Vibrations and reorientations of H2O molecules in [Sr(H2O)6]Cl2 studied by Raman light scattering, incoherent inelastic neutron scattering and proton magnetic resonance

doi:10.1016/j.saa.2014.01.054

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Volume 124, 24 April 2014, Pages 429-440

https://doi.org/10.1016/j.saa.2014.01.054 Get rights and content

Highlights

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Calculated FT-IR, Raman and neutron scattering spectra compared with experimental results.
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Molecular reorientations in [Sr(H₂O)₆]Cl₂ at phase transition studied by RS.
•
Dynamics of H₂O ligands is disturbed in the phase transition.
•
¹H NMR results revealed two kinds structurally not equivalent water molecules in the crystal lattice [Sr(H₂O)₆]Cl₂.

Abstract

Vibrational–reorientational dynamics of H₂O ligands in the high- and low-temperature phases of [Sr(H₂O)₆]Cl₂ was investigated by Raman Spectroscopy (RS), proton magnetic resonance (¹H NMR), quasielastic and inelastic incoherent Neutron Scattering (QENS and IINS) methods. Neutron powder diffraction (NPD) measurements, performed simultaneously with QENS, did not indicated a change of the crystal structure at the phase transition (detected earlier by differential scanning calorimetry (DSC) at $T_{C}^{h} = 252.9 K$ (on heating) and at $T_{C}^{c} = 226.5 K$ (on cooling)). Temperature dependence of the full-width at half-maximum (FWHM) of ν_s(OH) band at ca. 3248 cm⁻¹ in the RS spectra indicated small discontinuity in the vicinity of phase transition temperature, what suggests that the observed phase transition may be associated with a change of the H₂O reorientational dynamics. However, an activation energy value (E_a) for the reorientational motions of H₂O ligands in both phases is nearly the same and equals to ca. 8 kJ mol⁻¹. The QENS peaks, registered for low temperature phase do not show any broadening. However, in the high temperature phase a small QENS broadening is clearly visible, what implies that the reorientational dynamics of H₂O ligands undergoes a change at the phase transition. ¹H NMR line is a superposition of two powder Pake doublets, differentiated by a dipolar broadening, suggesting that there are two types of the water molecules in the crystal lattice of [Sr(H₂O)₆]Cl₂ which are structurally not equivalent average distances between the interacting protons are: 1.39 and 1.18 Å. However, their reorientational dynamics is very similar (τ_c = 3.3 ⋅ 10⁻¹⁰ s). Activation energies for the reorientational motion of these both kinds of H₂O ligands have nearly the same values in an experimental error limit: and equal to ca. 40 kJ mole⁻¹. The phase transition is not seen in the ¹H NMR spectra temperature dependencies. Infrared (IR), Raman (RS) and inelastic incoherent neutron scattering (IINS) spectra were calculated by the DFT method and quite a good agreement with the experimental data was obtained.

Graphical abstract

Introduction

Hexaaquastrontium chloride, with formula [Sr(H₂O)₆]Cl₂, is a particularly interesting molecular material because of occurrence of phase transition and fast reorientational motions of the H₂O ligands. At room temperature, (RT), hexaaquastrontium chloride crystallises in the trigonal system (P321 space group, No = 150) with unit cell parameter: a = b = 7.9596 Å, c = 4.1243 Å and one molecule per unit cell. There are two kinds o water molecules: H(1)-O(1)–H(1) and H(2)–O(2)–H(2), which are coordinated differently to Sr²⁺ cations. Each O(1) adjoining only one Sr²⁺ and each O(2) shared between two Sr²⁺ cations. Thus, there are two different metal–oxygen distances: Sr–O(1) = 2.570 and Sr–O(2) = 2.715 Å [1]. The ninefold coordination polyhedron can be described with six O(2) and three O(1) atoms. Exists a helical system of hydrogen bonding involves three H(1) and three H(2) atoms (arising from these two kinds of water molecules), and the Cl⁻ anions [2]. Each of Cl⁻ anion takes part in three of these bonding systems, thus forming six hydrogen bonds. The crystal structure of [Sr(H₂O)₆]Cl₂ is consistent with earlier X-ray diffraction studies [2], [3] and it is isostructural (isotopic) with [Ca(H₂O)₆]Cl₂ [4]. Spectroscopic studies [5], [6] also confirm that [Sr(H₂O)₆]Cl₂ includes two sets of differently bonded, but not much distorted H₂O molecules.

