Structural changes, thermodynamic properties, ¹H magic angle spinning NMR, and ¹⁴N NMR of (NH₄)₂CuCl₄·2H₂O

Abstract

The structural changes and thermodynamic properties of (NH₄)₂CuCl₄·2H₂O were studied by differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. In addition, the chemical shift, line width, and spin-lattice relaxation time of the crystals were also investigated by ¹H magic angle spinning nuclear magnetic resonance (MAS NMR), focusing on the role of NH₄ and H₂O near the phase transition temperature. The change at T_C2 (=406 K) and T_C3 (=437 K) seems to be a chemical change caused by thermal decomposition rather than a physical change such as a structural phase transition. The changes in the temperature dependence of these data near T_C2 are related to variations in the environments surrounding NH₄ and H₂O. The ¹⁴N NMR spectrum is also measured in order to investigate local phenomena related to the phase transition.

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I. Introduction

A number of compounds with the chemical formula A₂BX₄·2H₂O, where A = NH₄, K, Rb, Cs and B = Cu, Mn, Ca, Ni are monovalent and divalent metal ions, respectively, and X = Cl, Br is a halide ion, crystallize as perovskite-type two-dimensional layered structures.^1–7 Crystals with this arrangement can be divided into two classes according to their symmetry and structure.^8–10 The first class includes compounds containing Cu²⁺ ions that crystallize with tetragonal symmetry with the space group P4₂/mnm at room temperature. The tetrahedrons surrounding the divalent metal ions placed at the corners of the unit cell are rotated by exactly 90° with respect to the tetrahedron surrounding the ion at the center of the cell. The A⁺ ions are placed in the almost cubic cavities formed by the tetrahedrons.^11,12 Crystals containing Mn²⁺, Ca²⁺, and Ni²⁺ ions form the second of these two classes with triclinic symmetry and the space group P₁. The (NH₄)₂CuCl₄·2H₂O crystal, which is an example of the former, exhibits a structural phase transition from the point group 4/mmm to the point group 4(bar)2m at 200 K.^13–15 In addition, two different phase transitions at approximately 383 K and 413 K were observed in the DC electric conductivity measurements reported by Narsimlu et al.¹⁶ At room temperature, (NH₄)₂CuCl₄·2H₂O forms a tetragonal structure with space group P4₂/mnm, as shown in Fig. 1.¹⁴ The unit cell contains two formula units and has the lattice constants a = b = 7.596 Å, c = 7.976 Å.¹⁷ The unit cell contains two Cu²⁺ ions at equivalent positions (0, 0, 0) and (1/2, 1/2, 1/2) each surrounded by an approximate octahedron of four Cl⁻ ions and two water molecules. The line connecting the water molecules is parallel to the crystallographic c-axis.¹ The water molecule is trigonal coordinated and forms two equivalent O–H⋯Cl(1) hydrogen bonds with an H⋯Cl(1) length of 2.186 Å. The O–H distance in the water molecule is 0.965 Å. The NH₄⁺ is hydrogen bonded to the Cl(1) atoms at 3.357 Å, while in the other orientation, it is bonded to the Cl(2) atoms at 3.370 Å. The respective N–H distances are 1.011 Å and 1.021 Å.¹⁷

Fig. 1 Tetragonal structure of the (NH₄)₂CuCl₄·2H₂O crystal.

The purpose of this study is to investigate the structural changes and thermodynamic properties of (NH₄)₂CuCl₄·2H₂O single crystals using differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. Further, the chemical shift, line width, and spin-lattice relaxation time T_1ρ in the rotating frame of (NH₄)₂CuCl₄·2H₂O are measured by ¹H magic angle spinning/nuclear magnetic resonance (MAS/NMR) near the phase transition temperature, focusing on the role of NH₄ and H₂O. In addition, the ¹⁴N NMR spectrum in the laboratory frame is measured as a function of the temperature, in an attempt to understand the structural geometry near the phase transition temperature. Our findings represent the first report on the thermodynamic properties and NMR characteristics of (NH₄)₂CuCl₄·2H₂O, and are useful for understanding the phase transitions.

II. Experimental method

Single crystals of (NH₄)₂CuCl₄·2H₂O were grown by slowly evaporating aqueous solutions containing a stoichiometric mixture of NH₄Cl and CuCl₂·2H₂O in the molar ratio of 2 : 1 at room temperature. The obtained crystals were light blue. The phase transition temperature was determined using a Dupont 2010 DSC instrument. The rate of temperature change during heating was 10 °C min⁻¹.

