Bloembergen Purcell Pound Research Articles

Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation.

The molecular dynamics study of thermotropic mesogens exhibiting the crystal phases is valuable in unraveling the complex global (collective) and local (noncollective) motions executed by liquid crystal molecules, which would further advance the existing knowledge on orientationally disordered crystalline (ODIC) phases. Toward the fulfillment of such a task, a combined nuclear magnetic resonance (NMR) relaxometry approach employing the fast field cycling (FFC) NMR (10 kHz-30 MHz) and high-field pulsed NMR (400 MHz) techniques is utilized to sample the broad frequency range offered by molecular motions in the crystal phase of 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). The validity of the observed relaxation data is tested and interpreted by the Bloembergen-Purcell-Pound (BPP) model involving the superposition of four mutually independent Lorentzian spectral densities, reflecting molecular dynamical processes on different time scales. The salient feature of the detailed analysis reveals that the lengthening of temporal dynamics in the crystal phase due to molecular rotations by jumps, which are of intermolecular origin, is evident and further supports the presence of collective-like local dynamics. The analysis does permit decoupling of the molecular reorientations about their short axes (∼100 ns) as well as long axes (∼50 ns) and methyl group rotations (∼0.5 ns) on distinct time scales. The activation energies for reorientations about the short axes and methyl group rotations are found to be 27.3 ± 2.7 and 15.8 ± 1.1 kJ/mol, respectively. The fast methyl rotations in the crystal phase of 6CHBT obtained from FFC NMR are further well complemented by high-field NMR, where 1H NMR line shapes are relatively narrow when compared to those of the nematic phase.

Exploring Molecular Dynamics of Adsorbed CO2 Species in Amine-Modified Porous Silica by Solid-State NMR Relaxation.

Previous studies on CO2 adsorbents have mainly addressed the identification and quantification of adsorbed CO2 species in amine-modified porous materials. Investigation of molecular motion of CO2 species in confinement has not been explored in depth yet. This work entails a comprehensive study of molecular dynamics of the different CO2 species chemi- and physisorbed at amine-modified silica materials through the determination of the rotating frame spin–lattice relaxation times (T1ρ) by solid-state NMR. Rotational correlation times (τC) were also estimated using spin relaxation models based on the Bloch, Wangsness, and Redfield and the Bloembergen–Purcell–Pound theories. As expected, the τC values for the two physisorbed CO2 species are considerably shorter (32 and 20 μs) than for the three identified chemisorbed CO2 species (162, 62, and 123 μs). The differences in molecular dynamics between the different chemisorbed species correlate well with the structures previously proposed. In the case of the physisorbed CO2 species, the τC values of the CO2 species displaying faster molecular dynamics falls in the range of viscous liquids, whereas the species presenting slower dynamics exhibit T1ρ and τC values compatible with a CO2 layer of weakly interacting molecules with the silica surface. The values for chemical shift anisotropy (CSA) and 1H–13C heteronuclear dipolar couplings have also been estimated from T1ρ measurements, for each adsorbed CO2 species. The CSA tensor parameters obtained from fitting the relaxation data agree with the experimentally measured CSA values, thus showing that the theories are well suited to study CO2 dynamics in silica surfaces.

Freezing of molecular rotation in a paramagnetic crystal studied by P31 NMR

We present a detailed $^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) study of the molecular rotation in the compound $[\mathrm{Cu}{(\mathrm{pz})}_{2}{(2\ensuremath{-}\mathrm{HOpy})}_{2}]{({\mathrm{PF}}_{6})}_{2}$, where $\mathrm{pz}={\mathrm{C}}_{4}{\mathrm{H}}_{4}{\mathrm{N}}_{2}$ and $2\ensuremath{-}\mathrm{HOpy}={\mathrm{C}}_{5}{\mathrm{H}}_{4}\mathrm{NHO}$. Here, a freezing of the ${\mathrm{PF}}_{6}$ rotation modes is revealed by several steplike increases of the temperature-dependent second spectral moment, with accompanying broad peaks of the longitudinal and transverse nuclear spin-relaxation rates. An analysis based on the Bloembergen-Purcell-Pound (BPP) theory quantifies the related activation energies as ${E}_{a}/{k}_{B}=250$ and 1400 K. Further, the anisotropy of the second spectral moment of the $^{31}\mathrm{P}$ absorption line was calculated for the rigid lattice, as well as in the presence of several sets of ${\mathrm{PF}}_{6}$ reorientation modes, and is in excellent agreement with the experimental data. Whereas the anisotropy of the frequency shift and enhancement of nuclear spin-relaxation rates is driven by the molecular rotation with respect to the dipole fields stemming from the Cu ions, the second spectral moment is determined by the intramolecular interaction of nuclear $^{19}\mathrm{F}$ and $^{31}\mathrm{P}$ moments in the presence of the distinct rotation modes.