We have studied the melting and thermal decomposition of [Sr(H₂O)₆]Cl₂ using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) [7]. The gaseous products of the title compound’s decomposition process were identified online by a quadruple mass spectrometer (QMS). The thermal decomposition of the title compound proceeds in two main stages. In first stage dehydration of hexaaquastrontium chloride to strontium chloride undergoes in three steps and all (2/6, 3/6 and 1/6) of H₂O molecules are liberated. In the second stage the investigated anhydrous SrCl₂ remains unchanged up to ca. 900 K.

The phase transition of [Sr(H₂O)₆]Cl₂ was detected using differential scanning calorimetry (DSC), Fourier transform middle and far infrared absorption (FT-MIR and FT-FIR) spectroscopes [7]. One phase transition (PT) at $T_{C}^{h} = 252.9 K$ (on heating) and at $T_{C}^{c} = 226.5 K$ (on cooling) was detected by DSC for [Sr(H₂O)₆]Cl₂ in 123–295 K range. Thermal hysteresis of this PT is relatively large and equals to 26.4 K. Entropy change (ΔS) value at this first-order type phase transition equals to ca. 1.5 J mol⁻¹ K⁻¹. The temperature dependences of the full width at half maximum (FWHM) values of the infrared bands associated with ρ_t(H₂O)E and δ_as(HOH)E modes (at ca. 417 and 1628 cm⁻¹, respectively) suggest that the observed phase transition is associated with a sudden change of a speed of the H₂O reorientational motions. The H₂O ligands in the high temperature phase reorientate fast (correlation times 10⁻¹¹–10⁻¹³ s) with the activation energy of ca. 2 kJ mol⁻¹. Below $T_{C}^{c}$ probably a part of the H₂O ligands stopped their fast reorientation, while the remainders continued their fast reorientation, but with the activation energy of ca. 8 kJ mol⁻¹. Far and middle infrared spectra indicated characteristic changes at the vicinity of PT with decreasing of temperature, which suggested lowering of the crystal structure symmetry. Splitting of the band connected with v_as(OH) mode (at ca. 3600 cm⁻¹) near the $T_{C}^{c}$ suggested lowering of the crystal lattice symmetry. All these facts suggest that the discovered PT is connected both with a change of the reorientational dynamics of the H₂O ligands and with a change of the crystal structure [7].

The aim of the present study is to find connections between the previously recorded phase transition [7] and eventual changes in the rate of stochastic reorientational motions of the H₂O ligands and/or of the crystal structure, by means of Raman light scattering (RS), inelastic/quasi-elastic incoherent neutron scattering (IINS/QENS) and neutron powder diffraction (NPD) methods. IINS/QENS methods may inform us about fast (correlation time τ_c: 10⁻¹²–10⁻¹³ s) reorientational molecular motions. NPD method can find the connection of PT with the changes in the crystal structure. We would like also to compare the results obtained by the Raman scattering spectroscopy with the data obtained earlier [7] by Fourier transform middle and far-infrared spectroscopy (FT-MIR and FT-FIR). We hope that these studies make it possible to obtain more precise picture of the phase polymorphism of the title compound. Moreover, employed to these investigations also ¹H NMR method we would like to confirm additionally the existence of the two kinds of water molecules in [Sr(H₂O)₆]Cl₂ which are both structurally and dynamically not equivalent.

Section snippets

Experimental section

The sample of the title compound investigated in the present study was employed earlier in DSC and IR measurements [7]; the details of the synthesis and chemical analysis of the examined compound were also described in that paper.

