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1.
The transverse relaxation rates R 2 = 1/T 2 of protons can be determined by spin-echo sequences with multiple refocusing pulses using moderate radio-frequency field strengths and properly chosen inter-pulse delays so as to suppress echo modulations due to homonuclear scalar couplings. Combination with 2D heteronuclear correlation spectroscopy (HSQC) allows one to measure R 2 of arbitrary protons attached to nitrogen-15 or carbon-13 nuclei. Decays of six amide protons in the protein Ubiquitin that is nitrogen-15 enriched (but not deuterated) were measured at different temperatures.  相似文献   

2.
The 13C–1H CPMAS with flip-back pulse NMR experiment is revisited in view of applications to pharmaceutical mixtures. The analysis of the kinetics of relaxation and CP transfer with and without the flip-back pulse shows that a significant gain in 13C signal can be expected (thus in experimental time) from the flip-back pulse for protons with long T1. The gain is of the order of T1 of the protons expressed in seconds. The experiment is applied on samples with highly contrasted spin-lattice relaxation times T1 for protons, situation encountered in pharmaceutical mixtures. The application of the flip-back increases significantly the relative signal intensity of the component with the longer T1, making this component detectable even after using short recycle delays. Therefore, this CPMAS with flip-back experiment could be used routinely to get 13C CPMAS NMR spectra of mixtures in constant experimental time and signal-to-noise ratio without the need for optimization of the recycle delays, and for whatever may be the degree of crystallinity of the active principal ingredient (API) and/or excipients.  相似文献   

3.
The spin-lattice relaxation times of the various nuclei in methyl iodide, methyl iodide-d 3, and carbon-13 methyl iodide (13C, 1H, 2D) were measured between 210 and 350 K. The separation of the proton-proton intermolecular relaxation was accomplished by a dilution study in methyl iodide-d 3; the resulting intermolecular contribution agreed well with the existing theories for this mechanism. It was found that the spin-rotation interaction contributed significantly to the intramolecular relaxation of both the protons and the carbon-13. For both nuclei the separation of the spin-rotation interaction from the intramolecular dipole-dipole interaction was accomplished without making any assumptions about the temperature dependence of the spin-rotation relaxation time. The resulting spin-rotation relaxation times for both carbon-13 and protons offer evidence that the large spin-rotation effects are due to the methyl group reorientation.  相似文献   

4.
A protocol is presented for correcting the effect of non-specific cross-polarization in CHHC solid-state MAS NMR experiments, thus allowing the recovery of the 1H–1H magnetization exchange functions from the mixing-time dependent buildup of experimental CHHC peak intensity. The presented protocol also incorporates a scaling procedure to take into account the effect of multiplicity of a CH2 or CH3 moiety. Experimental CHHC buildup curves are presented for l-tyrosine·HCl samples where either all or only one in 10 molecules are U–13C labeled. Good agreement between experiment and 11-spin SPINEVOLUTION simulation (including only isotropic 1H chemical shifts) is demonstrated for the initial buildup (tmix < 100 μs) of CHHC peak intensity corresponding to an intramolecular close (2.5 Å) H–H proximity. Differences in the initial CHHC buildup are observed between the one in 10 dilute and 100% samples for cases where there is a close intermolecular H–H proximity in addition to a close intramolecular H–H proximity. For the dilute sample, CHHC cross-peak intensities tended to significantly lower values for long mixing times (500 μs) as compared to the 100% sample. This difference is explained as being due to the dependence of the limiting total magnetization on the ratio Nobs/Ntot between the number of protons that are directly attached to a 13C nucleus and hence contribute significantly to the observed 13C CHHC NMR signal, and the total number of 1H spins into the system. 1H–1H magnetization exchange curves extracted from CHHC spectra for the 100% l-tyrosine·HCl sample exhibit a clear sensitivity to the root sum squared dipolar coupling, with fast buildup being observed for the shortest intramolecular distances (2.5 Å) and slower, yet observable buildup for the longer intermolecular distances (up to 5 Å).  相似文献   

