determined by X-ray diffraction, it is highly possible that the calculated value based on the X-ray analysis is overestimated. This anticipation was supported by the experimental second moment of 22 ± 3 G2 determined below 200 K. Keeping this point in mind, we proceeded to estimate the T1 minimum due to ring-puckering motion. For ring-puckering motion, we assumed a motion between the configuration obtained at 123 K and the mirror-inversion configuration by the mean molecular plane (see Figure 5). For the motionally averaged second

⎛E ⎞ τ = τ∞exp⎜ a ⎟ ⎝ RT ⎠

(3)

From least-squares-fitting of the observed T1 minimum in phase I (T > 299 K), ΔM2 = 2.5 × 109 Hz2, τ∞ = 4.1 × 10−13 s, and Ea = 25 kJ mol−1 were obtained. The activation energy Ea can be assigned to that of the ring-puckering motion in the crystal and is much higher than the energy barrier reported for the ring-puckering motion of a four-membered ring molecule in the gas phase (i.e., 6 kJ mol−1 (518 cm−1) for cyclobutane18,19). However, this is not surprising, taking into account that the present case is for motion in a solid rather than in a gas or liquid phase. Below 299 K, the ring-puckering motion is expected to be frozen, as is the case for [(CH2)3NH2][Cu(HCOO)3]. We propose an order−disorder model for the phase transition. The potential curve for the ring-puckering motion can be assumed to be symmetric in phase I, as shown in Figure 6a. The energy barrier can be evaluated based on the activation energy. In the intermediate- and low-temperature phases of II and III, the ring-puckering occurs in an asymmetric potential well, as shown in Figure 6b,c. NMR relaxation due to molecular motion in an asymmetric potential well has been well established.20−23 The correlation time τ of molecular motion in the asymmetric potential well of Figure 6b is given by

Figure 5. Mean molecular plane of the azetidinium cation in [(CH2)3 NH2 ][Cu(HCOO) 3] at 123 K and the two related configurations.

moment by this motion, we obtained M2motion = 24.8 G2. Then, ΔM2 = M2rigid − M2motion = 7.7 G2 = 5.5 × 109 Hz2. Substituting this ΔM2 into eq 2 gave T1, min = 57 ms for the NMR frequency of 47.35 MHz. If we consider that a fraction of 8/11 of the total protons in the sample belong to the cation, while the observed relaxation rate is usually the average of the cationic and formate protons, then T1, min is expected to be 78 ms. However, if the contribution from intermolecular proton−proton dipolar interactions is taken into account, the minimum will become shorter. Figure 4 shows a T1, min of 0.12 s, which was observed at around 340 K using the Larmor frequency of 47.35 MHz. The observed value is longer than that expected; however, if the true interproton distance is longer than that derived from X-ray

τ=

⎛E ⎞ 1 τ∞ exp⎜ a ⎟ ⎝ RT ⎠ 1+a

(4)

where

⎛ ΔE ⎞ ⎟ a = exp⎜ ⎝ RT ⎠

(5)

and the spin−lattice relaxation rate is given by T1−1 ∝

⎛ ⎞ a τ 4τ ⎜ ⎟ + 2 2 2 2 2 (1 + a) ⎝ 1 + ω0 τ 1 + 4ω0 τ ⎠

(6)

Figure 6. Potential curves for the ring-puckering motion. The abscissa is the ring-puckering angle θ (the dihedral angle between the CNC plane and the other CCC plane of the azetidinium cation). 12425



Article

In the slow-motion limit of ω0τ ≫ 1 T1 ∝

⎛E ⎞ (1 + a)2 2 1+a 2 ω0 τ = ω0 τ∞ exp⎜ a ⎟ ⎝ RT ⎠ a a

(7)