Raman spectra were recorded on a MultiRAM FT-Raman spectrometer equipped with a 1064 nm laser line (laser power set on 250 mW) and with a germanium detector. All spectra were collected in a 4000–50 cm⁻¹ range, with 4 cm⁻¹ resolution, and a total of 64 scans were

Computational details

The main goal of these calculations was to obtain harmonic vibrational frequencies, which could be used to support the interpretation of the experimental spectra. The optimization of the geometry and frequency calculations were carried out with Gaussian 09 package, Revision C.01 [9], implemented on the ZEUS computer in the Academic Computer Centre, Cyfronet AGH, Kraków. The calculations were performed for two models: the first (named as Model 1) was based on the isolated equilibrium model of

Molecular structure and vibrational spectra

The equilibrium geometries computed with B3LYP/LANL2DZ ECP/6-311 + G(d,p) level of theory are shown in Fig. 1a and b for Model 1 and 2, respectively. Model 1 belongs to the C_s point group, possessing only σ mirror plane, whereas Model 2 has the C₂ point group. The symmetry is in agreement with the high-temperature crystalline phase [1].

In the crystal structure of [Sr(H₂O)₆]Cl₂ hydrogen bonds exist. In our simple model employed in the calculation the hydrogen bond interactions were not introduced

Conclusions

1.
The neutron diffraction pattern for [Sr(H₂O)₆]Cl₂ at 294 K is nearly exactly the same as that at 210 K, which implies that the phase transition at $T_{C}^{h} = 252.9 K$ (on heating) and at $T_{C}^{c} = 226.5 K$ (on cooling) does not display a structural character. The space group (P321, No. = 150) is the same for the high and low temperature phases.
2.
Optical spectra calculated by the DFT method (B3LYP functional, LANL2DZ ECP basis set (on Sr atom) and 6-311 + G(d,p) basis set (on H and O atoms) for the isolated [Sr(H₂O)₉]²⁺

Acknowledgements

The research was carried out with the equipment (FT-IR and NMR spectrometers) purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). This research was also supported in part by PL-Grid Infrastructure.

