1H Spin-lattice Relaxation Rates Research Articles

Universal 1H Spin-Lattice NMR Relaxation Features of Sugar-A Step towards Quality Markers.

1H fast field-cycling and time-domain nuclear magnetic resonance relaxometry studies have been performed for 15 samples of sugar of different kinds and origins (brown, white, cane, beet sugar). The extensive data set, including results for crystal sugar and sugar/water mixtures, has been thoroughly analyzed, with a focus on identifying relaxation contributions associated with the solid and liquid fractions of the systems and non-exponentiality of the relaxation processes. It has been observed that 1H spin-lattice relaxation rates for crystal sugar (solid) vary between 0.45 s-1 and 0.59 s-1, and the relaxation process shows only small deviations from exponentiality (a quantitative measure of the exponentiality has been provided). The 1H spin-lattice relaxation process for sugar/water mixtures has turned out to be bi-exponential, with the relaxation rates varying between about 13 s-1-17 s-1 (for the faster component) and about 2.1 s-1-3.5 s-1 (for the slower component), with the ratio between the amplitudes of the relaxation contributions ranging between 2.8 and 4.2. The narrow ranges in which the parameters vary make them a promising marker of the quality and authenticity of sugar.

Anion and cation dynamics in a carbon-substituted cesium closo-hydroborate CsCB11H12: 1H and 133Cs NMR studies

Cesium monocarba-hydroborate CsCB11H12 belongs to the family of alkali-metal closo- hydroborate salts which are considered as promising solid electrolytes for battery applications. To study the dynamical properties of this compound at the microscopic level, we have measured the 1H and 133Cs nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 80–434 K. Our measurements have revealed a coexistence of two types of reorientational jump processes of the complex [CB11H12]− anions. For the faster of these processes characterized by the average activation energy of 310 meV, the anion reorientations are not “frozen” at the NMR frequency scale down to 120 K. The behavior of the 133Cs NMR line width in CsCB11H12 is consistent with the onset of long-range diffusive motion of Cs+ ions at the frequency scale of ∼104 s−1 above 340 K, and the distinct effects of the diffusive Cs+ jumps on both the 1H and 133Cs spin-lattice relaxation rates become observable above 380 K.

Dynamical properties of the magnesium borohydride – ethylenediamine compound Mg(en)1.2(BH4)2: A nuclear magnetic resonance study

The magnesium borohydride – ethylenediamine (en) compound Mg(en)1.2(BH4)2 exhibits high Mg-ion conductivity above room temperature. To study the dynamical properties of this compound and its partially deuterium-substituted counterpart, we have measured the 1H, 2D, and 11B nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over a wide temperature range (6–324 K). Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. For the fastest of these processes characterized by the activation energy of 47(3) meV, the unusually high reorientational mobility is retained down to low temperatures, so that the corresponding H jump rate is of the order of 108 s−1 near 70 K. The presence of fast reorientational motion of [BH4]− anions in Mg(en)1.2(BH4)2 supports the idea that anion reorientations may facilitate the cation diffusion. In the studied temperature range, we have not found any distinct signs of localized motion of the ethylenediamine molecules or their fragments at the NMR frequency scale.

Dynamical properties of lithium borohydride – ammine composite LiBH4·NH3: A nuclear magnetic resonance study

The lithium borohydride – ammine composite LiBH4·NH3 exhibits a high hydrogen density and combines [BH4]− anions and neutral NH3 molecules within the common crystal structure. To study the dynamical properties of this compound, we have measured the 1H, 11B, and 7Li nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 18 – 293 K. Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. One of these processes corresponds to extremely fast BH4 reorientations with the jump rate reaching ~108 s−1 already at 95 K; this fast process is characterized by the activation energy of 78(2) meV. The local coordination of [BH4]− anions in LiBH4·NH3 suggests that the fastest process is represented by rotations around a single 3-fold symmetry axis of the BH4 tetrahedron. The slower processes characterized by the activation energies of 152(5) meV, 216(7) meV, and 297(10) meV may be attributed to reorientations around other symmetry axes of the BH4 tetrahedron. In the studied temperature range up to 293 K, we have not found any signs of diffusive Li+ jumps in LiBH4·NH3 at the frequency scale of ~104 s−1 or higher.

