Acceleration of carbon-13 spin-lattice relaxation times in amino acids by electrolytes

Measurements of the enhancement, by various electrolytes, of the spin-lattice relaxation time of carbon-13 at different locations in a number of amino acids are reported. Spin-lattice relaxation times T1 of all the carbons in amino acids generally tend to decrease with increase in the concentration of electrolytes, the largest effects often being observed for the charged carboxylate groups of the amino acids. Carboxylic carbons in amino acids are the sensitive 'acceptor' of the 13C spin-lattice relaxation accelerating effects offered by electrolytes, and the 13C spin-lattice relaxation accelerating ability of electrolytes decreases in the order Mg(ClO4)2 > MgCl2 > CaCl2 > NaCl > KCl > LiClO4 > NaOH. The mechanisms of the observed phenomena are discussed in terms of intermolecular interaction, paramagnetic impurities in electrolytes and other mechanisms; large contributions of intermolecular interactions with electrolytes are present on complex formation between amino acids and metal ions and the incoming 'unsaturation' of the primary solvation shell of cations with the increase in electrolyte concentration.     

Temperature-dependent 11B spin-lattice relaxation time for BF4 and CF3BF3 anions in room-temperature ionic liquids

Temperature-dependent (11)B T(1) values were measured for the BF(4) anion and BF(3) in the CF(3)BF(3) anion in room-temperature ionic liquids (RTILs) composed of the cation N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME). Including the lithium-salt-doped samples, two neat and two binary ionic liquids were studied. Arrhenius plots of the (11)B T(1) showed T(1) minima for BF(4) in the temperature range between 243 (or above freezing) and 373 K. Using the Bloembergen, Pound, and Purcell(BPP) equations for the (11)B quadrupolar and (11)B-(19) F dipolar relaxation mechanisms, the correlation times for motions of BF(4) were calculated. Since the internal rotation of BF(3) is assumed in CF(3)BF(3), T(1) minimum was not observed. The effects of the addition of the lithium salt on the (11)B correlation time and (11)BT(1) for the anions in the ILs are discussed.     

29Si spin-lattice NMR relaxation in microporous silica-based materials with high Mn2+ concentrations

The 29Si T1 time measurements have been performed under magic angle spinning for supermicroporous Si/Mn materials 1-6 containing Mn2+ in concentrations of 0 (1), 1.2 (2), 2.5 (3), 3.8 (4), 15 (5) and 19.8 wt% (6). It has been found that the central lines, corresponding to isotropic chemical shifts, and their sidebands relax differently in samples 2-4 with relatively small Mn2+ contents. Because the relaxation curves for 1-6 are nonexponential and identical for 6 at spinning rates of 5, 10 and 12 kHz, dipolar relaxation via the paramagnetic centers is dominant. The relaxation data have been interpreted as a function of Mn2+ concentrations in terms of incorporation of the Mn2+ ions into the silica lattice when their concentration is small (相似文献   

Solid State NMR Study of Polystyrene Nanolatex Particles(I) ~(13)C Spin-Lattice Relaxation Time

A wide range of physical methods has been used for studying the sintering behavior of bulk polystyrene, which has been the subject of AFM imaging, DSC measurement, Dilatometer meadurement, Fourier IR, and solid state NMR studies to investigate the physicochemical change and its relation to the electronic and magnetic performance. In recent years, more attention was paid to the size effect on annealing temperature for the latex particles with the diameter of less than 100 nm, which present…     

各种孔结构活性炭吸附苯、环己烷、正戊烷和正己烷的~1H和~(13)C NMR研究

~1H和~(13)C NMR研究证明被吸附在不同孔结构活性炭中的烃类以毛细管凝聚和吸附在固体表面两种状态存在。链状烷烃平铺地吸附在固体表面。被吸附烃与活性炭表面酸性基团的质子交换在弛豫过程中起着重要作用。     

Frequency and molecular-mass-dependent nonexponential proton NMR relaxation in polymer melts