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Temperature accuracy was equal to ±0.1 K. The incoherent inelastic/quasielastic neutron scattering spectra and neutron powder diffraction (NPD) patterns were measured using the time-of-flight method on a NERA spectrometer [9] at the high flux pulsed reactor IBR-2 in Dubna (Russia) at temperatures: 5, 200, 250 and 295 K. Experimental details are the same as in our previous paper [10]. Spin-lattice relaxation time T1 of a polycrystalline sample of [Ca(H2O)2](ReO4)2 was measured by a pulse NMR spectrometer working at 25 MHz using the saturation method.
A ¹H nuclear magnetic resonance study of [Ca(H₂O)₂](ReO₄)₂ has confirmed the existence of one phase transition at $T_{C}^{h} = 261.2 K$ (on heating) and $T_{C}^{c} = 231.2 K$ (on cooling), detected earlier by the DSC method. These transitions were reflected by changes in the temperature dependences of second moments of NMR line M₂^H. The study revealed anisotropic reorientations of whole [Ca(H₂O)₂]²⁺ cations, and the reorientations by 180° jumps of H₂O ligands. It was found that the phase transition at $T_{C}$ is associated with the reorientation of H₂O signed by oxygen O10 in crystal structure in high temperature phase. The oscillations of the cation as a whole unit around the axis crossing Ca and the middle of O9–O10 distance take place in both phases. From the temperature dependence of the full-width at half-maximum values (FWHM) of the bands connected with ρ_r (H₂O) mode in IR spectra, we can conclude that the reorientational motions of H₂O ligands contribute to the phase transition mechanism. The ligands perform fast (τ_R ≈ 10⁻¹¹ to 10⁻¹³ s) stochastic reorientational motions in the high temperature phase. The estimated mean value of activation energy for the reorientation of the H₂O ligands is ca. 11.1 kJ mol from Raman spectroscopy (RS) and 11.6 kJ mol from infrared spectroscopy, which is consistent with NMR results.
The proton-weighted phonon density functions G(ν) obtained at 5 K show some separate peaks characteristic for ordered phase. Additionally, IR, RS and IINS spectra were calculated by the DFT method and an excellent agreement with the experimental data was obtained using CASTEP code.
Phase transition, structure and reorientational dynamics of H <inf>2</inf> O ligands and ReO <inf>4</inf>− anions in [Ba(H <inf>2</inf> O) <inf>3</inf> ](ReO <inf>4</inf> ) <inf>2</inf> ⋅H <inf>2</inf> O
2019, Journal of Molecular Structure
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The G(ν) functions were calculated in the one-phonon-approximation from the time-of- flight IINS spectra [31]. The procedure for calculating the function G(ν) is described in the paper [32]. The calculated spectra have not been scaled.
One, low-temperature phase transition (PT) was detected for [Ba(H₂O)₃](ReO₄)₂⋅H₂O, in the temperature range of 135–295 K by means of differential scanning calorimetry (DSC). The mean value of the PT temperature is $T_{C}^{h}$  = 275.0 K (onset on heating) and $T_{C}^{c}$  = 235.5 K (onset on cooling). Sharpness of the DSC peak as well as associated with this transformation hysteresis indicate that the detected PT is of the first-order type. X-ray single crystal diffraction (XRSCD) and neutron powder diffraction (ND) results showed that during the PT discovered at $T_{C}^{c}$ the crystal structure of the studied compound does not change. Vibrational and reorientational motions of H₂O ligands and ReO₄⁻ anions, in the high (I) and low (II) temperature phases, were investigated by Fourier transform middle-infrared and Raman light scattering spectroscopies (FT-MIR and RS). The temperature dependences of the full-width at half-maximum values (FWHM) of the band associated with δ(OReO) mode at 350 cm⁻¹ in Raman spectra, suggests that the observed PT is not associated with a change of the perrhenate reorientational dynamics. However, the FT-MIR measurements showed that in the observed PT the reorientation dynamics of H₂O ligands do contribute to the PT mechanism. Below the temperature of 225 K, a constant value of FWHM is observed, demonstrating the inhibition of reorientation of H₂O molecules, and is associated only with so-called vibration relaxation. QENS measurements furnished evidence that H₂O motion in high temperature phase (phase I) can be fairly accurately described by a simple model of 180° jumps around a twofold axis within a picosecond time scale with an assumption that three from four H₂O molecules in [Ba(H₂O)₃]²⁺⋅H₂O unit perform fast stochastic reorientations. Below PT QENS broadening is not observed. The QENS and IR methods clearly confirm that the observed PT is connected with slowing down of the reorientation dynamics of water molecules. The density functional theory plane wave calculations of the normal modes were performed in order to support band assignment. A good agreement between calculated and experimental data (IR and RS spectra) was obtained.
Vibrational and reorientational dynamics and thermal properties in [Mg(H<inf>2</inf>O)<inf>4</inf>](ReO<inf>4</inf>)<inf>2</inf> supported by periodic DFT study
2018, Vibrational Spectroscopy
Citation Excerpt :
This behaviour suggests that the H2O ligands perform fast stochastic reorientations (τR ≈ 10−11-10−13 s) in both phases of [Mg(H2O)4](ReO4)2 with the average energy: Ea(I/II) ≈ 8.50 ± 0.37 kJmol−1. This is a general feature for many aqua complexes of divalent metal cations [1,3,4,40,41]. Fig. 8(b) demonstrate temperature dependence of the peak position of the band at 345 cm−1.
The polymorphism of the [Mg(H₂O)₄](ReO₄)₂ compound was investigated for the first time by us by means of differential scanning calorimetry (DSC). The measurements were performed in the temperature range of 295–130 K on cooling and heating of the sample at different rates. One reversible phase transition of the investigated compound has been found at: $T_{c}^{h}$ = 285.1 K (onset on heating) and $T_{c}^{c}$ = 256.5 K (onset on cooling). The large thermal hysteresis of the phase transition temperature T_c equal to ca. 28.6 K and the heat flow anomaly sharpness suggest that the detected phase transition is a first-order one. The moderate entropy change (ΔS ≈ 4.3 J mol⁻¹ K⁻¹) connected with observed phase transition indicates some kind of dynamical disorder of the high temperature phase.
X-ray single crystal and neutron powder diffraction results revealed that the phase transition discovered at $T_{C}^{c}$ is associated with a small change of the crystal structure.
Vibrational and reorientational motions of H₂O ligands and ReO₄⁻ anions, in the high (I) and low (II) temperature phases, were investigated by Fourier transform far and middle-infrared and Raman light scattering spectroscopies (FT-IR and RS). The temperature dependences of the full-width at half-maximum values (FWHM) of the bands associated with: 2δ(H₂O) mode (at 3236 cm⁻¹ in IR spectra) and ν_as(HO) mode (at 3463 cm⁻¹ in Raman spectra), suggest that the observed phase transition is not associated with a change of the H₂O reorientational motions. Analysis of the FWHM vs temperature of the Raman band at 345 cm⁻¹ connected with the δ(OReO) mode, indicate that the observed phase transition is not connected with a change of the ReO₄⁻ reorientational dynamics. From the IR and RS band shape analysis it was found that H₂O ligands and ReO₄⁻ anions perform fast (correlation time scale, τ_R ≈ 10⁻¹¹–10⁻¹³ s) motions in the both phases and their reorientational motions do not contribute to the phase transition mechanism. The estimated mean value of activation energy for H₂O ligands and $Re O_{4}^{-}$ anions in the both phases (high and low) is: E_a(I/II) = 8.50 ± 0.37 kJmol⁻¹ and E_a(I/II) = 6.78 ± 0.37 kJmol⁻¹, respectively. The density functional theory plane wave calculations of the normal modes within the periodic boundary conditions (CASTEP code) were also performed in order to support band assignment. We have obtained good agreement between calculated and experimental data (IR and RS spectra).
Raman light scattering, infrared absorption and neutron scattering studies of the phase transition and reorientational dynamics of H<inf>2</inf>O ligands and ClO<inf>4</inf>- anions in [Ca(H<inf>2</inf>O)<inf>4</inf>](ClO<inf>4</inf>)<inf>2</inf>
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Citation Excerpt :
Temperature accuracy was equal to 0.1 °C. The incoherent inelastic/quasielastic neutron scattering spectra and neutron powder diffraction (NPD) patterns were measured using the time-of-flight method on a NERA spectrometer [8] at the high flux pulsed reactor IBR-2 in Dubna (Russia) at temperatures: 20.5, 100, 180, 225 and 290 K. Experimental details are the same as in our previous paper [9]. The main goal of these calculations was to obtain harmonic vibrational wavenumbers, which could be used to support the interpretation of the experimental IR, RS and IINS spectra.
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Thermal, structural, and spectroscopic characterization of an ionic [Cu(H<inf>2</inf>O)<inf>4</inf>](ReO<inf>4</inf>)<inf>2</inf> compound in a wide temperature range
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental section

