NMR Relaxation Measurements Research Articles

Characterization of water in hydrated Bombyx mori silk fibroin fiber and films by 2H NMR relaxation and 13C solid state NMR.

The mechanical properties of Bombyx mori silk fibroin (SF) change remarkably upon hydration. However, the microscopic interaction between SF and water is not currently well understood on a molecular level. We were able to identify four distinct components in the relaxation times for water in SF fiber by 2H solution NMR relaxation and exchange measurements. In addition, the effects of hydration on the conformation of Ser and Tyr residues in the site-specific crystalline and non-crystalline domains of 13C selectively labeled SF, respectively, could be determined independently. Thus, our measurements provide new insight relating the characteristics of water and the hydration structure of silk, which are relevant in light of current interest in the design of novel silk-based biomaterials.

Effect of solvent model when probing protein dynamics with molecular dynamics

We probe the dynamics of the Bpti and Galectin-3 proteins using molecular dynamics simulations employing three water models at different levels of resolution, viz. the atomistic TIP4P-Ewald, the coarse-grained Elba and an implicit generalised Born model. The dynamics are quantified indirectly by model-free order parameters, S2 of the backbone NH and selected side-chain bond vectors, which also have been determined experimentally through NMR relaxation measurements. For the backbone, the order parameters produced with the three solvent models agree to a large extent with experiments, giving average unsigned deviations between 0.03 and 0.06. For the side-chains, for which the experimental data is incomplete, the deviations are considerably larger with mean deviations between 0.13 and 0.17. However, for both backbone and side-chains, it is difficult to pick a winner, as all models perform equally well overall. For a more complete set of side-chain vectors, we resort to analysing the variation among the estimates from different solvent models. Unfortunately, the variations are found to be sizeable with mean deviations between 0.11 and 0.15. Implications for computational assessment of protein dynamics are discussed.

Step‐growth thiol–thiol photopolymerization as radiation curing technology

ABSTRACTThis report introduces a novel UV‐curing technology based on thiol–thiol coupling for polydisulfide network formation. Beginning with a model tris(3‐mercaptopropionate) trithiol monomer and xanthone propionic acid‐protected guanidine as photobase generator, a comprehensive characterization based on spectroscopic techniques supports the reaction of thiols into disulfides without side reactions. The best experimental conditions are described as regards to film thickness, irradiance, emission wavelength, and atmosphere composition. The results shed light on a step‐growth photopolymerization mechanism involving two steps: first, the formation of thiyl radicals by thiolate air oxidation or/and thiol photolysis, and second, their recombination into disulfide. By varying thiol functionality and structure, oligomer chain length and monomer/oligomer ratio, the network architecture can be finely tuned. The molecular mobility of the polydisulfide network is crucial to high thiol conversion rates and yields as revealed by 1H T2 NMR relaxation measurements. Ultimately, spatial control enables the formation of a photopatterned poly(disulfide) film, used as next‐generation high refractive index photoresist. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 117–128.

Ion Dynamics in a Mixed-Cation Alkoxy-Ammonium Ionic Liquid Electrolyte for Sodium Device Applications.

The ion dynamics in a novel sodium-containing room-temperature ionic liquid (IL) consisting of an ether-functionalised quaternary ammonium cation and bis(trifluoromethylsulfonyl)amide [NTf2 ] anion with various concentrations of Na[NTf2 ] have been characterised using differential scanning calorimetry, impedance spectroscopy, diffusometry and NMR relaxation measurements. The IL studied has been specifically designed to dissolve a relatively large concentration of Na[NTf2 ] salt (over 2 mol kg-1 ) as this has been shown to improve ion transport and conductivity. Consistent with other studies, the measured ionic conductivity and diffusion coefficients show that the overall ionic mobility decreases with decreasing temperature and increasing salt content. NMR relaxation measurements provide evidence for correlated dynamics between the ether-functionalised ammonium and Na cations, possibly with the latter species acting as cross-links between multiple ammonium cations. Finally, preliminary cyclic voltammetry experiments show that this IL can undergo stable electrochemical cycling and could therefore be potentially useful as an electrolyte in a Na-based device.

