Proton NMR Spin-lattice Relaxation Research Articles

NMR and NQR study of polymorphism in carbamazepine

Four polymorphic forms of carbamazepine have been simultaneously investigated by 1H NMR and 14N NQR. The results show that the proton spin-lattice relaxation time and the 14N NQR spectra can be used to differentiate between various polymorphic forms. Spontaneous transformations from Form II to Form III and from Form IV to Form III have been investigated through their influence on the 14N NQR spectrum and the proton NMR signal and spin-lattice relaxation. The 14N NQR spectra prove that in the observed polymorphic forms of carbamazepine the hydrogen bonded dimers of carbamazepine molecules are the basic elements of the crystal structure. The dimers are centrosymmetric in Forms III and IV and in metastable polymorphic form occurring during the transformation of Form IV to Form III. Two non-equivalent molecular positions are observed in Form II with the occupation ratio 1:1 and in Form I with the occupation ratio either 2:1 or 3:1. The 14N NQR data are related to the published crystal structures. Possible reasons for the mismatch of the X-ray and NQR data for Forms I and II are discussed.

Metal-Ligand Recognition Index Determination by NMR Proton Relaxation Study.

In this study, we developed and validated a new proposed parameter quantifying the interaction strength between natural and/or synthetic molecules with paramagnetic metal ions. The Metal ion Recognition Index, Miri, is a quantitative parameter to describe the proton environment and to define their involvement in the inner and/or outer sphere of the paramagnetic metal ion. The method is based on the analysis of NMR proton spin-lattice relaxation rates of a specific ligand in both the diamagnetic and paramagnetic conditions. The proposed procedure is also useful to calculate the ligand proton spin-lattice relaxation rate in the paramagnetic bound conditions, which is typically very difficult to determine experimentally. Miri was used to compare the ligand proton involvement toward different paramagnetic species, in particular the Copper(II)-Piroxicam system. Copper(II)-Piroxicam complex is one of the most active anti-inflammatory and anti-arthritic species. Miri provides an opportunity to improve our knowledge of metal-ligand complexes that play a fundamental role in bioinorganic interactions.

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.

Dispersion of Water Proton Spin–Lattice Relaxation Rates in Aqueous Solutions of Multiwall Carbon Nanotubes (MWCNTs) Stabilized via Alkyloxymethylimidazolium Surfactants

Carbon nanotubes and a number of other carbon nanomaterials have a tendency to aggregate, which often resulted in difficulties of dispersion of these nanomaterials in aqueous solutions. The ability of dicationic (gemini) surfactants to disperse multiwall carbon nanotubes in water and the dynamic processes taking place at the water–multiwall carbon nanotubes (MWCTs) interface are studied. Stable dispersions of multiwall carbon nanotubes with selected gemini surfactants (1,1′-(1,6-hexanediyl)bis(3-alkyloxymethylimidazolium) dichlorides) were prepared and characterized by nuclear magnetic relaxation dispersion (NMRD), NMR diffusometry, scanning and transmission electron microscopy, and Fourier transform infrared spectroscopy. The addition of multiwall carbon nanotubes to aqueous solutions of studied gemini surfactants leads to significant paramagnetic enhancement of the spin–lattice relaxation processes, which gets more pronounced with increasing concentration of well-dispersed MWNTs in water. The dominant role of outer sphere (OS) relaxation mechanism in total observed R1, governed by two-dimensional diffusion of water on the carbon nanotube surface in the vicinity of paramagnetic centers incorporated in the MWCNTs’ sidewalls (mainly of iron origin), was assumed to explain NMRD data. The NMR diffusion experiments confirm the existence of restricted water diffusion in the studied supernatants. The NMR diffusion results are consistent with the FTIR and NMR proton spin–lattice relaxation dispersion in which the more effective R1 dispersion noticed for the sample with IMIC6C12 was ascribed to the better accessibility of water molecules to the surface of the MWCNTs.

(1)H NMR Relaxometric Study of Molecular Dynamics in a "de Vries" Liquid Crystal.