Anion reorientations and cation diffusion in a carbon-substituted sodium nido-borate Na-7,9-C2B9H12: 1H and 23Na NMR studies

Abstract Polyhydroborate-based salts of lithium and sodium have attracted much recent interest as promising solid-state electrolytes for energy-related applications. A member of this family, sodium dicarba-nido-undecahydroborate Na-7,9-C2B9H12 exhibits superionic conductivity above its order-disorder phase transition temperature, ∼360 K. To investigate the dynamics of the anions and cations in this compound at the microscopic level, we have measured the 1H and 23Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 148–384 K. It has been found that the transition from the low-T ordered to the high-T disordered phase is accompanied by an abrupt, several-orders-of-magnitude acceleration of both the reorientational jump rate of the complex anions and the diffusive jump rate of Na+ cations. These results support the idea that reorientations of large [C2B9H12]− anions can facilitate cation diffusion and, thus, the ionic conductivity. The apparent activation energies for anion reorientations obtained from the 1H spin-lattice relaxation data are 314 meV for the ordered phase and 272 meV for the disordered phase. The activation energies for Na+ diffusive jumps derived from the 23Na spin-lattice relaxation data are 350 and 268 meV for the ordered and disordered phases, respectively.

Glassy and Polymer Dynamics of Elastomers by 1H-Field-Cycling NMR Relaxometry: Effects of Fillers.

1H spin–lattice relaxation rate (R1) dispersions were acquired by field-cycling (FC) NMR relaxometry between 0.01 and 35 MHz over a wide temperature range on polyisoprene rubber (IR), either unfilled or filled with different amounts of carbon black, silica, or a combination of both, and sulfur cured. By exploiting the frequency–temperature superposition principle and constructing master curves for the total FC NMR susceptibility, χ″(ω) = ωR1(ω), the correlation times for glassy dynamics, τs, were determined. Moreover, the contribution of polymer dynamics, χpol″(ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χglass″(ω), well represented by the Cole–Davidson spectral density. Glassy dynamics resulted moderately modified by the presence of fillers, τs values determined for the filled rubbers being slightly different from those of the unfilled one. Polymer dynamics was affected by the presence of fillers in the Rouse regime. A change in the frequency dependence of χpol″(ω) at low frequencies was observed for all filled rubbers, more pronounced for those reinforced with silica, which suggests that the presence of the filler particles can affect chain conformations, resulting in a different Rouse mode distribution, and/or interchain interactions modulated by translational motions.

Rotational correlation times, diffusion coefficients and quadrupolar peaks of the protic ionic liquid ethylammonium nitrate by means of 1H fast field cycling NMR relaxometry

The protic ionic liquid (PIL) ethylammonium nitrate (EAN) is characterized by hydrogen bonding between the NH bonds of the cations and the oxygen atoms of the anions. The three-dimensional H-bond network of EAN very much resembles that of water. Although this first ionic liquid is known for more than 100 years, there is still lack of information about the rotational and translational dynamics, and how both are related to each other. For that purpose, we measured the 1H nuclear magnetic relaxation dispersion curves (NMRD) of EAN by means of Fast Field Cycling (FFC) NMR relaxometry. We were able to analyze the measured 1H spin-lattice relaxation rates R1 covering a large temperature range between 248 and 333 K simultaneously. Applying the well-known Bloembergen-Purcell-Pound (BPP) approach, we decomposed the total relaxation rates into intramolecular and intermolecular contributions, describing the rotational and translational dynamics of the ethylammonium cation. Here, we report rotational correlation times, τR, and translational diffusion coefficients, DT, both derived from the total 1H spin-lattice relaxation rates as a function of temperature and frequency. Self-diffusion coefficients, DT, were also determined from the total relaxation rates in the low frequency range, wherein only the intermolecular relaxation contribution is frequency dependent. Then, DT results from the slope of a linear fit of the spin-lattice relaxation rate R1 as a function of the square root of frequency, ν. The diffusion coefficients obtained from both methods are in good agreement with self-diffusion coefficients measured by pulsed-field-gradient (PFG) NMR. The rotational correlation times, τR, derived from the intramolecular 1H relaxation contribution were compared to correlation times obtained from high-field NMR deuteron relaxation experiments, dielectric spectroscopy (DS) and femto-second-infrared (fs-IR) spectroscopy. For EAN, we also observed a local enhancement of the 1H spin-lattice relaxation rates R1 at high frequencies. This so-called quadrupole relaxation enhancement (QRE) results from the interference of the Zeeman transition energy of the 1H spins and the energy difference of two levels of the quadrupole 14N nuclei. QRE is usually known for solids, but rarely for the liquid state as observed here for EAN. Overall, we show that FFC relaxometry provides access to rotational correlation times, translational diffusion coefficients and quadrupole relaxation enhancement at the same time.