Band Ordering and Dynamics of Cu2–xTe and Cu1.98Ag0.2Te

63Cu, 65Cu, and 125Te NMR measurements are reported for Cu2–xTe and Cu1.98Ag0.2Te. The results demonstrate an onset of Cu-ion hopping below room temperature, including an activation behavior consistent with high-temperature transport measurements but with a significant enhancement of the hopping barriers with Ag substitution. We also separated the Korringa behavior by combining NMR line shape and relaxation measurements, thereby identifying large negative chemical shifts for both nuclei, as well as large Cu and Te s-state contributions in the valence band. Further comparison was obtained through heat capacity measurements and chemical shifts computed by density functional methods. The large diamagnetic chemical shifts coincide with behavior previously identified for materials with topologically nontrivial band inversion, and in addition, the large metallic shifts point to analogous features in the valence band density of states, suggesting that Cu2Te may have similar inverted features.

Micellar crowding and branching in a versatile catanionic system

The catanionic system didodecyldimethylammonium bromide (DDAB)-sodium taurodeoxycholate (STDC)-D2O is characterized by an exceptionally extended L1 region. The comparison of self-diffusion coefficient of the solvent and the DDAB embedded in the micelles provided information about hydration of the aggregates. Moreover, correlating self-diffusion and 14N NMR relaxation measurements new insight could be obtained regarding the translational and rotational micellar motions in the crowded solutions of systems with 0.2 DDAB-STDC molar ratio. 1H 2D NMR spectra gave some hints about the mutual arrangement of DDAB and STDC within the aggregates. For samples with 1.8 and 2.6 DDAB/STD molar ratios the 14N data were in agreement with the presence of somewhat branched, interconnected micelles. 23Na and 81Br dynamic parameters resulted particularly sensitive to the surrounding environment.The peculiar rheological behaviour of the, highly concentrated, branched micelles samples, namely the steady oscillations in the step tests, was found to be an example of instability originated by the combined effect of material elasticity and slippage at the fixed wall.

The Fine Line Between a Two-Phase and Solid-Solution Transformation in Li4+XTi5O12

The working of Li-ion batteries is based on the reversible insertion and extraction of Li-ions in the crystal structure of the positive and negative electrode materials resulting in highly efficient and high density energy storage. The nature of the electrode phase transitions induced by the insertion of Li-ions is of large practical importance for Li-ion battery performance. First order phase transitions result in an attractive flat electrode potential, however the phase boundaries formed are associated with poor rate performance and typically lead to poor cycle performance by mechanical failure. However, two of the most studied important electrode materials, olivine LiFePO4 1 and spinel Li4Ti5O12.2 , 3 both displaying a first order phase transition, appear exceptions showing excellent rate performance and a long cycle life. Interestingly, even in equilibrium the fundamental nature of the structural change during the first order phase transition of spinel Li4Ti5O12, is under debate, where it has been proposed to display either a complete solid solution, a phase-separation on the nano-scale or macroscopic phase separation.4-8 Despite the large number of first principle studies9-11 performed, mainly aiming to elucidate the diffusion pathways of Li, few efforts have been made to accurately describe the bulk insertion at finite temperatures from first principles. Here we report an exhaustive Li-vacancy configurational density functional theory (DFT) investigation of Li1+xTi5O12 (0 ≥ x ≥ 9) performed with different Li on 16d arrangements. The results indicate that the configurational interactions allow phase segregation between 8a and 16c occupation on a sub nanometer length scale induced by the partial replacement of Li on the 16d sites. This intimate mixing of the 8a and 16c occupancy structurally appears as a solid solution, however it should be considered as a first order phase transition with a sub nanometer domain size, the existence of which is due to the very small energy associated with the coherent interfaces. Based on our predictions of a sub-nano phase separating system we investigate kinetic properties of these phase boundaries by molecular dynamic simulations. We initialized molecular dynamic calculations in supercells that contain two phases (Li4Ti5O12 and Li7Ti5O12) directly in contact with each other. The results clearly display ultra-fast interface kinetics that rationalizes the excellent rate performance of this material. Calculated activation barriers are in good agreement with NMR relaxation measurement. The interface width between the coexisting phases is basically the distance separating the 16c and 8a atomic positions creating mixed 16c/8a occupation condition very similar to a solid solution description. In retrospect, the current work not only explains the apparent phase separation vs.solid solution contradiction in literature, it also explains the excellent rate performance of this intriguing material. (1) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Journal of the Electrochemical Society 1997, 144, 1188. (2) Deschanv, A.; Raveau, B.; Sekkal, Z. Mater. Res. Bull. 1971, 6, 699. (3) Colbow, K. M.; Dahn, J. R.; Haering, R. R. J. Power Sources 1989, 26, 397. (4) Wagemaker, M.; Simon, D. R.; Kelder, E. M.; Schoonman, J.; Ringpfeil, C.; Haake, U.; Lützenkirchen-Hecht, D.; Frahm, R.; Mulder, F. M. Adv. Mater. 2006, 18, 3169.(5) Wagemaker, M.; van Eck, E. R. H.; Kentgens, A. P. M.; Mulder, F. M. J. Phys. Chem. B 2009, 113, 224.(6) Lu, X.; Zhao, L.; He, X.; Xiao, R.; Gu, L.; Hu, Y.-S.; Li, H.; Wang, Z.; Duan, X.; Chen, L.; Maier, J.; Ikuhara, Y. Adv. Mater. 2012, 24, 3233.(7) Kitta, M.; Akita, T.; Tanaka, S.; Kohyama, M. J. Power Sources 2014, 257, 120.(8) Schmidt, W.; Bottke, P.; Sternad, M.; Gollob, P.; Hennige, V.; Wilkening, M. Chem. Mater. 2015, 27, 1740.(9) Ziebarth, B.; Klinsmann, M.; Eckl, T.; Elsässer, C. Phys. Rev. B 2014, 89, 174301. (10) Chen, Y.; Ouyang, C.; Song, L.; Sun, Z. Electrochim. Acta 2011, 56, 6084. (11) Bhattacharya, J.; van der Ven, A. Phys. Rev. B 2010, 81, 104304.