Liquid crystals that exhibit de Vries smectic A phases are promising materials for new generations of ferroelectric liquid crystal displays and other electro-optical devices. We investigated the molecular dynamic properties of a rod-like de Vries liquid crystal compound, namely the 9HL, a (S)-hexyl lactate derivative, in the whole mesophasic range. This is the first molecular dynamics' investigation on a de Vries phase, and the interest of this system is related to the understanding of the structural and supramolecular organization of de Vries SmA phases, which has been a subject of a broad scientific debate in the last years. Proton NMR spin-lattice relaxation times, T1, were measured in the range 100 MHz to 5 kHz, thus covering a very wide range of motional regimes. (1)H NMR dispersion curves and temperature-dependent relaxation rates were analyzed with a global minimum target fitting approach and the main molecular motions, namely reorientational diffusion, translational self-diffusion, layer undulation, and tilting director fluctuations were fully characterized. The molecular dynamics' behavior observed across the SmA-SmC* phase transitions of 9HL was consistent with the proposed cluster diffuse cone model for the de Vries SmA phase.

Probing the structural evolution during the geopolymerization process at an early age using proton NMR spin-lattice relaxation

Proton nuclear magnetic resonance (1H NMR) was used to characterize the geopolymerization process of a metakaolin-based geopolymer. In the spin-lattice relaxation time (T1) distribution curve obtained for the first 15-min time point, two relaxation peaks were observed, which were assigned to two different water phases in the initial paste. Based on the evolution curve of the average mean value of T1, the geopolymerization process in the early age can be divided into four successive stages: the induction period, acceleration period, deceleration period and stabilization period. This work reveals that 1H NMR is a noninvasive and non-destructive method for the analysis of structural evolution during the geopolymerization process.

Oxygen in S phase of a cell cycle

It isgenerally agreed that the S phase is radioresistant because of its deficiency of molecularoxygen. This conclusion is based on the oxygenenhancement ratio (OER) and not by measur-ing the actual molecular oxygen content in eachphase of the cell cycle (Table 1).The OER is calculated by radiation dosedelivered under hypoxic and under aeratedcondition to achieve the same biological endpoint. The OER at high doses yield a valuebetween 2?3 and 3?5 for X-rays and g rays.We abstracted the data from the literature. It isinteresting to note in Figure 1, as the cell ages,water proton NMR Spin Lattice relaxation time(T

Proton NMR Relaxation Study on the Nematic–Nematic Phase Transition in A131 Liquid Crystal

A study of the proton NMR spin-lattice relaxation time, T(1), of the A131 liquid crystal compound as a function of temperature and Larmor frequency, using a combination of fast field-cycling and standard NMR techniques, is presented. The frequency dispersion in a wide range (from 10 kHz to 300 MHz) at different temperatures and the temperature variation of T(1), in several frequency conditions, were analyzed considering the contributions of the molecular movements generally detected in liquid crystals. In the case of nematic phases of calamitic liquid crystals, the nuclear spin relaxation is dominated by collective movements and local molecular reorientations. The experimental results clearly show a transition within the nematic range of this compound, previously identified as one from the uniaxial to the biaxial phase. This transition can be associated with a slowing down of the molecular rotations around the long molecular axis, where the preferred orientation defines the principal director as detected in the T(1) dispersion analysis.

Probing the hydration of composite cement pastes containing fly ash and silica fume by proton NMR spin-lattice relaxation

Proton NMR spin-lattice relaxation (T1) was used as a prober for observing the hydration process of composite cement pastes blending fly ash and silica fume during the early age. The distribution at initial time, evolution curves and signals intensity of T1 were shown in this paper. Results demonstrate that the T1 distribution curves at initial time exhibit two peaks, which are regarded as two different water phases in the pastes. The evolution curves of T1 are in good agreement with the hydration process of composite pastes and could be roughly divided into four stages: initial period, dormant period, acceleration period and steady period. The hydration mechanism for each stage was discussed. The hydration of the composite cement pastes was retarded by the addition of fly ash and silica fume when compared to that of pure cement. However, the hydration degree of the cement in the blends was promoted.