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.

NMR 1H-1H Dipole Relaxation in Fluids: Relaxation of Individual 1H-1H Pairs versus Relaxation of Molecular Modes.

The intramolecular 1H NMR dipole-dipole relaxation of molecular fluids has traditionally been interpreted within the Bloembergen-Purcell-Pound (BPP) theory of NMR intramolecular relaxation. The BPP theory draws upon Debye's theory for describing the rotational diffusion of the 1H-1H pair and predicts a monoexponential decay of the 1H-1H dipole-dipole autocorrelation function between distinct spin pairs. Using molecular dynamics (MD) simulations, we show that for both n-heptane and water this is not the case. In particular, the autocorrelation function of individual 1H-1H intramolecular pairs itself evinces a rich stretched-exponential behavior, implying a distribution in rotational correlation times. However, for the high-symmetry molecule neopentane, the individual 1H-1H intramolecular pairs do conform to the BPP description, suggesting an important role of molecular symmetry in aiding agreement with the BPP model. The intermolecular autocorrelation functions for n-heptane, water, and neopentane also do not admit a monoexponential behavior of individual 1H-1H intermolecular pairs at distinct initial separations. We suggest expanding the autocorrelation function in terms of modes, provisionally termed molecular modes, that do have an exponential relaxation behavior. With care, the resulting Fredholm integral equation of the first kind can be inverted to recover the probability distribution of the molecular modes. The advantages and limitations of this approach are noted.

Probing relaxation models by means of Fast Field-Cycling relaxometry, NMR spectroscopy and molecular dynamics simulations: Detailed insight into the translational and rotational dynamics of a protic ionic liquid

A combination of temperature dependent Fast Field-Cycling (FFC) relaxometry and high-resolution NMR spectroscopy provides more than five orders of magnitude in frequency range for studying translational and rotational dynamics of ionic liquids. However, to make use of this broad frequency range, certain requirements have to be met: The viscosity of the liquid has to be of the right order of magnitude, and the material has to exhibit a sufficiently broad liquid range. In addition, NMR sensitive nuclei have to be present (ideally) on both, cation and anion, and appropriate relaxation models properly describing the molecular dynamics have to be available. For the latter, Molecular Dynamics (MD) simulations are ideally suited to suggest meaningful relaxation models. In this study we employ the protic ionic liquid triethylammonium bis(trifluoromethylsulfonyl)-imide [TEA][NTf2] as model compound to gain insight into details of the molecular dynamical processes. By addressing different NMR sensitive nuclei on both ions, 1H nuclei on the triethylammonium cation, and 19F on the NTf2 anion, we are able to obtain translational dynamics as well as rotational dynamics for both species at the same time. The obtained temperature dependent translatoric diffusion coefficients are consistent with our MD simulations, and are found to be in agreement with data reported in the literature. In addition, two types of NMR relaxation processes are employed to investigate the intramolecular relaxation: (1) dipolar relaxation at low frequencies (employing FFC) addressing 1H and 19F nuclei, and (2) quadrupolar relaxation at high frequencies addressing 2H nuclei. In near quantitative agreement with MD simulations, we show that both, dipolar relaxation obtained via FFC and quadrupolar relaxation obtained via high-field NMR are leading to consistent rotational correlation times, suggesting that the rotational motion of the cation is dominantly isotropic. Hence, the intramolecular 1H relaxation rate of the cation determined via FFC can be appropriately expressed by a single rotational correlation time, compatible with the Bloembergen Purcell Pound (BPP) approach. MD simulations of the NTf2 anion, however, suggest that its reorientational dynamics is strongly anisotropic. Consequently, the proper description of the intramolecular 19F relaxation rate of the anion requires a more complex model beyond the BPP approach employing at least two correlation times: By taking internal rotation of the CF3 groups into account, we are able to determine correlation times compatible with our MD simulations.