The ¹H MAS NMR spectra of (NH₄)₂CuCl₄·2H₂O in a rotating frame were obtained using the Bruker DSX 400 FT NMR spectrometer at the Korea Basic Science Institute, Western Seoul Center. The static magnetic field used was 9.4 T, and the central radio frequency was set at ω₀/2π = 400.13 MHz for the ¹H nucleus. The powder sample was placed in a 4 mm MAS probe, and the MAS rate was set to 5 kHz to minimize spinning sideband overlap. The spin-lattice relaxation times in the rotating frame were measured using a saturation recovery pulse sequence called sat–t–π/2; the nuclear magnetizations of the ¹H nuclei at time t after the sat pulse, a combination of one hundred π/2 pulses applied at regular intervals, were determined following the π/2 excitation pulse. The width of the π/2 pulse was 3.45 μs below 410 K and 6.7 μs above 420 K for ¹H.

In addition, the ¹⁴N NMR spectra of the (NH₄)₂CuCl₄·2H₂O single crystals in the laboratory frame were measured using a Unity INOVA 600 NMR spectrometer at the Korea Basic Science Institute, Western Seoul Center. The static magnetic field was 14.1 T, and the Larmor frequency was set to ω₀/2π = 43.342 MHz. The ¹⁴N NMR experiments were performed using the solid-state echo sequence: 3.7 μs–t–3.7 μs–t. The NMR measurements were obtained in the temperature range of 180–430 K. Unfortunately, the chemical shift and resonance frequency could not be measured above 430 K because the NMR spectrometer did not have adequate temperature control at high temperature. The temperatures of all the samples were maintained at constant values by controlling the helium gas flow and heater current, which yielded an accuracy of ±0.5 K.

III. Results and discussion

The structure of the (NH₄)₂CuCl₄·2H₂O crystals at room temperature was determined by X-ray diffraction (XRD) (PANalyical, X'pert Pro MPD) with a Cu-Kα (λ = 1.5418 Å) radiation source at the Korea Basic Science Institute, Western Seoul Center. Measurement was taken in a θ–2θ geometry from 10° to 60° at 45 kV and 40 mA tube power. The (NH₄)₂CuCl₄·2H₂O crystals were determined to have a tetragonal structure with the lattice constants a = b = 7.5991 Å, c = 7.9661 Å, α = β = γ = 90°. This result is consistent with those of previous studies.^1,17 The DSC analysis of the (NH₄)₂CuCl₄·2H₂O crystals revealed three endothermic peaks during heating, as shown in Fig. 2. The mass of the powdered samples used in the DSC experiment is 6.6 mg. The endothermic peak enlarged in Fig. 2 near 200 K (=T_C1) is consistent with the phase transition temperature reported previously, and is very small relative to the other endothermic peaks. The endothermic peaks near 406 K (=T_C2) and 437 K (=T_C3) are related to the thermal dehydration according to the below-mentioned loss of H₂O. TG analysis was used to determine whether these high-temperature transformations were structural phase transitions or melting temperatures. The TG curve of (NH₄)₂CuCl₄·2H₂O is shown in Fig. 3. The first occurrence of mass loss begins at approximately 364 K, and at 406 K, 96.75% of mass remains, yielding (NH₄)₂CuCl₄·1.5H₂O. The mass loss at 430 K and 437 K reaches 6.49% and 9.74% for (NH₄)₂CuCl₄·H₂O and (NH₄)₂CuCl₄·0.5H₂O, respectively. The bulk mass of (NH₄)₂CuCl₄·2H₂O decreases at 364 K (T_d), which is ascribed to the onset of partial thermal decomposition, and reaches complete thermal decomposition, thereby turning into (NH₄)₂CuCl₄, at approximately 443 K. The transformation anomalies at 406 (=T_C2) and 437 K (=T_C3) in the DSC experiment are related to the phase transitions from (NH₄)₂CuCl₄·2H₂O to (NH₄)₂CuCl₄·1.5H₂O and from (NH₄)₂CuCl₄·2H₂O to (NH₄)₂CuCl₄·0.5H₂O, respectively. Optical polarizing microscopy showed that the crystals are light blue in color at room temperature and that they undergo color changes as the temperature increases. As the temperature increases, the color of the crystal varies from light blue (295 K and 373 K) to light green (386 K) to green (393 K) to dark yellow (433 K) and finally to brown (448 K), as shown in the inset of Fig. 3. This color change may be related to the partial loss of H₂O. The DSC peaks at 406 K and 437 K are related to chemical changes through thermal dehydration, based on the TG and optical polarizing microscopy results. The weight loss evidenced by the TG curve suggests that T_C2 and T_C3 in (NH₄)₂CuCl₄·2H₂O are not related to physical changes such as structural phase transitions. Rather, they are related to a chemical change through thermal dehydration.