A theoretical treatment of the nonexponential relaxation behavior of the different proton nuclear magnetic resonance (NMR) relaxation processes in polymer melts is presented. Formulas are derived for a three-component model given by two versions and a homogeneous distribution of correlation times. The theoretical results were tested with measurements of T₁, T_2e, and T₂ as functions of frequency and molecular mass in linear fractionated polyethylene samples. While the T₁ relaxation always yields exponential magnetization decays, the T_2e and T₂ measurements show biexponential relaxation behavior. From the calculations it was found that the correlation time of the local motion is independent of the molecular mass, whereas the correlation time of the slowest motional process increases with M^2.8_w for the three-component model and with M^2.2_w for the distribution of correlation times, respectively. © 1992 John Wiley & Sons, Inc.     

Biexponential C spin-lattice relaxation behavior of the crystalline region of ethylene copolymers and its origin

In this work, two series of ethylene-dimethylaminoethylmethacrylate (EDAM) and ethylene-methyl acrylate copolymers (EMA) with different comonomer content were studied by high-resolution solid-state ¹³C NMR spectroscopy. Biexponential ¹³C spin-lattice relaxation behaviors of the crystalline region were observed for all copolymer samples either melt-quenched or isothermally crystallized. The relative content of the shorter ¹³C T₁ component to that of the longer ¹³C T₁ component was found to increase with comonomer content. By employing a new pulse sequence which can be considered as a combination of Goldman-Shen's and Torchia's pulse sequences, it was demonstrated that the shorter ¹³C T₁ component is corresponding to the intermediate part of the crystalline region. The thickness of the intermediate part is estimated to be about 0.85 nm.     

NMR studies of molecular motion of ultradispersed polytetrafluoroethylene

Molecular motion in ultradispersed polytetrafluroethylene obtained by special gas-phase technology has been studied experimentally and theoretically based on a temperature dependence of the second moment of ¹⁹F NMR spectra and the time of spin-lattice relaxation. The results of observations are interpreted as the consequence of reorientation motion of CF₂ groups around the axis of macromolecules at low temperature and of translational motion of macromolecules in the high temperature region. Qualitative differences from the molecular motion in industrial polytetrafluoroethylene (teflon-4) were detected and parameters of dynamic processes determined.     

1H NMR relaxation study of polymer-solvent interactions during thermotropic phase transition in aqueous solutions

The dynamic-structural changes and polymer - solvent interactions during the thermotropic phase transition in poly(vinyl methyl ether) (PVME)/D₂O solutions in a broad range of polymer concentrations (c = 0.1-60 wt.-%) were studied combining the measurements of ¹H NMR spectra, spin-spin (T₂) and spin-lattice (T₁) relaxation times. Phase separation in solutions results in a marked line broadening of a major part of polymer segments, evidently due to the formation of compact globular-like structures. The minority (∼15%) mobile component, which does not participate in the phase separation, consists of low-molecular-weight fractions of PVME, as shown by GPC. Measurements of spin-spin relaxation times T₂ of PVME methylene protons have shown that globular structures are more compact in dilute solutions in comparison with semidilute solutions where globules probably contain a certain amount of water. A certain portion of water molecules bound at elevated temperatures to (in) PVME globular structures in semidilute and concentrated solutions was revealed from measurements of spin-spin and spin-lattice relaxation times of residual HDO molecules.     

NMR relaxation study of thermal degradation in cured PMR polyimides

We have studied cross-linking and thermal degradation of high-performance first-and second-generation PMR-15 polyimides, both thermoset and thermoplastic versions, by performing nonspectroscopic NMR solid echo T^*₂ relaxation measurements at temperatures up to 430°C using probes built for this purpose. We employ signal averaging and automated decomposition of the relaxation decays into two Gaussian components, the slower of which gradually appears above 300°C. Tracking the molecular mobility spectrum in terms of the relative intensity of the components and their relaxation times as temperature is cycled, we detect essentially no irreversible effects below the glass transition, measure permanent mobility reductions attributable to completion of cure, and find that exposure to temperatures above 380°C on the order of 1 h is required for substantial thermal degradation to occur. These results are closely supported by thermal and mechanical measurements on parallel specimens. Second-generation PMR resins appear to have higher microscopic rigidity and reduced viscous fraction at high temperatures. ©1995 John Wiley & Sons, Inc.     