Glassy and Polymer Dynamics of Elastomers by 1H Field-Cycling NMR Relaxometry: Effects of Cross-Linking.

1H spin lattice relaxation rate (R1) dispersions were acquired by field-cycling (FC) NMR relaxometry between 0.01 and 35 MHz over a wide temperature range on polyisoprene (IR), polybutadiene (BR), and poly(styrene-co-butadiene) (SBR) rubbers, obtained by vulcanization under different conditions, and on the corresponding uncured elastomers. By exploiting the frequency–temperature superposition principle, χ″(ωτs) master curves were constructed by shifting the total FC NMR susceptibility, χ″(ω) = ωR1(ω), curves along the frequency axis by the correlation times for glassy dynamics, τs. Longer τs values and, correspondingly, higher glass transition temperatures were determined for the sulfur-cured elastomers with respect to the uncured ones, which increased by increasing the cross-link density, whereas no significant changes were found for fragility. The contribution of polymer dynamics, χpol″(ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χglass″(ω), well represented using a Cole–Davidson spectral density. For all elastomers, χpol″(ω) was found to represent a small fraction, on the order of 0.05–0.14, of the total χ″(ω), which did not show a significant dependence on cross-link density. In the investigated temperature and frequency ranges, polymer dynamics was found to encompass regimes I (Rouse dynamics) and II (constrained Rouse dynamics) of the tube reptation model for the uncured elastomers and only regime I for the vulcanized ones. This is clear evidence that chemical cross-links impose constraints on chain dynamics on a larger space and time scale than free Rouse modes.

NMR Relaxometry and Diffusometry Analysis of Dynamics in Ionic Liquids and Ionogels for Use in Lithium-Ion Batteries.

We have investigated the charge transport dynamics of a novel solid-like electrolyte material based on mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM] TFSI) and various concentrations of lithium salt bis(trifluoromethylsulfonyl)imide (LiTFSI) confined within a SiO2 matrix, prepared via a sol-gel method. The translational diffusion coefficients of BMIM+, TFSI-, and Li+ in ILs and confined ILs (ionogels, IGs) with different concentrations of lithium salt have been measured at variable temperatures, covering the 20-100 °C range, using nuclear magnetic resonance (NMR) pulsed field gradient diffusion spectroscopy. The mobility of BMIM+, TFSI-, and Li+ was found to increase with the [BMIM] TFSI/LiTFSI ratio, exhibiting an almost liquid-like mobility in IGs. Additionally, the effect of confinement on IL rotational dynamics has been analyzed by measuring 1H, 19F, and 7Li spin-lattice relaxation rate dispersions of IGs at different temperatures, using fast field-cycling NMR relaxometry. The analysis of the experimental data was performed assuming the existence of two fractions of the liquid: a bulk fraction (at least several ionic radii from the silica particles) and a surface fraction (close to the silica particles) and using two different models based on translational and rotational diffusion and reorientation mediated by translational displacements. The existence and weighting of these two fractions of ions were obtained from the direct diffusion measurements. The results show that the ion dynamics slowed only modestly under confinement, which evidences that IGs preserve IL transport properties, and this behavior is an encouraging indication for using IGs as a solid electrolyte for Li+ batteries.