Fig. 2 DSC thermogram of (NH₄)₂CuCl₄·2H₂O upon heating.

Fig. 3 Thermogravimetric analysis (TGA) of (NH₄)₂CuCl₄·2H₂O (inset: color changes according to the temperature: (a) 295 K, (b) 373 K, (c) 386 K, (d) 393 K, (e) 433 K, and (f) 448 K).

The ¹H MAS NMR spectra of (NH₄)₂CuCl₄·2H₂O, which were obtained as a function of temperature, only exhibit one peak ascribed to chemical shift, as shown in Fig. 4. The spinning sidebands of the peak are marked with asterisks. There are two kinds of protons in (NH₄)₂CuCl₄·2H₂O: ammonium protons and water protons. The current experiment was unable to distinguish the signals resulting from these two types of protons, because the eight protons from ammonium and the four protons from water are expected to yield two superimposed lines. Thus, the signal generated by the ammonium protons might include the signal caused by the water protons.

Fig. 4 ¹H MAS NMR spectrum of (NH₄)₂CuCl₄·2H₂O at several temperatures.

The chemical shifts for the ¹H nuclei in (NH₄)₂CuCl₄·2H₂O with respect to tetramethylsilane (TMS) at a frequency of 400.13 MHz are presented in Fig. 5 as a function of temperature. The chemical shifts of the ¹H nucleus change abruptly near T_C2, whereas those near T_C1 are continuous. The change in the chemical shift with temperature indicates that the configuration of the atoms neighboring the ¹H nuclei is undergoing change. The full width at half maximum (FWHM) of the ¹H MAS NMR signal is shown in the inset of Fig. 5 as a function of temperature. As the temperature increases, the FWHM near T_C1 is continuous, and that near T_C2 decreases in a step-like shape. This stepwise narrowing is generally considered to be caused by internal motions, which have a temperature dependence related to that observed for the chemical shift. The shape of the line changes progressively with increasing temperature from the Gaussian-like shape of a rigid lattice to a Lorentzian shape. Near T_C2 the line width undergoes an abrupt drop, after which the line width becomes considerably narrower.

Fig. 5 Chemical shift of ¹H MAS NMR spectrum of (NH₄)₂CuCl₄·2H₂O as a function of temperature. (inset: line width of the ¹H MAS NMR spectrum of (NH₄)₂CuCl₄·2H₂O).

The nuclear magnetization recovery traces for ¹H MAS NMR are usually represented by a single-exponential function. However, the magnetization recovery traces for ¹H MAS NMR in (NH₄)₂CuCl₄·2H₂O could be described by the following double-exponential function:^18–20

M(t)/M(∞) = a exp(−t/T_1ρ(s)) + b exp(−t/T_1ρ(l)), (1)

where T_1ρ(s) and T_1ρ(l) are the short and long spin-lattice relaxation times, respectively. The magnetization recovery traces showing the delay time of the ¹H resonance signal at temperatures of 200 K, 420 K, and 430 K are shown in the inset of Fig. 6. The obtained spin-lattice relaxation curves were well fitted by the abovementioned double-exponential function, with the slope of the recovery trace decreasing as the temperature increased. Note that the occurrence of a double-exponential spin-lattice relaxation pattern is unusual for a strongly dipolar-coupled proton system, whereas spin diffusion is expected to afford a single-exponential relaxation pattern.²¹ Therefore, we concluded that the proton system comprises two spatially well-separated nuclei. The values of T_1ρ for ¹H in (NH₄)₂CuCl₄·2H₂O at several temperatures are shown in Fig. 6. As mentioned above, two different sets of T_1ρ values were obtained from the double-exponential function: the larger values, T_1ρ(l), correspond to the longer N–H⋯Cl chain systems in the NH₄ groups, whereas the smaller ones, T_1ρ(s), correspond to the shorter O–H⋯Cl chain systems in the H₂O groups. Here, T_1ρ(l) decreases slightly with increasing temperature, as opposed to T_1ρ(s), which increases with increasing temperature. Furthermore, T_1ρ(s) and T_1ρ(l) for ¹H in these groups were continuous in close proximity to T_C1, whereas T_1ρ(s) and T_1ρ(l) of ¹H in both the H₂O and NH₄ groups were discontinuous close to T_C2, corresponding to an abrupt increase with increasing temperature. The abrupt rise in T_1ρ near T_C2 contributes to the increased mobility of the protons in H₂O and NH₄. The changes observed for the ¹H nucleus near T_C2 are related to variations in the symmetry of the environments of H, i.e., to changes in the symmetry of the NH₄ and H₂O groups. The forms of the tetrahedrons of water molecules are probably disrupted by the loss of H₂O.