Just a Flexible Linker? The Structural and Dynamic Properties of CBP-ID4 Revealed by NMR Spectroscopy

Here, we present a structural and dynamic description of CBP-ID4 at atomic resolution. ID4 is the fourth intrinsically disordered linker of CREB-binding protein (CBP). In spite of the largely disordered nature of CBP-ID4, NMR chemical shifts and relaxation measurements show a significant degree of α-helix sampling in the protein regions encompassing residues 2–25 and 101–128 (1852–1875 and 1951–1978 in full-length CBP). Proline residues are uniformly distributed along the polypeptide, except for the two α-helical regions, indicating that they play an active role in modulating the structural features of this CBP fragment. The two helical regions are lacking known functional motifs, suggesting that they represent thus-far uncharacterized functional modules of CBP. This work provides insights into the functions of this protein linker that may exploit its plasticity to modulate the relative orientations of neighboring folded domains of CBP and fine-tune its interactions with a multitude of partners.

In situ determination of surface relaxivities for unconsolidated sediments

AbstractNMR relaxometry has developed into a method for rapid pore‐size determination of natural porous media. Nevertheless, it is prone to uncertainties because of unknown surface relaxivities which depend mainly on the chemical composition of the pore walls as well as on the interfacial dynamics of the pore fluid. The classical approach for the determination of surface relaxivities is the scaling of NMR relaxation times by surface to volume ratios measured by gas adsorption or mercury intrusion. However, it is preferable that a method for the determination of average pore sizes uses the same substance, water, as probe molecule for both relaxometry and surface to volume measurements. One should also ensure that in both experiments the dynamics of the probe molecule takes place on similar length scales, which are in the order of some microns. Therefore, we employed NMR diffusion measurements with different observation times using bipolar pulsed field gradients and applied them to unconsolidated sediments (two purified sands, two natural sands, and one soil). The evaluation by Mitra's short‐time model for diffusion in restricted environments yielded information about the surface to volume ratios which is independent of relaxation mechanisms. We point out that methods based on NMR diffusometry yield pore dimensions and surface relaxivities consistent with a pore space as sampled by native pore fluids via the diffusion process. This opens a way to calibrate NMR relaxation measurements with other NMR techniques, providing information about the pore‐size distribution of natural porous media directly from relaxometry.

Thermodynamics, size, and dynamics of zwitterionic dodecylphosphocholine and anionic sodium dodecyl sulfate mixed micelles

The thermodynamic properties of micellization for dodecylphosphocholine (DPC), sodium dodecyl sulfate (SDS), and their mixtures were studied using isothermal titration calorimetry. NMR relaxation measurements were used to explore molecular mobility of the DPC-containing micelles, whereas the diffusion measurements were taken to determine the micelle size. The DPC/SDS mixed systems reveal a tendency to form two kinds of micelles in buffered solution at lower temperatures. An increase in temperature as well as the transfer of the DPC/SDS mixed micelles from buffered to unbuffered solution results in only a single-step micellization process. The average size of the DPC-containing micelles is only slightly dependent on the SDS fraction. Examination of the data of spin–spin relaxation (T 2) shows that methylene protons on the polar headgroup of DPC and methylene protons (H1) on the hydrocarbon chain in the micellar systems studied reveal a heterogeneous dynamic behavior reflected in a two-component T 2 relaxation in the whole temperature range. The latter is the main constituent of the rigid interfacial layer core protecting the penetration of water into the hydrophobic interior.