Thermodynamic, Physical, and Structural Characteristics in Layered Hybrid Type (C2H5NH3)2MCl4 (M = 59Co, 63Cu, 65Zn, and 113Cd) Crystals.

The thermal, physical, and molecular dynamics of layered hybrid type (C2H5NH3)2MCl4 (M = 59Co, 63Cu, 65Zn, and 113Cd) crystals were investigated by thermogravimetric analysis (TGA) and magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The temperatures of the onset of partial thermal decomposition were found to depend on the identity of M. In addition, the Bloembergen–Purcell–Pound curves for the 1H spin-lattice relaxation time T1ρ in the rotating frames of CH3CH2 and NH3, and for the 13C T1ρ of CH3 and CH2 were shown to exhibit minima as a function of the inverse temperature. These results confirmed the rotational motion of 1H and 13C in the C2H5NH3 cation. Finally, the T1ρ values and activation energies Ea obtained from the 1H measurements for the H‒Cl···M (M = Zn and Cd) bond in the absence of paramagnetic ions were larger than those obtained for the H‒Cl···M (M = Co and Cu) bond in the presence of paramagnetic ions. Moreover, the Ea value for 13C, which is distant from the M ions, was found to decrease upon increasing the mass of the M ion, unlike in the case of the Ea values for 1H.

Dynamics of NH3(CH2)2NH3 cation in perovskite layer crystal NH3(CH2)2NH3CuCl4 by M

We replaced Pb with nontoxic Cu metal for lead-free perovskite solar cells and investigated their potential for solar cell applications. The ionic dynamics of the NH3(CH2)2NH3CuCl4 crystal are investigated as a function of temperature, with a focus on the role of the NH3(CH2)2NH3+ cation. This investigation was conducted by 1H and 13C MAS NMR experiments. The contributions to the NMR chemical shifts are correlated to the distribution of the spin density in the ligand moiety. The 1H NMR chemical shift for NH3 changes more significantly with the temperature than that for CH2. This change in the cationic motion is enhanced at the N-end of the organic cation, which is fixed at the inorganic layer by N–H⋯Cl hydrogen bonds. In addition, the molecular motions are discussed in terms of the spin-lattice relaxation time, which reveal that the CH2 ion located in the middle of the N–C–C–N bond undergoes tumbling motion according to the Bloembergen–Purcell–Pound theory.

Thermal decomposition and structural dynamics in perovskite (C2H5NH3)2CdCl4 crystals

In this study, (C2H5NH3)2CdCl4 single crystals were grown, and their thermal properties were studied by the thermogravimetric analysis and differential scanning calorimetry. Phase transition temperature above room temperature was observed at 470 K. In addition, the cation dynamics in this crystal were investigated by 1H magic-angle spinning nuclear magnetic resonance (MAS NMR) and 13C cross-polarization (CP)/MAS NMR experiments. The Bloembergen–Purcell–Pound curves for 1H T1ρ and 13C T1ρ in C2H5NH3 cation exhibited minimum values, and these curves represent C2H5NH3 and C2H5 rotational motions. The activation energy for 1H in the C2H5NH3 cation is Ea = 22.63 kJ mol−1, whereas those for 13C in CH3 and CH2 are Ea = 18.05 and 19.14 kJ mol−1, respectively. Furthermore, the proton dynamics of C2H5NH3 undergo faster rotation than carbons. This implies that the molecular motion for 1H is enhanced at the C-end and N-end of an organic cation, whereas the molecular motion is not free at the main chain carbons of the organic cation.

Thermal and structural properties, and molecular dynamics in organic-inorganic hybrid perovskite (C2H5NH3)2ZnCl4.

The thermal and structural properties and molecular dynamics of layered perovskite-type (C2H5NH3)2ZnCl4 are investigated by differential scanning calorimetry, thermogravimetric analysis, and magic angle spinning nuclear magnetic resonance spectroscopy. The thermal properties and phase transitions are studied. Additionally, the Bloembergen–Purcell–Pound (BPP) curves for the 1H spin–lattice relaxation time T1ρ in the C2H5NH3 cation and for the 13C T1ρ in C2H5 are shown to have minima as a function of inverse temperature. This observation implies that these curves represent the rotational motions of 1H and 13C in the C2H5NH3 cation. The activation energies for 1H and 13C in the C2H5NH3 cation are obtained; the molecular motion of 1H is enhanced at the C-end and N-end of the organic cation, and that at the carbons of the main chain is not as free as that for protons at the C-end and N-end.