Fig. 6 Temperature dependences of the ¹H spin-lattice relaxation time in the rotating frame, T_1ρ(s) and T_1ρ(l) for H₂O and NH₄ in (NH₄)₂CuCl₄·2H₂O (inset: the saturation recovery traces for delay time for ¹H in (NH₄)₂CuCl₄·2H₂O).

The NMR spectra of ¹⁴N of (NH₄)₂CuCl₄·2H₂O single crystals were recorded in order to investigate local phenomena related to the phase transition. The ¹⁴N nucleus has a spin number 1. Spectra were obtained by the solid-state echo method using static NMR at a Larmor frequency of ω₀/2π = 43.342 MHz in the laboratory frame. The ¹⁴N NMR spectra of (NH₄)₂CuCl₄·2H₂O single crystals at 200, 300, and 400 K are shown in Fig. 7. Two resonance lines are expected because of the quadrupole interaction of the ¹⁴N (I = 1) nucleus.¹⁹ With respect to the crystal orientation, the magnetic field was applied along the crystallographic c-axis. The resonance frequencies of the ¹⁴N signals are plotted in Fig. 8, as a function of temperature. In the vicinity of T_C1, the frequencies of both signals are discontinuous, whereas those near T_C2 are continuous, and the resonance frequency increases with increasing temperature. In addition, the splitting of the ¹⁴N resonance lines above 200 K increases slightly with increasing temperature. These temperature-dependent changes in the ¹⁴N resonance frequencies are attributed to changes in the structural geometry of the NH₄⁺ ion.²² In all temperature, all nitrogen is physically equivalent, and the ¹⁴N quadrupole parameter is slowly increased.²³ In this case, the electric field gradient (EFG) tensors at the N sites are varied, reflecting the changing atomic configurations around the ¹⁴N nuclei in NH₄.

Fig. 7 ¹⁴N NMR spectrum of a (NH₄)₂CuCl₄·2H₂O single crystal at 200 K, 300 K, and 400 K.

Fig. 8 Resonance frequencies of ¹⁴N NMR spectrum of a (NH₄)₂CuCl₄·2H₂O single crystal as a function of temperature.

IV. Conclusion

The thermodynamic properties and structural mechanisms near the phase transition temperatures in (NH₄)₂CuCl₄·2H₂O were studied through DSC, TG, NMR chemical shift, and the spin-lattice relaxation time T_1ρ. The DSC and TG results indicate that the endothermic peak near 200 K (=T_C1) is a structural phase transition, and the endothermic peaks near 406 K (=T_C2) and 437 K (=T_C3) are related to a chemical change through thermal dehydration due to the escape of H₂O. On the other hand, the changes in the temperature dependence of the chemical shift, linewidth, and spin-lattice relaxation time T_1ρ near T_C2 are related to variations in the symmetry of the environments of NH₄ and H₂O. The mechanism of these changes at high temperature is related to hydrogen bond proton transfer involving the breakage of a weak hydrogen bond. The change at T_C2 and T_C3 seems to be a chemical change caused by thermal decomposition rather than a physical change such as a structural phase transition, and the tetrahedrons formed by the water molecules are probably disrupted by the loss of H₂O. From the DSC, TG, and NMR data, it is clear that the structural change at high temperature arises due to the loss of the two water molecules coordinated to the Cu²⁺ ion along the c-axis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Basic Science Research program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2016R1A6A1A03012069 and 2015R1A1A3A04001077).

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