Acceleration of carbon-13 spin-lattice relaxation times in amino acids by electrolytes

A series of amino acids and carboxylic acids were determined by 13C NMR spectroscopy.The results showed that addition of 3M MgCl2 led to the 13C NMR integral area of samples being well proportional to number of carbon atoms that produce the particular signal with reliability over 95%. Measurements of 13C spin-lattice relaxation times (T1's) are reported for a number of amino acids. T1's of all the carbons in amino acids generally tend to decrease with the increase of the concentration of electrolytes, and the presence of magnesium slats is of significant. Carboxylic carbons in amino acids are the most sensitive "acceptor" of the 13C spin-lattice relaxation accelerating effects in electrolytes, and the 13C spin-lattice relaxation accelerating ability of electrolytes is Mg(ClO4)2 ＞MgCl2 ＞CaCl2 ＞NaCl ＞KCl ＞LiClO4 ＞NaOH. In general, T1's of C1 carbons in nonpolar a-amino acids are higher than those in polar and basic a-amino acids both in aqueous and 3M MgCl2 medium. In aliphatic straight-chain amino acids, a-, a-, a-, ai- and a- amino acids, T1's of C1 carbons tend to reduce with the increase of inserted carbon numbers between amino and carboxylic groups compared with Gly. T1's can be decreased even more when amino acids are mixed in 3M MgCl2, but T1's of carbons in amino acids decrease slightly with increase of the concentration of amino acids in 3M MgCl2. The mechanisms of the observed phenomena are discussed in terms of intermolecular interaction and paramagnetic impurity in electrolytes, large contributions of intermolecular interaction which is enhanced in electrolytes concentrate on the incoming "unsaturation" of the primary solvation shell of cations with the increase of electrolytes concentration and complexes formation of amino acids with metal ions. In electrolytes, amino acids are "anchored" to cations and molecule tumbling is slowed down, molecular rigidity is increased and molecular size is "enlarged", all of these are helpful to accelerate the 13C spin-lattice relaxation. Atlast, MgCl2 is proposed as an efficient relaxation agent for analysis of amino acids and some carboxylic acids.Samples were dissolved with the aid of supersonic which has the effect of degassing, and they were degassed again with supersonic for 30 seconds right before determination. All of the 13C NMR was obtained with a Bruker DPX-300 NMR instrument, using NOE-suppressed inverse gated decoupling with a recycle delay 8.00s and a sweep width 30120.48Hz, experiment temperature is integral of the carbon with the smallest chemical shift is calibrated as 10.00. Spin-lattice relaxation times (T1's) were determined by using inversion recovery according to Bruker avance user's guide.The pulse sequence is (T-90.°-T-180°-o-90t°) n.     

Peculiarities of NMR relaxation in micellar gels of Pluronic F-127

Based on the ¹H relaxation of transverse nuclear magnetization of triblock-copolymer Pluronic F-127 in D₂O, we proposed a model of the associated pluronic structure in which the polyethylene oxide of molecules in neighboring micelles are intertwined in regions of overlapping micellar coronas, while the polypropylene oxide cores of the micelles play a role of nodes in the 3D network.     

Proton NMR relaxation studies on several nitroxyl-water systems

Proton spin-lattice and spin-spin relaxation times of several nitroxyl radicals in aqueous solutions have been measured between 10 kHz and 90 MHz. There are two regions where the relaxation times of the solvent water protons are frequency dependent. It is possible to extract from these experimental data structural and dynamical parameters such as the number of solvated water protons and/or their mean lifetime in the first hydration sphere around the paramagnetic center. These results were found to differ considerably from corresponding data reported in the literature which have been deduced mainly from chemical shift measurements. To whom correspondence should be addressed.     