Proton dynamics in antiferroelectric CsH3(SeO3)2 single crystal investigated by 1H NMR measurements

1H nuclear magnetic resonance (NMR) measurements have been performed to study the proton dynamics associated with the antiferroelectric transition of a hydrogen-bonded single crystal of CsH3(SeO3)2. Herein, 1H NMR spectrum, shift, linewidth, and spin-lattice relaxation rate 1/T1 are measured in the temperature range of 80–296 K with the c-axis parallel to a magnetic field of ~4.85 T. The spectrum exhibits a composite structure with two narrow peaks at 296 K; at a low temperature, this structure evolves into a single broad shape with three humps. This complex shape and evolution are deconvoluted into five or six components based on the number of inequivalent and disordered hydrogen sites. By estimating the chemical shift and linewidth for each proton site, we identify all peaks. The spin-lattice relaxation recovery exhibits a double-exponential behavior with two relaxation times, short T1S and extremely large T1L. Both T1S and T1L follow Arrhenius behavior. From the respective 1/T1(T), the activation energies for proton motion are measured to be small: 1.16 ± 0.1 and 0.83 ± 0.06 kJ/mol for T1S and T1L, respectively. While the static NMR data, chemical shift and linewidth, show no evidence for the transition, the dynamic data 1/T1L highlights a clear increase across TN = 145 K, which is possibly a signature of the transition.

Nuclear magnetic resonance study of atomic motion in the mixed borohydride-amide Li2(BH4) (NH2)

In order to study the reorientational motion of the anions and the translational diffusion of Li+ cations in the borohydride-amide Li2(BH4) (NH2) showing fast-ion conduction, we have measured the 1H, 7Li, and 11B NMR spectra and spin-lattice relaxation rates over the temperature range of 80–341 K. Our measurements have revealed a coexistence of several motional processes in this mixed-anion compound. The fastest processes correspond to two thermally-activated types of BH4 reorientations. These reorientations give rise to two peaks of the 1H, 7Li, and 11B spin-lattice relaxation rates observed near 220 K and 290 K; they are characterized by the activation energies of 196 (2) meV and 498 (5) meV, respectively. For each of the two processes, the reorientational jump rate reaches ∼108 s−1 near the corresponding peak temperature. The diffusive jump motion of Li+ ions is found to be much slower than both reorientational processes; it leads to the 7Li NMR line narrowing above 300 K that indicates the onset of Li+ jumps at the frequency scale of ∼104 s−1. As the temperature approaches the melting point of Li2(BH4) (NH2), Tm ≈ 365 K, we have also observed the onset of diffusive motion of H-containing species at the same frequency scale.

Comprehensive Picture of Water Dynamics in Nafion Membranes at Different Levels of Hydration.

1H NMR spectroscopy is employed to study the long-range (diffusion) and short-range (relaxation) motions of water in the hydrophilic channels of Nafion at different stages of hydration, λwater. From Bloembergen, Purcell, and Pound analysis of the temperature-dependent 1H spin-lattice relaxation rates, the coexistence of two motional modes for λwater < 9 was observed, which can be attributed to the nonfreezing behavior of water molecules. At higher hydration levels, a clear transition between two different motional modes was detected associated with the freezing of bulk water. The hydration-level-dependent diffusion 1H NMR studies showed a steep increase in diffusion coefficients at 6 ≤ λwater ≤ 9 followed by a linear increase with the increasing hydration level (λwater ≥ 12). Taken together the correlation of long- and short-range motion studies allowed us to establish a comprehensive picture of the hydration shell formation for the sulfonic acid groups in Nafion dependent on λwater that can be divided into three characteristic regions. These include the formation of the first hydration shell at λwater ≈ 3 (region I), the second hydration shell at λwater ≈ 8 (region II), and finally (iii) the presence of bulk water above λwater ≥ 10 (region III).

Comparison of anion and cation dynamics in a carbon-substituted closo-hydroborate salt: 1H and 23Na NMR studies of solid-solution Na2(CB9H10)(CB11H12)

The hexagonal mixed-anion solid solution Na2(CB9H10)(CB11H12) shows the highest room-temperature ionic conductivity among all known Na-ion conductors. To study the dynamical properties of this compound, we have measured the 1H and 23Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates in Na2(CB9H10)(CB11H12) over the temperature range of 80–435 K. It is found that the diffusive motion of Na+ ions can be described in terms of two jump processes: the fast localized motion within the pairs of tetrahedral interstitial sites of the hexagonal close-packed lattice formed by large anions and the slower jump process via octahedral sites leading to long-range diffusion. Below 350 K, the slower Na+ jump process is characterized by the activation energy of 353(11) meV. Although Na+ mobility in Na2(CB9H10)(CB11H12) found from our NMR experiments is higher than in other ionic conductors, it appears to be an order-of-magnitude lower than that expected on the basis of the conductivity measurements. This result suggests that the complex diffusion mechanism and/or correlations between Na+ jumps should be taken into account. The measured 1H spin-lattice relaxation rates for Na2(CB9H10)(CB11H12) are consistent with a coexistence of at least two anion reorientational jump processes occurring at different frequency scales. Near room temperature, both reorientational processes are found to be faster than the Na+ jump process responsible for the long-range diffusion.