133Cs Nuclear Magnetic Resonance Relaxometry as a Probe of the Mobility of Cesium Cations Confined within Dense Clay Sediments

133Cs nuclear magnetic resonance is used to quantify the mobility of cesium counterions confined within the interlamellar space of synthetic Hectorite clay under controlled hydration conditions. The degree of lamellae ordering within the macroscopic clay sediment is determined by analyzing the variation of the 133Cs NMR spectra as a function of the sample orientation in the static magnetic field. Furthermore, a detailed analysis of the 133Cs NMR spectra allows one to determine the order of magnitude of the quadrupolar and heteronuclear dipolar couplings monitoring the NMR relaxation of the cesium cations confined within clay sediments. Finally, a lower limit to the average residence time of the cesium cations within the clay interlamellar space is determined by using 133Cs NMR relaxation measurements under spin-locking conditions.

On the relationship between NMR-derived amide order parameters and protein backbone entropy changes.

Molecular dynamics simulations are used to analyze the relationship between NMR-derived squared generalized order parameters of amide NH groups and backbone entropy. Amide order parameters (O(2) NH ) are largely determined by the secondary structure and average values appear unrelated to the overall flexibility of the protein. However, analysis of the more flexible subset (O(2) NH < 0.8) shows that these report both on the local flexibility of the protein and on a different component of the conformational entropy than that reported by the side chain methyl axis order parameters, O(2) axis . A calibration curve for backbone entropy vs. O(2) NH is developed, which accounts for both correlations between amide group motions of different residues, and correlations between backbone and side chain motions. This calibration curve can be used with experimental values of O(2) NH changes obtained by NMR relaxation measurements to extract backbone entropy changes, for example, upon ligand binding. In conjunction with our previous calibration for side chain entropy derived from measured O(2) axis values this provides a prescription for determination of the total protein conformational entropy changes from NMR relaxation measurements.

Site-specific protonation kinetics of acidic side chains in proteins determined by pH-dependent carboxyl (13)C NMR relaxation.

Proton-transfer dynamics plays a critical role in many biochemical processes, such as proton pumping across membranes and enzyme catalysis. The large majority of enzymes utilize acid-base catalysis and proton-transfer mechanisms, where the rates of proton transfer can be rate limiting for the overall reaction. However, measurement of proton-exchange kinetics for individual side-chain carboxyl groups in proteins has been achieved in only a handful of cases, which typically have involved comparative analysis of mutant proteins in the context of reaction network modeling. Here we describe an approach to determine site-specific protonation and deprotonation rate constants (kon and koff, respectively) of carboxyl side chains, based on (13)C NMR relaxation measurements as a function of pH. We validated the method using an extensively studied model system, the B1 domain of protein G, for which we measured rate constants koff in the range (0.1-3) × 10(6) s(-1) and kon in the range (0.6-300) × 10(9) M(-1) s(-1), which correspond to acid-base equilibrium dissociation constants (Ka) in excellent agreement with previous results determined by chemical shift titrations. Our results further reveal a linear free-energy relationship between log kon and pKa, which provides information on the free-energy landscape of the protonation reaction, showing that the variability among residues in these parameters arises primarily from the extent of charge stabilization of the deprotonated state by the protein environment. We find that side-chain carboxyls with extreme values of koff or kon are involved in hydrogen bonding, thus providing a mechanistic explanation for the observed stabilization of the protonated or deprotonated state.

Phase Behavior, Diffusion, Structural Characteristics, and pH of Aqueous Hydrophobic Ionic Liquid Confined Media: Insights into Microviscosity and Microporsity in the [C4C4im][NTf2] + Water System