Molecular Dynamic Evaluation of starch-PLA blends nanocomposite with organoclay by proton NMR relaxometry

Starch and PLA were used alone and in blends to prepare nanostructured materials using both hydrophilic and organophilic clays, and PVA. All nanostructured materials were obtained by the solution intercalation method using water and chloroform as solvents. These systems were characterized by using conventional X-ray diffraction (XRD), conventional NMR and the non-conventional fast field cycling (FFC) NMR technique. The spin-lattice relaxation times were measured as a function of the Larmor frequency. The FFC results showed that the starch has only one relaxation time related to the amorphous region. PLA hybrids presented two distinct spin-lattice relaxation times. The blends of the two polymers also showed two relaxation times. The renormalized Rouse formalism was applied to describe the polymer molecular dynamics behavior in the studied systems containing starch. By adding clay or PVA, differences could be observed in relaxation time corresponding to the more amorphous region, indicating that, when adding clay and PVA, the effect that each has on the dynamics of the mixture is cancelled out.     

On the effect of temperature on the dielectric relaxation time of some benzene derivatives and certain of their binary mixtures

Dielectric relaxation behaviour of polar molecules in a non-polar solvent, or mixtures of these substances at different microwave frequencies and over a range of temperatures and concentrations give an idea about inter- and intra-molecular forces. Also such studies enable one to calculate thermodynamic parameters such as, the change of activation energy for dipole orientation (Δ G?), the enthalpy (Δ H?) and entropy (Δ S?) of activation. Such studies in the case of binary, ternary, etc. mixtures of polar molecules in pure liquid phase or in dilute solution phase of them in a non-polar solvent help in drawing certain quantitative conclusions regarding their relaxation behaviour as to whether a single component is responsible for observed microwave absorption, or a cooperative phenomenon (average) by all the dipoles of the mixture contribute to it. An experimental investigation is here performed on typical systems. With this in view, systematic dielectric measurements in a range of temperatures are carried out at a single microwave frequency on a single weight fraction in benzene of the four substituted phenols, namely, p-fluorophenylacetonitrile, p-bromonitrobenzene, m-bromonitrobenzene and 2-chloro-6-fluoro-benzaldehyde and on binary (1?:?1) mixtures of [p-2-chloro-6-fluoro-benzaldehyde?+?o-ethylphenol] and [p-fluorophenylacetonitrile?+?2-n-butyl phenol] in benzene as solvent at different temperatures. The results are presented and discussed.     

2H NMR spin relaxation study of the soft segment motion in alternating ethylene–propylene copolymers

We synthesized three partially deuterated polymer samples, namely a poly(ethylene‐alt‐propylene) (EP) alternating copolymer, a poly(styrene‐b‐EP) diblock copolymer (SEP) and a poly(styrene‐b‐EP‐b‐styrene) triblock copolymer (SEPS). The ²H spin–lattice relaxation time, T₁, of EP soft segments above their glass transition temperature was measured by solid‐state ²H NMR spectroscopy. It was found that the block copolymers had a fast and a slow T₁ component whereas EP copolymer had only a fast component. The fast T₁ components for SEP and SEPS are similar to the T₁ value of EP above ca 20°C. The slow T₁ component for SEP and SEPS exhibited a minimum at 60°C and approached the value of the fast component near the T_g of polystyrene. The motional behavior of the EP units for SEP is similar to that of SEPS over the entire range of temperature. Copyright © 2001 John Wiley & Sons, Ltd.     

Fast measurement of heteronuclear relaxation: frequency-domain analysis of NMR accordion spectroscopy

NMR relaxation parameters are usually derived from series of 2D experiments. The whole procedure can be very time consuming, especially for the study of the relaxation of nuclei at natural abundance. Palmer and Mandel have proposed the use of accordion spectroscopy to determine one relaxation parameter using two experiments only. In this paper, we show that the experimental time can be further reduced, by recording only three experiments for the determination of both the longitudinal and transverse relaxation rates. The analysis of the experiments is performed in the frequency domain, the relaxation rates being deduced from the linewidth of the peaks of interest. A detailed statistical analysis of errors introduced by the line fitting procedure on derived relaxation parameters was used to derive guidelines for the choice of experimental parameters. This procedure was applied to the study of the Cα relaxation parameters of a six-residue unlabeled peptide. The results were compared with those obtained by classical accordion spectroscopy. Copyright © 2001 John Wiley & Sons, Ltd.