Molecular Dynamics and Hyperpolarization Performance of Deuterated β-Cyclodextrins.

We discuss the temperature dependence of the 1H and 13C nuclear spin-lattice relaxation rate 1/ T1 and dynamic nuclear polarization (DNP) performance in β-cyclodextrins with deuterated methyl groups. It is shown that 13C DNP-enhanced polarization is raised up to 10%. The temperature dependence of the buildup rate for nuclear spin polarization and of 1/ T1, below 4.2 K, is analyzed in the framework of the thermal mixing regime and the origin of the deviations from the theoretical behavior discussed. 13C 1/ T1 is determined at low temperature by the glassy dynamics and at high temperature by the rotational molecular motions of the deuterated methyl groups. Thanks to deuteration, relaxation times approaching 30 s are achieved at room temperature, making this material interesting for molecular imaging applications. The effect of molecular dynamics on the line width of the NMR spectra is also discussed.

Nuclear magnetic resonance study of anion and cation dynamics in CsSiH3

In order to study the dynamical properties of cesium silanide CsSiH3, we have measured the 1H and 133Cs nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates in this compound over the temperature range of 5–354 K. The results of the 1H NMR measurements indicate that [SiH3]‒ anions in CsSiH3 retain unusually high reorientational mobility down to low temperatures. In particular, a significant narrowing of the proton NMR spectrum due to [SiH3]‒ reorientations is observed in the range ∼ 10–14 K. The order-disorder (β→α) phase transition accompanied by the strong acceleration of [SiH3]‒ reorientations and by the change in the reorientational mechanism is observed above 200 K; according to the 1H spin-lattice relaxation data, this transition appears to be gradual. For the high-temperature disordered α-phase, the activation energy for [SiH3]‒ reorientations is found to be 41(4) meV. The 133Cs NMR results are consistent with the onset of diffusive motion of Cs+ cations at the frequency scale of ∼104 s−1 above 300 K.

Field-cycling NMR relaxometry: the benefit of constructing master curves

ABSTRACTThe currently available field-cycling NMR relaxometers still suffer from a rather narrow frequency range; they facilitate measurements of the 1H spin–lattice relaxation rate in the frequency of 10 kHz–30 MHz. This limit may be overcome by constructing master curves via exploiting frequency-temperature superposition, the latter being an intrinsic feature of the collective dynamics in many soft matter systems. It states that the shape of the motional time-correlation function is essentially temperature independent over a large temperature interval. As will be demonstrated, master curves may be built in the susceptibility or the spectral density representation of spin–lattice relaxation data. As a result, the effective frequency range is expanded up to ten decades, and the applied shift factors provide the temperature dependence of the corresponding correlation time. Three examples are presented: cyano adamantane in its plastically crystalline phase, the liquid glycerol, and the melt of poly(ethylene propylene). Advantages and limitations of the approach are discussed.

Static Modulation Wave of Arrays of Halogen Interactions Transduced to a Hierarchy of Nanoscale Change Stimuli of Crystalline Rotors Dynamics.

Here we present a study where what can be seen as a static modulation wave encompassing four successive arrays of interacting iodine atoms in crystalline 1,4-Bis((4'-(iodoethynyl)phenyl) ethynyl)bicyclo[2,2,2]octane rotors changes the structure from one-half molecule to three-and-a-half molecules in the asymmetric unit below a phase transition at 105 K. The remarkable finding is that the total 1H spin-lattice relaxation rate, T1-1, of unprecedented complexity to date in molecular rotors, is the weighted sum of the relaxation rates of the four contributing rotors relaxation rates, each with distinguishable exchange frequencies reflecting Arrhenius parameters with different activation barriers ( Ea) and attempt frequencies (τo-1). This allows us to show in tandem with rotor-environment interaction energy calculations how the dynamics of molecular rotors are able to decode structural information from their surroundings with remarkable nanoscale precision.