Therefore, at a temperature for which a ≫ 1 can be assumed, the slope of the ln T1 vs 103 K/T plot is determined from the activation energy Ea, i.e., the energy barrier obtained for the stable configuration. Let us assume that, for a phase transition from I to II, an asymmetry ΔE is introduced by ΔE/2 stabilization of one configuration and ΔE/2 destabilization of the other configuration and that the potential energy at the transition state from one configuration to the other does not change. The activation energies Ea,I and Ea,II for phases I and II, respectively, are thus related by Ea,II = Ea,I + ΔE/2. The slope of the ln T1 vs 103 K/T plot (Figure 4) increased with decreasing temperature through Tc (I−II). The activation energy Ea,II = 33 kJ mol−1 was obtained from the slope in the temperature range 294−278 K, shown as the red line in Figure 4. Comparing this value with Ea,I = 25 kJ mol−1 obtained for phase I, the potential asymmetry can be estimated to be ΔE = 16 kJ mol−1. It is worth noting that the slope for phase II tends to decrease again with decreasing temperature, as shown by the blue line in Figure 4, from which Ea,II = 25 kJ mol−1 was obtained. This result suggests that the potential energy at the transition state also decreases with decreasing temperature (see Figure 6c). If the potential energy decreases further with decreasing temperature, then the potential will eventually change from a double to a single minimum potential. A very small disagreement in the T1 data was observed at the 254 K phase transition, which is dependent on the thermal history of the sample; measurements for increasing and decreasing temperature gave slightly different values, which corresponds to a hysteresis of ca. 15 K, as shown in Figure 4. In this hysteresis temperature region, the slopes of the ln T1 vs 103 K/T plots for phases II and III are parallel, which implies that the change in the potential curve through Tc (II−III) is small. The transition from a double to a single minimum potential will occur in phase III with further decrease in the temperature. In a broad single minimum potential well, as shown in Figure 6d, molecular libration will give a dominant relaxation mechanism. It was concluded that the potential energy for the transition state of ring-puckering decreases with decreasing temperature for phases II and III, and this may be related to the onset of the extraordinarily large dielectric anomaly in the temperature range of phases II and III (200−295 K). In the crystal, all azetidinium cations are crystallographically equivalent. However, there are two physically nonequivalent cations deformed in the opposite configuration of phases II and III. When an electric field is applied to the crystal, the field will represent a forward bias for half of the deformed cations and a reverse bias for the remaining half. An asymmetric double minimum potential with a low potential energy at the transition state may easily transform to a single minimum potential at the cationic site where the electric field acts as a forward bias, as shown in Figure 7. In contrast, the effect of the electric field on the potential curve may be negligible at the other cationic site. Thus, a local electric polarization may be induced by an electric field in the region where such a potential deformation occurs. The high susceptibility of the potential curve to the electric field is expected to be a cause for the extraordinarily large dielectric anomaly.

Figure 7. Expected potential change by application of an electric field.

To confirm these phase transitions and obtain structural information for the azetidinium cation, 14N NQR measurements were conducted using double resonance methods. The NQR data were obtained for phase III (162−211 K) by sample shuttling between two magnets and around the transition from phase II to phase I (295−317 K) using the FFC relaxometer. In the intermediate temperature region (211−295 K) and above, the proton spin−lattice relaxation time in the low magnetic field (ca. 10 mT) is shorter than the minimum shuttling time between the magnets; therefore, the double resonance spectrometer based on sample shuttling cannot be used. The quadrupole dips observed above 295 K with the FFC relaxometer disappear below room temperature. This is due to an increase of the proton spin−lattice relaxation rate in the range of the 14N NQR frequencies with respect to the 14N spin−lattice relaxation rates, so that the 14N contribution to the proton spin−lattice relaxation rate in resonance (νL = νQ) is decreased and the quadrupole dips are hidden within the noise. The cross-relaxation spectra of [(CH2)3NH2][Zn(HCOO)3] measured with the FFC relaxometer are presented in Figure 8.

Figure 8. Quadrupole dips in the cross-relaxation spectra of [(CH2)3NH2][Zn(HCOO)3] for phases I (317 and 309 K) and II (296 K).

Three dips at the 14N NQR frequencies ν+, ν−, and ν0 (ν+ > ν− > ν0) are observed with an additional dip at the frequency ν0/2, which is associated with two-quantum transitions in the proton spin system and single-quantum transitions in the 14N spin system. The intensities of the quadrupole dips decreased with decreasing temperature. The temperature dependence of the 14N NQR frequencies as determined using the double resonance technique is presented in Figure 9. The 14N NQR frequencies are dependent on two parameters: the quadrupole coupling constant e2qQ/h and the asymmetry parameter η of the electric-field-gradient (EFG) 12426



Article

Figure 9. Temperature dependence of the 14N NQR frequencies ν+, ν−, and ν0 for [(CH2)3NH2][Zn(HCOO)3].