Study of H1 spin lattice relaxation and dielectric relaxation in Poly(propylene glycol) system

Proton NMR spin-lattice relaxation time (T1) and dielectric relaxation time (τ0) have been used to correlate the experimental and calculated values of T1 for Poly (propylene glycol) at 300 K. Also, the contributions of dielectric relaxation time (τ0) of each constituent group of PPG-425 to the effective dielectric relaxation time due to the whole chain of the polymer have been calculated using NMR spin–lattice relaxation time. The H1 NMR data and dielectric data of Poly (propylene glycol) HO [CH(CH3)CH2O]nH with number average molecular weight 425 (n = 7) g mol−1have been used to understand the conformation and basic mechanism of relaxation in polymer in its pure liquid state. Conformational analysis using Hartree-Fock formalism was performed for the polymer of n = 7 and the average bond distances obtained from the computation were used to assign NMR relaxation times to specific subgroups. Free energy and enthalpy of activation associated with methyl and other groups in the chain of PPG molecules have been calculated. BPP (Bloembergen Purcell Pound) model gives better correlation for molecular tumbling motion for PPG-425 in pure liquid state than Kubo & Tomita equation.

Low-field spin dynamics of Cr7Ni and Cr7Ni−Cu−Cr7Ni molecular rings as detected by μSR

Muon spin rotation measurements were used to investigate the spin dynamics of heterometallic ${\mathrm{Cr}}_{7}\mathrm{Ni}$ and ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ molecular clusters. In ${\mathrm{Cr}}_{7}\mathrm{Ni}$ the magnetic ions are arranged in a quasiplanar ring and interact via an antiferromagnetic exchange coupling constant $J$, while ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ is composed of two ${\mathrm{Cr}}_{7}\mathrm{Ni}$ linked by a bridging moiety containing one Cu ion, that induces an inter-ring ferromagnetic interaction ${J}^{\ensuremath{'}}\ensuremath{\ll}J$. The longitudinal muon relaxation rate $\ensuremath{\lambda}$ collected at low magnetic fields ${\ensuremath{\mu}}_{0}Hl0.15$ Tesla, shows that the two systems present differences in spin dynamics vs temperature. While both samples exhibit a main peak in the muon relaxation rate vs temperature, at $T\ensuremath{\sim}10$ K for ${\mathrm{Cr}}_{7}\mathrm{Ni}$ and $T\ensuremath{\sim}8$ K for ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$, the two compounds have distinct additional features: ${\mathrm{Cr}}_{7}\mathrm{Ni}$ shows a shoulder in $\ensuremath{\lambda}(T)$ for $Tl8$ K, while ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ shows a flattening of $\ensuremath{\lambda}(T)$ for $Tl2$ K down to temperatures as low as $T=20$ mK. The main peak of both systems is explained by a Bloembergen-Purcell-Pound (BPP)-like heuristic fitting model that takes into account of a distribution of electronic spin characteristic times for $Tg5$ K, while the shoulder presented by ${\mathrm{Cr}}_{7}\mathrm{Ni}$ can be reproduced by a BPP function that incorporates a single electronic characteristic time theoretically predicted to dominate for $Tl5$ K. The flattening of $\ensuremath{\lambda}(T)$ in ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ occurring at very low temperature can be tentatively attributed to field-dependent quantum effects and/or to an inelastic term in the spectral density of the electronic spin fluctuations.

The transport and conductivity properties of the ionic liquid EMIMTCM

The ionic liquid 1-ethyl-3-methylimidazolium tricyanomethanide (EMIMTCM) is a very interesting, yet so far poorly studied, ionic liquid which has a low viscosity, high conductivity, and acceptable electrochemical stability for use as an electrolyte. In this study the self-diffusion coefficients, spin–lattice relaxation times, and spin–spin relaxation times of EMIMTCM were measured over the temperature range 263 to 343K. To gain insight into the origin of the physical properties of EMIMTCM, the temperature dependence of the measured cation diffusion coefficient was compared with two structurally related EMIM-based ionic liquids which differed only in the choice of anion: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide (EMIMFSA) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)amine (EMIMTFSA). The experimentally determined diffusion coefficient, conductivity, and viscosity values were analysed using the Stokes–Einstein–Sutherland, Stokes–Einstein–Debye and Vogel–Fulcher–Tamman equations, respectively. The correlation time (τcation) of the cations were calculated from the relaxation data using the Bloembergen–Purcell–Pound equation. The overall isotropic molecular reorientational correlation time, τc, and translational correlation time, τD, of the cations were calculated from the viscosity and translational diffusion coefficients, respectively. The results of this study provide information on reorientational and translational motions of EMIMTCM and the correlation times were found to be in the following order τD>τc>τcation and EMIMTCM diffused faster than EMIMTFSA and EMIMFSA, due to the smaller size of the TCM anion and its lower viscosity.