Nuclear magnetic resonance study of hydrogen dynamics in Al(BH4)4-based hypersalts M[Al(BH4)4] (M = Na, K, Rb, Cs)

In order to study the dynamical properties of the novel series of complex hydrides M[Al(BH4)4] (M = Na, K, Rb, Cs), we have measured the 1H and 11B spin-lattice relaxation rates and the 1H nuclear magnetic resonance spectra in these compounds over broad temperature ranges (6–384 K) and resonance frequency ranges (14–90 MHz). For all the studied compounds, the behavior of the spin-lattice relaxation rates is governed by the reorientational motion of BH4 groups. For Na[Al(BH4)4], the temperature dependences of the measured 1H and 11B spin-lattice relaxation rates suggest a coexistence of two reorientational processes with different characteristic jump rates and the activation energies of 186(7) and 262(9) meV. For K[Al(BH4)4], Rb[Al(BH4)4], and Cs[Al(BH4)4], the relaxation data are satisfactorily described by the model with a Gaussian distribution of the activation energies and the average activation energies of 393(6), 360(5), and 353(5) meV, respectively. The barriers for reorientational motion in M[Al(BH4)4] are discussed on the basis of changes in the local environment of BH4 groups.

Molecular dynamics of 1-ethyl-3-methylimidazolium triflate ionic liquid studied by 1H and 19F nuclear magnetic resonances.

The molecular dynamics of an ionic liquid (IL) composed of a 1-ethyl-3-methylimidazolium cation and a triflate (trifluoromethanesulfonate) anion, abbreviated as [Emim][TfO], were studied by NMR spectroscopy. By measuring the temperature-dependent high-field 1H and 19F spin-lattice relaxation (SLR) rates, the frequency-dependent 1H and 19F SLR dispersion curves using fast-field-cycling relaxometry, and the temperature-dependent 1H and 19F diffusion constants, and by utilizing the fact that the primary NMR-active nucleus on the Emim cation is 1H, whereas on the TfO anion it is 19F, the cationic and anionic dynamics were studied separately. A single theoretical relaxation model successfully reproduced all the experimental data of both types of resonant nuclei by fitting all the data simultaneously with the same set of fit parameters. Upon cooling, [Emim][TfO] exhibited a supercooled liquid phase between TSL = 256 K and the crystallization temperature TCr ≈ 227-222 K, as confirmed by differential scanning calorimetry (DSC) experiments. Theoretical analysis revealed that within the liquid and the supercooled liquid states of [Emim][TfO], the 1H and 19F relaxation rates are affected by both the rotational and translational diffusional processes with no discontinuous change at TSL. While the rotational diffusion is well described as an Arrhenius thermally activated process, the translational diffusion undergoes strong freezing dynamics that are well described by the Vogel-Fulcher model assuming a freezing temperature of T0 = 157 K. The existence of the supercooled liquid region in the [Emim][TfO] IL should be taken into account when using this IL for a specific application.

Effects of partial halide anion substitution on reorientational motion in NaBH4: A nuclear magnetic resonance study

To study the effects of partial halide anion substitution on the reorientational motion of [BH4]− anions in NaBH4, we have measured the 1H and 11B NMR spectra and spin–lattice relaxation rates in the cubic solid solutions Na(BH4)0.5Cl0.5 and Na(BH4)0.5I0.5 over broad ranges of temperature (8–324K) and the resonance frequency (14–90MHz). For both solid solutions, the measured 1H spin–lattice relaxation rates are governed by reorientations of BH4 groups, and the experimental data can be satisfactorily described by the model with a Gaussian distribution of the activation energies. The average values of the activation energies derived from the 1H spin–lattice relaxation data are 192±7meV for Na(BH4)0.5Cl0.5 and 120±3meV for Na(BH4)0.5I0.5. At a given temperature, the reorientational jump rates are found to increase in the order of Na(BH4)0.5Cl0.5–NaBH4–Na(BH4)0.5I0.5, in agreement with the quasielastic neutron scattering (QENS) results. This trend correlates with the lattice expansion reflecting the increase in the size of the corresponding anions (in the order of Cl−–[BH4]−–I−).