Computational details

Molecular structure and vibrational spectra

Conclusions

Acknowledgements

References (32)

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Proc. Nucl. Phys. Solid State Phys. Symp.

J. Chem. Phys.

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Cited by (9)

Complex water dynamics in crystalline [Ca(H<inf>2</inf>O)<inf>2</inf>](ReO<inf>4</inf>)<inf>2</inf>, studied by the vibrational spectroscopy and proton magnetic resonance

Phase transition, structure and reorientational dynamics of H <inf>2</inf> O ligands and ReO <inf>4</inf><sup>−</sup> anions in [Ba(H <inf>2</inf> O) <inf>3</inf> ](ReO <inf>4</inf> ) <inf>2</inf> ⋅H <inf>2</inf> O

Vibrational and reorientational dynamics and thermal properties in [Mg(H<inf>2</inf>O)<inf>4</inf>](ReO<inf>4</inf>)<inf>2</inf> supported by periodic DFT study

Raman light scattering, infrared absorption and neutron scattering studies of the phase transition and reorientational dynamics of H<inf>2</inf>O ligands and ClO<inf>4</inf><sup>-</sup> anions in [Ca(H<inf>2</inf>O)<inf>4</inf>](ClO<inf>4</inf>)<inf>2</inf>

Thermal, structural, and spectroscopic characterization of an ionic [Cu(H<inf>2</inf>O)<inf>4</inf>](ReO<inf>4</inf>)<inf>2</inf> compound in a wide temperature range

Dynamics of H<inf>2</inf>O ligands and ReO<inf>4</inf><sup>−</sup> anions at the phase transition in [Mn(H<inf>2</inf>O)<inf>2</inf>](ReO<inf>4</inf>)<inf>2</inf> studied by complementary spectroscopic methods