Figure 10. Temperature dependence of the principal values of the quadrupole coupling tensor.

tensor V. The quadrupole coupling constant is a product of the 14 N nuclear electric quadrupole moment eQ and the largest (by magnitude) principal value eq of the EFG tensor divided by the Planck constant h. The EFG tensor V is a symmetric traceless second rank tensor composed of the second derivatives of the electrostatic potential produced at the position of the atomic nucleus by surrounding electric charges with respect to the coordinates. The EFG tensor has three principal values: VZZ = eq, VYY, and VXX (|VZZ| ≥ |VYY| ≥ |VXX|). The asymmetry parameter η is defined as η = (VXX − VYY)/VZZ, and it ranges between 0 and 1. The principal values of the quadrupole coupling tensor q, which is the product of the EFG tensor V and the nuclear electric quadrupole moment eQ, divided by the Planck constant h, can be determined using NQR. The principal values qZZ, qYY, and qXX of the quadrupole coupling tensor are calculated from the 14N NQR frequencies as

Figure 11. Ring-puckering motion of azetidinium ion in phase I.

perpendicular to the H−N−H plane. qXX does not change at Tc (I−II), which shows that the reorientation axis perpendicular to the H−N−H plane is parallel to the principal X-axis of the quadrupole coupling tensor. The reorientation angle can be estimated, assuming that the reorientation is fast on the NQR time scale, by taking the data just above Tc (I−II) and at the lowest temperature (162 K) where the NQR frequencies have been measured. At 162 K, qZZ = ± 1065 kHz and qYY = ∓ 730 kHz were obtained. At 308 K, ⟨qZZ⟩ = ± 795 kHz and ⟨qYY⟩ = ∓ 525 kHz were obtained. Assuming only reorientation around the principal X-axis for the angle ± α and no other thermal motions

qZZ = ±(2ν+ + 2ν−)/3 qYY = ±(2ν− − 4ν+)/3 qXX = ±(2ν+ − 4ν−)/3

⟨qZZ ⟩ = qZZ cos 2 α + qYY sin 2 α

(8)

(9)

Taking the obtained data into account, we obtain α = 23°. The angle α is overestimated and is approximately twice as large as that in cyclobutane in the gas phase18,19 or in the azetidinium cation of the [(CH2)3NH2][Cu(HCOO)3] crystal6 because the dihedral angle of puckering (∼2α) is reported to be 29−35° or 26°, respectively. The reason may be that other thermal motions were not taken into account, which results in an additional decrease in the principal values of the quadrupole coupling tensor with increasing temperature. In the temperature interval 162−308 K, qXX, which is independent of the reorientation, changes for 70 kHz. The changes of qZZ and qYY due to other thermal motions may be expected to be approximately the same size. The magnitudes of both increase with decreasing temperature. If α = 13° is assumed, as derived from the X-ray structural analysis, then ⟨qZZ⟩ = ± 974 kHz is obtained from eq 9 with qZZ = ± 1065 kHz and qYY = ∓ 730 kHz. This value is larger in magnitude by 179 kHz than the observed value of ⟨qZZ⟩ = ± 795 kHz at 308 K. This difference is approximately 2.6 times as large as the change of 70 kHz for qXX. However, this is possible because |qZZ/qXX| ≈ 3, i.e., the change in the principal value due to librations will be larger when the principal value is larger. Moreover, a libration around the X-axis is expected to be much more pronounced than those

2

The sign of the quadrupole coupling constant qZZ = e qQ/h cannot be determined from NQR. The absolute values of the principal values of the 14N quadrupole coupling tensor in [(CH2)3NH2][Zn(HCOO)3] are presented in Figure 10 as a function of temperature. An almost linear temperature dependence of qXX is evident through the three crystallographic phases. Such dependence is often observed in NQR and is mainly due to thermal motions, such as librations. The two other principal values, qZZ and qYY, exhibit a stronger change below 299 K, which may be the result of the ring-puckering motion that gradually freezes below Tc (I−II) = 299 K, which is concluded on the basis of the proton spin− lattice relaxation. The ring-puckering motion is presented in Figure 11. This motion produces a random exchange of the 14N quadrupole coupling tensor between two orientations. The reorientation axis is perpendicular to the plane of Figure 11. The 14N quadrupole coupling tensor is mainly of local origin and is produced in the present case by electrons in the C−N and N−H bonds. In a reasonable approximation, one of the principal axes (polar axis) passes the bisector of the C−N−C and H−N−H angles. The two other principal axes are perpendicular to the polar axis, and one of them is also perpendicular to the C−N−C plane, while the other is 12427



Article

around the Z- or Y-axes because the azetidinium cation is fixed mainly on the NH2 side.6,7 This will result in a larger decrease in magnitude for qZZ and qYY than for qXX due to libration. Taking into account the averaging effect due to librations, the effect of the puckering-motion is smaller than that assumed in the calculation, and also the angle α is smaller. The 14N NQR data support the conclusions obtained by analysis of the proton spin−lattice relaxation data.