Roles of chemically inequivalent N(CH3)4 ions in phase transition temperatures in [N(CH3)4]2CoCl4 by single-crystal NMR and MAS NMR

The temperature dependences of the 1H and 13C spin–lattice relaxation time in the laboratory frame, T1, and in the rotating frame, T1ρ, in [N(CH3)4]2CoCl4 were measured by static nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR. In the ferroelastic phase, 1H T1ρ underwent molecular motion according to the Bloembergen–Purcell–Pound theory. Two inequivalent ions, a-N(CH3)4 and b-N(CH3)4, were identified by 13C cross polarization (CP)/MAS NMR. On the basis of the 13C NMR results, the existence of two chemically inequivalent a-N(CH3)4 and b-N(CH3)4 ions in the ferroelectric phase and the existence of the ferroelastic twin structure of the N(CH3)4 ions in the ferroelastic phase were discussed.

Nuclear Spin Relaxation Characteristic of Submonolayer $$^3$$ 3 He Films in Nanochannels

In order to obtain information on dynamics of helium films in the nondegenerate fluid region, we have performed a pulsed-NMR experiment at 3.29 MHz on \(^3\)He films adsorbed in straight 2.4 nm channels of FSM silicates down to 0.54 K. In general, the spin-lattice and spin-spin relaxation times \(T_1\) and \(T_2\) were explained in terms of the two-dimensional Bloembergen–Purcell–Pound model for dipolar relaxation. Temperature dependences of \(T_1\) in submonolayer \(^3\)He films show a minimum, indicating that the dipolar-field correlation time \(\tau _\mathrm {c}\) is about \(\omega ^{-1}=4.8\times 10^{-8}\) s. The temperature \(T_\mathrm {min}\) of the \(T_1\) minimum monotonically lowers with increasing coverage, suggesting that \(^3\)He adatoms become more mobile at higher coverages. The low-dimensional property of \(^3\)He adatoms is observed as the separation of \(T_1\) and \(T_2\) above \(T_\mathrm {min}\) where \(\omega \tau _\mathrm {c}<1\). On the other hand, several features specific to films in the nanochannel geometry were also found. Especially, the temperature dependence of \(T_2\) becomes very small just below \(T_\mathrm {min}\) and shows a shoulder at lower temperatures. This anomaly has not been observed in \(^3\)He adsorbed in wider pores or on flat surfaces, so that it is considered to be characteristic of \(^3\)He films confined in narrow channels with a diameter of a few nm.

Temperature dependence of relaxation times and temperature mapping in ultra-low-field MRI

Ultra-low-field MRI is an emerging technology that allows MRI and NMR measurements in microtesla-range fields. In this work, the possibilities of relaxation-based temperature measurements with ultra-low-field MRI were investigated by measuring T1 and T2 relaxation times of agarose gel at 50μT–52mT and at temperatures 5–45°C. Measurements with a 3T scanner were made for comparison. The Bloembergen–Purcell–Pound relaxation theory was combined with a two-state model to explain the field-strength and temperature dependence of the data. The results show that the temperature dependencies of agarose gel T1 and T2 in the microtesla range differ drastically from those at 3T; the effect of temperature on T1 is reversed at approximately 5mT. The obtained results were used to reconstruct temperature maps from ultra-low-field scans. These time-dependent temperature maps measured from an agarose gel phantom at 50μT reproduced the temperature gradient with good contrast.

High-temperature behavior of NH4H2PO4 studied by single-crystal and MAS NMR

To confirm a high-temperature behavior of NH4H2PO4, the temperature dependences of the line-width, resonance frequency, and spin–lattice relaxation times in the laboratory frame, T1, and in the rotating frame, T1ρ, were investigated using a Fourier transform NMR spectrometer. The hydrogen bonds both in O–H–O between two PO4 groups and in N–H–O between NH4 and PO4 were distinguished, and the T1 values of both types of hydrogen-bond proton and 31P ions were described by the Bloembergen–Purcell–Pound theory. In addition, the T1ρ values of both types of hydrogen-bond proton and of 31P ions exhibited strong temperature dependences at high temperature; the changes in T1ρ at high temperature were related to variations in the symmetry.