5.
The advantages offered by ultra-fast (>60 kHz) magic angle spinning (MAS) rotation for the study of biological samples, notably containing paramagnetic centers are explored.It is shown that optimal conditions for performing solid-state 13C NMR under 60 kHz MAS are obtained with low-power CW 1H decoupling, as well as after a low-power 1H,13C cross-polarization step at a double-quantum matching condition. Acquisition with low-power decoupling highlights the existence of rotational decoupling sidebands. The sideband intensities and the existence of first and second rotary conditions are explained in the framework of the Floquet–van Vleck theory.As a result, optimal 13C spectra of the oxidized, paramagnetic form of human copper zinc superoxide dismutase (SOD) can be obtained employing rf-fields which do not exceed 40 kHz during the whole experiment. This enables the removal of unwanted heating which can lead to deterioration of the sample. Furthermore, combined with the short 1H T1s, this allows the repetition rate of the experiments to be shortened from 3 s to 500 ms, thus compensating for the sensitivity loss due to the smaller sample volume in a 1.3 mm rotor. The result is that 2D 13C–13C correlation could be acquired in about 24 h on less than 1 mg of SOD sample.  相似文献   

6.
The nanocrystalline material of 15 mol% Gd-doped ceria (Ce0.85Gd0.15O2−δ ) was prepared by citrate auto ignition method. The electrical study and dielectric relaxation technique were applied to investigate the ionic transport process in this nanocrystalline material with an average grain size of 13 nm and the dynamic relaxation parameters are deduced in the temperature range of 300–600°C. The ionic transference number in the material is found to be 0.85 at 500°C at ambient conditions. The oxygen ionic conduction in the nanocrystalline Ce0.85Gd0.15O2−δ material follows the hopping mechanism. The grain boundary relaxation is found to be associated with migration of charge carriers. The frequency spectra of modulus M″ exhibited a dielectric relaxation peak corresponding to defect associates (Gd-Vo\blacksquare \blacksquare)\blacksquare(\mathrm{Gd}\mbox{-}\mathrm{V}_{\mathrm{o}}^{_{_{{\blacksquare\,\blacksquare}}}})^{_{_{{\blacksquare}}}}. The material exhibits very low values of migration energy and association energy of the oxygen vacancies in the long-range motion, i.e., 0.84 and 0.07 eV, respectively.  相似文献   

7.
A five-channel (1H, 19F, 31P, 27Al, 13C) 2.5 mm magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) probe is used in combination with three separate receivers for the parallel acquisitions of one (1D) and two-dimensional (2D) NMR spectra in model fluorinated aluminophosphate and porous Al-based metal-organic framework (MOF). Possible combinations to record simultaneously spectra using this set-up are presented, including (i) parallel acquisitions of quantitative 1D NMR spectra of solids containing nuclei with contrasted T1 relaxation rates and (ii) parallel acquisitions of 2D heteronuclear NMR spectra. In solids containing numerous different NMR-accessible nuclei, the number of NMR experiments that have to be acquired to get accurate structural information is high. The strategy we present here, i.e. the multiplication of both the number of irradiation channels in the probe and the number of parallel receivers, offers one possibility to optimize this measurement time.  相似文献   

8.
An experimental comparison of sensitivity and resolution of satellite transition (ST) MAS and multiple quantum (MQ) MAS was performed for 27Al (I = 5/2) using several pulse sequences with a z-filter and SPAM, and two inorganic samples of kaolin (Al2Si2O5(OH)4) and glass (43.1CaO–12.5Al2O3–44.4SiO2). Six pulse sequences of STMAS (double-quantum filter-soft pulse added mixing = DQF-SPAM, double-quantum filter = DQF, double-quantum = DQ) and MQMAS (3QMAS-z-filter = 3Qz, 3QMAS-SPAM = 3Q-SPAM, 5QMAS-z-filter = 5Qz) are employed. All experiments have been conducted utilizing a static field of 16.4 T (700 MHz for 1H) and a rotor spinning frequency of 20 kHz. Dependence of S/N ratios as a function of radio frequency (r.f.) field strengths indicates that strong r.f. fields are essential to obtain a better S/N ratio in all experiments. High sensitivity is obtained in the following order: DQF-SPAM, DQF, DQ, 3QSPAM, and 3Qz, although the degree of sensitivity enhancement given by STMAS for glass is slightly smaller than that for kaolin. This might be due to the different excitation and conversion efficiencies of ST and MQ coherences as a function Cq values because quadrupolar interaction of the glass are widely distributed, or to motional broadening caused by framework flexibility in the structure of glass. With respect to resolution, the full widths at half maximum (FWHM) of F1 projections of DQF-STMAS and 3QMAS spectra for kaolin are found to be comparable, which agrees with a simulated result reported in a literature. For glass, the STMAS possess slightly wider line widths than 3QMAS. However, because such a difference in line widths of STMAS and 3QMAS spectra is substantially small, we have concluded that STMAS and 3QMAS have comparable resolution for crystalline and non-crystalline materials.  相似文献   