(9) Wang, Z.; Zhang, B.; Otsuka, T.; Inoue, K.; Kobayashi, H.; Kurmoo, M. Dalton Trans. 2004, 2209−2216. (10) Seliger, J.; Blinc, R.; Arend, H.; Kind, R. Z. Phys. B 1976, 25, 189−195. (11) Stephenson, D.; Smith, J. A. S. Proc. R. Soc. A 1988, 416, 149− 178. (12) Seliger, J.; Ž agar, V. J. Magn. Reson. 2008, 193, 54−62. (13) Seliger, J.; Ž agar, V.; Blinc, R. J. Magn. Reson. 1994, A106, 214− 222. (14) Bloembergen, N.; Purcell, E. M.; Pound, R. V. Phys. Rev. 1948, 73, 679−712. (15) Abragam, A. The Principles of Nuclear Magnetism; Oxford University Press: London, U.K., 1961. (16) Sjöblom, R. J. Magn. Reson. 1976, 22, 411−424. (17) Dunitz, J. D.; Schomaker, V. J. Chem. Phys. 1952, 20, 1703− 1707. (18) Miller, F. A.; Capwell, R. J. Spectrochim. Acta 1971, 27A, 947− 956. (19) Laane, J. Pure Appl. Chem. 1987, 59, 1307−1326. (20) Soda, G. J. Jpn. Chem. 1974, 28, 799−813. (21) Andrew, E. R.; Latanowicz, L. J. Magn. Reson. 1986, 68, 232− 239. (22) Latanowicz, L.; Reynhardt, E. C. Mol. Phys. 1997, 90, 107−118. (23) Takeda, S.; Inabe, T.; Benedict, C.; Langer, U.; Limbach, H. Ber. Bunsenges. Phys. Chem. 1998, 102, 1358−1369.

4. CONCLUSIONS (1) Successive phase transitions were determined for [(CH2)3NH2][Zn(HCOO)3] from DSC measurements to be Tc (I−II) = 299 K and Tc (II−III) = 254 K. These phase transitions are expected to be related to order−disorder in the configuration of the azetidinium cation. (2) The ring-puckering motion of the azetidinium cation was detected using 1H NMR spin−lattice relaxation measurements. In phase I (T > 299 K), the activation energy of the motion was estimated to be 25 kJ mol−1. (3) In phases II and III, the inequivalence ΔE of the two azetidinium cation configurations increases. In the high temperature range of phase II, ΔE was estimated to be 16 kJ mol−1. With decreasing temperature in phase II, it is expected that the potential energy at the transition state of ringpuckering decreases, which results in a decrease of the activation energy for the transition from the stable to metastable configurations. (4) With a further decrease in temperature in phase III, the double minimum potential transforms into a single minimum potential. (5) The onset of the extraordinarily large dielectric anomaly may be related to the high susceptibility of the potential curve to the electric field.

■

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +81 3 5317 9739. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Thanks are due to Professors Hayao Kobayashi and Akiko Kobayashi for private communications prior to the publication of paper, Associate Professor Hiroki Fujimori for the use of the DSC instrument, and Mr. Shoichiro Takegami for sample preparation and identification. This work was supported by the Japan Ministry of Education, Culture, Sports, Science, and Technology (MEXT)-Supported Program for the Strategic Research Foundation at Private Universities, 2009−2013.

■

REFERENCES

(1) Lines, M. E.; Glass, A. M. Principles and Applications of Ferroelectrics and Related Materials; Clarendon Press: Oxford, U.K., 1977. (2) Blinc, R. Advanced Ferroelectricity; Oxford University Press: Oxford, U.K., 2011. (3) Zhang, W.; Xiong, R. G. Chem. Rev. 2012, 112, 1163−1195. (4) Xu, G.-C.; Ma, X.-M.; Zhang, L.; Wang, Z.-M.; Gao, S. J. Am. Chem. Soc. 2010, 132, 9588−9590. (5) Jain, P.; Dalal, N. S.; Toby, B. H.; Kroto, H. W.; Cheetham, A. K. J. Am. Chem. Soc. 2008, 130, 10450−10451. (6) Zhou, B.; Imai, Y.; Kobayashi, A.; Wang, Z.-W.; Kobayashi, H. Angew. Chem., Int. Ed. 2011, 50, 11441−11445. (7) Imai, Y.; Zhou, B.; Ito, Y.; Fujimori, H.; Kobayashi, A.; Wang, Z.M.; Kobayashi, H. Chem.Asian J. 2012, 7, 2786−2790. (8) Hoch, M. J. R.; Rushworth, F. A. Proc. Phys. Soc. 1964, 83, 949− 958. 12428


Phase Transition and Ring-Puckering Motion in a MetalâOrganic

Recommend Documents

Phase Transition and Ring-Puckering Motion in a MetalâOrganic