Variable temperature NMR characterization of α-glycine

Proton NMR spin–lattice relaxation times in the laboratory frame ( T 1) and in the rotating frame ( T 1 ρ ) were measured as a function of temperature for a static sample of α-glycine. Both T 1 and T 1 ρ data can be fit quantitatively by a single thermally-activated motion (the modulation of the dipolar coupling by random hopping about the threefold axis of the –NH 3 group), with no addition of other mechanisms at any temperature between 173 and 415 K. An activation energy of 21.7 ± 1 kJ/mol was extracted and is compared with previously reported values for both α- and γ-glycine. Such comparisons allow the correction of glycine polymorphs misidentified in the literature. The minimum in T 1 at 325 K corresponds to a correlation time of 0.53 ns. Chemical shifts as a function of temperature were measured by 1H CRAMPS and by 13C and 15N CP/MAS experiments. These results are discussed relative to a previous report of anomalous electrical behavior in α-glycine within this temperature range.

1H NMR relaxation in urea

Proton NMR spin–lattice relaxation times T 1 were measured for urea as a function of temperature. An activation energy of 46.3 ± 4.7 kJ/mol was extracted and compared with the range of 38–65 kJ/mol previously reported in the literature as measured by different magnetic resonance techniques. In addition, proton NMR spin–lattice relaxation times in the rotating frame T 1 ρ were measured as a function of temperature. These measurements provide acquisition conditions for the 13C and 15N CP/MAS spectra of pure urea in the crystalline phase.

Miscibility with positive deviation in Tg–composition relationship in blends of poly(2-vinyl pyridine)- block-poly(ethylene oxide) and poly( p-vinyl phenol)

Phase behavior and miscibility with positive deviation from linear T g–composition relationship in a copolymer/homopolymer blend system, poly(2-vinyl pyridine)- block-poly(ethylene oxide) (P2VP- b-PEO)/poly( p-vinyl phenol) (PVPh), were investigated by differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FT-IR) and solid-state 13C nuclear magnetic resonance ( 13C NMR), optical microscopy (OM), and scanning electron microscopy (SEM). Optical and electron microscopy results as well as NMR proton spin–lattice relaxation times in laboratory frame ( T 1 H ) all confirmed the miscibility as judged by the T g criterion using DSC. In comparison to the literature result on a homopolymer/homopolymer blend of P2VP/PVPh, fitting with the Kwei equation on the T g–composition relationship for the block-copolymer/homopolymer blend of P2VP- b-PEO/PVPh blend system yielded a smaller q value ( q = 120) for P2VP- b-PEO/PVPh than that for P2VP/PVPh blend ( q = 160). The FT-IR and 13C NMR results revealed hydrogen-bonding interactions between the pendant pyridine group of P2VP- b-PEO and phenol unit in PVPh, which is responsible for the noted positive deviation of the T g–composition relationship. Comparison of the shifts of hydroxyl IR absorbance band, reflecting the average strength of H-bonding, indicates a decreasing order of P2VP/PVPh > P2VP- b-PEO/PVPh > PEO/PVPh blends. The PEO block in the copolymer segment tends to defray the interaction strength in the P2VP- b-PEO/PVPh blends because of relative weaker interaction between PEO and PVPh than that between P2VP and PVPh pairs. A comparative ternary (P2VP/PEO)/PVPh blend was also studied as the controlling experiments for comparison to the P2VP- b-PEO/PVPh blend. The thermal behavior and interaction strength in (P2VP/PEO)/PVPh ternary blends are discussed with those in the P2VP- b-PEO/PVPh copolymer/homopolymer blend.

Theoretical estimates for proton-NMR spin–lattice relaxation rates of heterometallic spin rings

Heterometallic molecular chromium wheels are fascinating new magnetic materials. We reexamine the available experimental susceptibility data on MCr 7 wheels in terms of a simple isotropic Heisenberg Hamiltonian for M=Fe, Ni, Cu, and Zn and find in that FeCr 7 needs to be described with an iron–chromium exchange that is different from all other cases. In a second step we model the behavior of the proton spin lattice relaxation rate as a function of applied magnetic field for low temperatures as it is measured in nuclear magnetic resonance (NMR) experiments. It appears that CuCr 7 and NiCr 7 show an unexpectedly reduced relaxation rate at certain level crossings.