9.
Spin-lattice relaxation processes in 13CH3 groups in methyl compounds are studied both theoretically and experimentally. The four spin-½ nuclei in such methyl groups give rise to 16 spin-rotational states, which are split by rotational tunnelling. From the corresponding populations (15 independent) five long lived combinations are formed: the 13C magnetization M C, proton magnetization M H, tunnelling energy TE, rotational polarization RP and dipolar energy DE. Their spin-lattice relaxation via the transitions induced by the 13C-proton dipolar interaction is studied in detail. Direct relaxation rates and coupling terms between these combinations are derived. Predictions are compared with experimental data for 13C spin-lattice relaxation at 75.4 MHz in 99% enriched (only methyl carbons enriched) single crystal of aspirin. Above 40 K, the M C recovery is exponential and describable in terms of the direct relaxation transitions without couplings. The same is true for the initial relaxation in the region of non-exponential relaxation between 30 K and 40 K. The orientation dependence of the initial relaxation rate agrees with the theoretical calculations. The non-exponentiality is related to resonant level-crossing transitions with ωt, + ωC = ωH, where the angular frequencies represent rotational tunnelling and carbon and proton resonances, respectively. The resonant transitions produce couplings between M C, M H and TE that are described quite accurately by the present model.  相似文献   

10.
The first carbon-13 shift thermometer for the temperature range of 100–300 K is based on the very rapid equilibration of a pair of semibullvalene valence tautomers. The temperature dependence of the equilibrium constant is reflected in strongly temperature-dependent shift differences Δδ between averaged signals, e.g.,d(Δδ)/dT= 0.051 ppm K−1at 300, 0.087 ppm K−1at 200, and 0.175 ppm K−1at 110 K for the quaternary carbon atoms C2 and C6. At 37 temperaturesT, which were measured with calibrated platinum resistance thermometers, shift differences Δδ were taken from nondecoupled carbon-13 spectra recorded from solutions of 1 in mixtures of chlorodifluoromethane and deuterated dimethyl ether without spinning. The least-squares fit of these Δδ vsTdata to a polynomial equation of the fourth degree (Eq. [5],r2= 0.9999) allows the calculation of temperatures from measured shift differences with a standard deviation of 0.46 K and an estimated error of about 1 K. The heating effects of WALTZ-16 decoupling and the influence of solvents on Δδ are investigated. A comparison with existing NMR thermometers demonstrates the superior performance of the new carbon-13 shift thermometer with respect to precision and the accessible temperature range.  相似文献   

11.
A class of well behaved charged analogues of Schwarzchild’s interior solution has been obtained using a particular electric intensity. The solutions of this class are utilized to depict a superdense star model with surface density 2×1014 g cm−3. The solution obtained is new and the pressure (p), density (c 2 ρ), velocity of sound and (p/(c 2 ρ)) are monotonically decreasing towards the pressure free interface. Moreover the adiabatic constant is found to be more than (4/3) which is necessary for stability under radial perturbation. Also the electric intensity increases monotonically towards the surface. The well behaved model has the maximum mass M=1.740793M Θ , Radius 12.130308 km. The redshift at the center and on the surface is given by z 0=0.384261 and z a =0.292489. Out of the models of superdense star obtained couple of models represent Vela Pulsar for (i) α 2=1.03, b=0.33, , Radius=10.8566 km, M=1.18331M Θ , I=0.642601×1045, (ii) α 2=1.1, b=0.3, , Radius=11.197533 km, M=1.311438M Θ , I=0.774508×1045. All the solutions mentioned above are reducible to Schwarzchild interior solution in the absence of charge.  相似文献   

12.
The effect of bulk magnetic susceptibility (BMS) on solid state NMR spectra of paramagnetic compounds was investigated theoretically and experimentally. The BMS shift was calculated for cylindrical and spherocylinderical containers with some ratios of the lengthLand the diameterD. The results show the best resolution can be obtained by using a long cylindrical sample container withL/D> 10 and by exciting only the region near the center of the container. The effect of the random orientations and distributions of crystallites in a powder sample was also calculated according to a model proposed by Schwerket al.[J. Magn. Reson. A119, 157 (1996)] with removing the Fermi contact term from their model. Static and the magic-angle spinning13C NMR spectra were recorded on two paramagnetic compounds of Ln(C2D5SO4)3· 8H2O where Ln = Pr, Yb. The modified theory predicts the BMS broadening of the experimental spectra very well.  相似文献   