Influence of magic angle spinning on T1H of SBR studied by solid state 1H NMR

We have investigated the influence of the high centrifugal pressure caused by fast magic-angle spinning (MAS) on the molecular motion of styrene–butadiene rubbers (SBR) filled with SiO2 (SBR/Si composite) though solid-state magic-angle spinning nuclear magnetic Resonance (1H MAS NMR) measurements. Because the 1H–1H dipolar interaction of elastomers is weak compared with that of glassy polymers, a narrower 1H linewidth is observed under fast MAS. The temperature dependence of the 1H spin-lattice relaxation time (T1H) revealed that the T1H minimum increases with the MAS rate. Furthermore, we observed a difference in the temperature dependence of T1H between end-chain-modified SBR and normal (unmodified) SBR in the SBR/Si composites. The temperature dependence of T1H is described by the Bloembergen–Purcell–Pound theory, with the assumption that the correlation time obeys the Williams–Landel–Ferry empirical theory. The fitting showed that the molecular motion does not change significantly until a MAS rate of 20 kHz, with the motional mode changing considerably at a MAS rate of 25 kHz. The motion of SBR in the unmodified SBR/Si composite was greatly affected by the fast MAS rates. Furthermore, the plot of the estimated centrifugal pressure versus the T1H minimum resembled the stress–strain curve. This result enables the detection of macroscopic physical deformation by the microscopic parameter T1H. Temperature-dependent curves of T1H for the SBR with and without end-chain modification in the SBR/Si composites showed the increment of the T1H values as the MAS rate increases and the considerable change at the fast MAS rate of 25 kHz. The increase of T1H values is represented by the factor f that decides the observed T1H minimum value. The plot of centrifugal pressure caused by MAS as a function of 1/f exhibited the similar trend to that of the stress–strain curve. The differences in the T1H between the end-chain-modified and normal SBR under fast MAS was attributed to the Payne effect.

^1H NMR Relaxation Study of Molecular Motion in the Paraelectric Phase of (NH4)2Cd2(SO4)3Single Crystals

The NMR spectrum and spin-lattice relaxation times, <TEX>$T_1$</TEX>, of the <TEX>$^{1}H$</TEX> nuclei in <TEX>$(NH_{4})_{2}Cd_{2}(SO_{4})_{3}$</TEX> single crystals were obtained. The two minima in <TEX>$T_1$</TEX> in the paraelectric phase are attributed to the reorientational motions of the <TEX>$NH_{4}^{+}$</TEX> groups. The <TEX>$^{1}H\;T_1$</TEX> of the <TEX>$(NH_{4})_{2}Cd_{2}(SO_{4})_{3}$</TEX> crystals can be described with Bloembergen- Purcell-Pound (BPP) theory. The experimental value of <TEX>$T_1$</TEX> can be expressed in terms of an isotropic correlation time <TEX>${\tau}_H$</TEX> for molecular motions by using the BPP theory, and determine the role of protons in these processes.

Nuclear Magnetic Resonance Relaxation Study of the Phase Transformations of LiNH4SO4and LiND4SO4Single Crystals: The Roles of Li, NH4and ND4Ions

The NMR spectrum and spin–lattice relaxation times, T 1 , of the 1 H, 2 H, and 7 Li nuclei in LiNH 4 SO 4 and LiND 4 SO 4 crystals were obtained, and from these measurements, the rotational correlation times were calculated. In addition, the activation energies for the reorientation of 1 H, 2 H, and 7 Li were obtained for each phase. For both types of crystal, each curve of T 1 versus inverse temperature exhibited a minimum value, which is attributed to the effects of reorientational motion. From the 1 H, 2 H, and 7 Li T 1 curves, we conclude that the relaxation process is affected by molecular motion, as described by the Bloembergen–Purcell–Pound theory. The differences in activation energy among the various phases indicate that the 1 H, 2 H, and 7 Li ions are significantly affected by the transitions between phases. The present results indicate that the contributions from the Li–O–S of the skeleton and the dipole moments of the ammonium groups change discontinuously at each phase transition temperature....