Peculiar Molecular Dynamics Behaviour in the Isotropic Phase of Some Liquid Crystalline Systems

NMR proton spin-lattice relaxation constitutes an important tool for the characterization of the molecular dynamics in liquid crystals (LC). For LCs, in general, the main difference between the proton relaxation dispersion in mesophases and in the isotropic phase appears in the low frequency regime: typically a plateau is observed in the isotropic phase and specific frequency dependences, due to collective movements, are found in the LC phases. In more complex systems, peculiar frequency dependencies, in the low frequency regime may be found in the isotropic phase. The results presented in this paper reveal the presence of particular slow motional mechanisms related to the specific characteristics of the investigated systems.

1H Nmr Study Of Molecular Motions In Thiourea Pyridinium Nitrate Inclusion Compound

The proton NMR second moment and spin-lattice relaxation time have been studied for polycrystalline thiourea pyridinium nitrate inclusion compound and its perdeuderated analogues in a wide temperature range. The reorientation of two dynamically different pyridinium cations around their pseudohexagonal symmetry axis taking place over inequivalent barriers have been revealed in the low-temperature phase. Activation parameters for these motions have been derived. A symmetrization of the potential barriers has been observed at the transition from intermediate to the high temperature phase. The motion of thiourea molecules has been also evidenced, but could not be unambiguously described.

1H NMR Study of Molecular Motions in Thiourea Pyridinium Halide Inclusion Compounds

The proton NMR second moment and spin-lattice relaxation time have been studied for polycrystalline inclusion compounds of thiourea pyridinium chloride, bromide, iodide and their perdeuterated analogues in a wide temperature range. The pyridinium cation reorientation around the pseudohexagonal C6’ symmetry axis over inequivalent barriers and hindered rotation of the thiourea molecule around its C=S bond have been revealed. The activation parameters of the both motions have been found.

1H Spin-Lattice Relaxation in a NH4HSO4Single Crystal

The proton NMR line width and spin-lattice relaxation times for the NH 4 HSO 4 single crystal were studied over a wide temperature range. The proton spin-lattice relaxation time for NH 4 HSO 4 has ...

NMR Study of the Polymer Electrolyte Formed by Ethylene Glycol, Citric Acid and LiClO 4

In this work we report a proton ( 1 H) and lithium ( 7 Li) NMR lineshape and spin-lattice relaxation time investigation of this polymer electrolyte obtained from the polymerization of ethylene glycol and citric acid with lithium salt. The 7 Li data indicates that the lineshape and spin-lattice relaxation is affected by the coupling of its quadrupolar moment to the surrounding electric field gradients. The activation energy for the lithium motion extracted from the 7 Li relaxation data is 0.23 eV. The dynamics parameters obtained from the NMR data indicate that the lithium mobility in the EG:CA:LiClO 4 is comparable to those found in polymer gel electrolytes and in plasticized polymer electrolytes.

NMR Study on (CH 3 ) 2 NH 2 H 2 AsO 4

Proton NMR line width and spin-lattice relaxation time T 1 for the ferroelectric (CH 3 ) 2 NH 2 H 2 AsO 4 have been measured at the temperature range between 77 K and 300 K. The temperature dependence of T 1 is well described by the expressions based on rotation of the methyl group using the value of activation energy E a = 99 meV and elementary correlation time 0 = 1.5 2 10 m 13 sec. Moreover, a sharp dip of T 1 near ferrroelectric phase transition temperature ( T c = 274.5 K) results from the ordering of protons. In addition, the temperature dependence of T 1 deviates from the expression for molecular rotation above around 200 K. It is deduced that the deviation is caused by the suppression of some kind of proton motion.

Proton magnetic resonance study of the low-temperature phase transition in a LiNH4SO4 single crystal

The proton NMR line width and spin-lattice relaxation times for LiNH4SO4 single crystal were studied at low temperature range of 6 and 280K. The changes in the proton relaxation behavior near the phase transition temperature indicates a change in the state of internal motion at the transition. The molecular motions obtained by the spin-lattice relaxation processes were found to be determined by molecular reorientation of the NH4 ions in phases III, IV, and V. We also confirmed that the phase transitions occur at 26 and 133K.