We present our studies on the physicochemical properties of water confined in Dibutylimidazolium bis(trifluoromethanesulfonylimide) ([C4C4im][NTf2]) reverse micelles through the NMR relaxation measurements that provide us an understanding of microviscosity and pH in the confined condition. We present experimental results on phase behavior, diffusion, structural characteristics and pH in aqueous ionic liquid-confined media. The ternary phase diagram was constructed by the cloud point measurements and the microheterogeneous regions were detected by the measurement of bulk viscosity and diffusion coefficients of K4[Fe(CN)6] inside the homogeneous microemulsion systems through the cyclic voltammetric (CV) measurements. The size of the microemulsion systems was characterized by the dynamic light scattering (DLS) method. The (1)H NMR spectra of homogeneous microemulsion systems were taken which indicates the presence of bound and free water molecules inside the microemulsion system. The NMR spin-lattice relaxation time (T1) of water molecules in its homogeneous microemulsion systems were measured and the reorientational correlation time (τc) of water molecules obtained from it indicates that the fluidity of homogeneous confined media decreases with the decrease in the composition of water. Microviscosity of the aqueous confined media was calculated from the measured T1 relaxation time values by applying the Debye-Stokes equation and correlated with the bulk viscosity of the samples. It was observed that both the microviscosity and bulk viscosity show inverse relationship. The fraction of bound and free water molecules were calculated from the measured T1 values. NMR spin-spin relaxation time (T2) of water molecules in its homogeneous microemulsion systems were measured with the varying pH of the aqueous core. A change in the T2 relaxation time of the water proton was observed proposing an exchange of proton between the H2O and -OH group of the TX-100 molecules. Finally, methyl orange (MO) was used as a UV-vis spectrophotometric molecular probe and the measured λmax values of the probe were used for the detection of micropolarity of the homogeneous aqueous confined media and was found to be increase with the increase in the size of the confined media.

Unraveling the complexity of protein backbone dynamics with combined (13)C and (15)N solid-state NMR relaxation measurements.

Typically, protein dynamics involve a complex hierarchy of motions occurring on different time scales between conformations separated by a range of different energy barriers. NMR relaxation can in principle provide a site-specific picture of both the time scales and amplitudes of these motions, but independent relaxation rates sensitive to fluctuations in different time scale ranges are required to obtain a faithful representation of the underlying dynamic complexity. This is especially pertinent for relaxation measurements in the solid state, which report on dynamics in a broader window of time scales by more than 3 orders of magnitudes compared to solution NMR relaxation. To aid in unraveling the intricacies of biomolecular dynamics we introduce (13)C spin-lattice relaxation in the rotating frame (R1ρ) as a probe of backbone nanosecond-microsecond motions in proteins in the solid state. We present measurements of (13)C'R1ρ rates in fully protonated crystalline protein GB1 at 600 and 850 MHz (1)H Larmor frequencies and compare them to (13)C'R1, (15)N R1 and R1ρ measured under the same conditions. The addition of carbon relaxation data to the model free analysis of nitrogen relaxation data leads to greatly improved characterization of time scales of protein backbone motions, minimizing the occurrence of fitting artifacts that may be present when (15)N data is used alone. We also discuss how internal motions characterized by different time scales contribute to (15)N and (13)C relaxation rates in the solid state and solution state, leading to fundamental differences between them, as well as phenomena such as underestimation of picosecond-range motions in the solid state and nanosecond-range motions in solution.

Insights into the molecular flexibility of θ-defensins by NMR relaxation analysis.

θ-Defensins are mammalian cyclic peptides that have antimicrobial activity and show potential as stable scaffolds for peptide-based drug design. The cyclic cystine ladder structural motif of θ-defensins has been characterized using NMR spectroscopy and is important for their structure and stability. However, the effect of the pronounced elongated topology of θ-defensins on their molecular motion is not yet understood. Studies of molecular motion by NMR relaxation measurements have been facilitated by the recent development of a semirecombinant method for producing cyclic peptides that allows for isotopic labeling. Here we have undertaken a multifield (15)N NMR relaxation analysis of the anti-HIV θ-defensin, HTD-2, and interpreted the experimental data using various models of overall and internal molecular motion. We found that it was necessary to apply a model that includes internal motion to account for the variations in the experimental T1 and NOE data at different backbone amide sites in the peptide. Although an isotropic model with internal motion was the simplest model that provided a satisfactory fit with the experimental data, we cannot exclude the possibility that overall motion is anisotropic, especially considering the strikingly elongated topology of θ-defensins. The presence of flexible side chains, self-association, interactions with solvent, and internal motions are all potential contributors to the observed relaxation data. Internal motion consistent with the constraints imposed by the cyclic cystine ladder was observed in that the order parameters, S(2), show that residues in the turns are more flexible than those in the β-sheet. This study provides insights into the dynamics of θ-defensins and information that might be useful in their application as scaffolds in drug design.

Solid Electrolytes Based on Fast Fluorine Ion Motions — 19F T1ρ NMR Relaxation vs Conductivity Measurements of Mechanosynthesized Nanocrystalline Ba0.6La0.4F2.4

The use of mechanosynthesis for the preparation of ion conductors provides access to new compounds whose properties are governed by chemical metastability, defects introduced and size effects present. The benefits of this valuable synthesis method manifest in enhanced ion transport properties of the prepared nanocrystalline ceramics. Such materials are useful candidates to act as solid electrolytes, e.g., in fluorine ion batteries [1]. Here, the introduction of large amounts of La3+ into the cubic structure of BaF2 was carried out by using high-energy ball-milling. The replacement of Ba2+ with a trivalent cation leads to the formation of an increased number fraction of point defects in the metastable crystal structure, which resulted in an increase of the dc-conductivity when compared to pure, nano-sized BaF2 [2]. Until now, only little is known about activation energies and jump rates of the elementary hopping processes of such materials. In the present study, we used broadband impedance spectroscopy and 19F NMR relaxometry to get to the bottom of ion jump diffusion proceeding on short-range and long-range length scales in Ba0.6La0.4F2.4. The dielectric measurements revealed an activation energy of 0.57 eV for macroscopic ion transport, whilst NMR, which is sensitive to both long-range motion and localized jumps, gave much smaller activation energies [3]. This was confirmed by high-frequency impedance measurements carried out at frequencies as high as 3 GHz that enabled us to compare the results from complementary methods applied in the same time window.[1] C. Rongeat, M. A. Reddy, R. Witter, and M. Fichtner, J. Phys. Chem. C, 117, 4943 (2013).[2] A. Düvel, J. Bednarcik, V. Šepelák, and P. Heitjans, J. Phys. Chem. C, in press (2014), DOI: 10.1021/jp410018t.[3] F. Preishuber-Pflügl, and M. Wilkening, Phys. Chem. Chem. Phys., in press (2014), DOI: 10.1039/C4CP00422A.

Ultra-low-field NMR relaxation and diffusion measurements using an optical magnetometer.

Nuclear magnetic resonance (NMR) relaxometry and diffusometry are important tools for the characterization of heterogeneous materials and porous media, with applications including medical imaging, food characterization and oil-well logging. These methods can be extremely effective in applications where high-resolution NMR is either unnecessary, impractical, or both, as is the case in the emerging field of portable chemical characterization. Here, we present a proof-of-concept experiment demonstrating the use of high-sensitivity optical magnetometers as detectors for ultra-low-field NMR relaxation and diffusion measurements.

Ultra‐Low‐Field NMR Relaxation and Diffusion Measurements Using an Optical Magnetometer

AbstractNuclear magnetic resonance (NMR) relaxometry and diffusometry are important tools for the characterization of heterogeneous materials and porous media, with applications including medical imaging, food characterization and oil‐well logging. These methods can be extremely effective in applications where high‐resolution NMR is either unnecessary, impractical, or both, as is the case in the emerging field of portable chemical characterization. Here, we present a proof‐of‐concept experiment demonstrating the use of high‐sensitivity optical magnetometers as detectors for ultra‐low‐field NMR relaxation and diffusion measurements.

Physical State of di-Nitrogen in the Presence of SBA and Various Zeolites Probed by 15N Spin–Lattice Relaxation of Pure 15N2 in the 30–300 K Temperature Range

15N NMR spin–lattice relaxation times of N2 in the presence of mesoporous SBA-80 and selected zeolites (Y, OMEGA, MORDENITE, ZSM-5, ZSM-11, AlPO-5, RHO) have been measured at 30 MHz and at temperatures ranging from ambient temperature to about 30 K. The interpretation of these experimental data rests on two relaxation mechanisms: spin-rotation (mainly active when N2 molecules reorient rapidly) and dipolar interaction with surface protons (depending essentially on the N2 translational diffusion mobility). Both mechanisms have to be taken into account for all samples except SBA-80 (for which the interpretation of experimental data requires two different spin-rotation interactions), and RHO (for which two different dipolar interactions are required). For all zeolites investigated here it has been indispensable to define a cutoff temperature below which spin-rotation is no longer active, indicating the quasi-disappearance of the gaseous state to the benefit of a condensed state. This cutoff temperature is 77 K for Y, OMEGA, and possibly MORDENITE, whereas it is higher for those zeolites with pore diameters approaching the dinitrogen molecular length. It is thus concluded that these NMR relaxation measurements are able to reveal the physical state of dinitrogen in the presence of SBA-80 and zeolites possessing very different structures.