13.
Nitrogen-14 (spin I = 1) has always been a nucleus difficult to observe in solid-state NMR and until recently its observation was restricted to one-dimensional (1D) spectra. We present here the first 3D 1H–13C–14N NMR correlation spectrum. This spectrum was acquired on a test sample l-histidine·HCl·H2O using a recently developed technique, which consists in indirectly observing 14N nuclei via dipolar recoupling with an HMQC-type experiment.  相似文献   

14.
The interaction of three common herbicides, paraquat, acifluorfen and alachlor, with spinach chloroplast photosystem II (PS II) was investigated by measuring1H nuclear magnetic resonance spin-lattice relaxation rates, transient nuclear Overhauser effect (NOE) and NOE spectroscopy (NOESY) spectra. Binding to PS II was detected by (i) the enhancement of single-selective relaxation rates and (ii) the decrease in the optimal mixing time providing maximal cross-peak intensity in NOESY spectra. Titration of relaxation enhancements was used to calculate the dissociation constants (K d) from the bound state for paraquat (K d = 292 ± 71 μM−1) and acifluorfen (K d = 311 ± 58 μM−1). A similarK d was apparent for alachlor. Double-selective relaxation rates allowed the isolation of dipolar relaxation terms between selected proton pairs wherefrom dynamic features of the bound state were evaluated. In all cases the motional correlation time of bound herbicide (τc = 0.1−0.4 ns at 300 K) was found two orders of magnitude slower than in the free-solution state. In the case of alachlor the E and Z isomers were observed to bind differently to PS II and a change in conformation could be hypothesized.  相似文献   

15.
Dynamics in a DNA decamer duplex,d(CATTTGCATC) ·d(GATGCAAATG), were investigated via a detailed13C NMR relaxation study. Every 2′-deoxyadenosine and 2′-deoxyguanidine was chemically enriched with 15%13C and 98%15N isotopes. Six nuclear relaxation parameters [R(13Cz),R(1Hz),R(21Hz13Cz),R(13Cx),R(21Hz13Cx) and steady-state13C{1H} NOE] were measured at 600 MHz and three were measured at 500 MHz (1H frequency) for the CH spin systems of sugar 1′, 3′, and 4′ as well as base 8 and 2 positions. A dependence of relaxation parameter values on chemical position was clearly observed; however, no sequence-specific variation was readily evident within our experimental error of ∼5–10%, except for 3′ and 5′ termini. It was demonstrated that the random 15%13C enrichment effectively suppressed both scalar and dipolar contributions of the neighboring carbons and protons on the relaxation parameters. To analyze dynamics via all observed relaxation parameters, full spectral density mapping (1992, J. W. Peng and G. Wagner,J. Magn. Reson.98, 308) and the “model-free” approach (1982, Lipari and Szabo,J. Am. Chem. Soc.104, 4546) were applied complementarily. A linear correlation between three spectral density values,JC),JH− ωC), andJH+ ωC) was observed in plots containing all measured values, but not for the other spectral density terms includingJ(0). These linear correlations reflect the effect of overall motion and similar internal motions for each CH vector in the decamer. The correlations yielded two correlation times, 3–4 ns and 10–200 ps. One value, 3–4 ns, corresponds to the value of 3.3 ns obtained for the overall isotropic tumbling correlation time determined from analysis of13C T1/T2 ratios. The possibility of overall anisotropic tumbling was examined, but statistical analysis showed no advantage over the assumption of simple isotropic tumbling. Lack of correlations entailingJ(0) implies that a relatively slow chemical exchange contributes to yielding of effectiveJeff(0) values. Based on spectral density mapping and the T1/T2 ratio analysis, three basic assumptions were initially employed (and subsequently justified) for the model-free calculation: isotropic overall tumbling, one internal motion, and the presence of chemical exchange terms. Except for terminal residues, the order parameterS2and the corresponding fast internal motion correlation time were determined to be about 0.8 ± 0.1 and 20 ± 20 ps, respectively, for the various CH vectors. Only a few differences were observed between or within sugars and bases. The internal motion is very fast (ps–ns time scale) and its amplitude restricted; e.g., assuming a simple wobble-in-a-cone model, the internal motion is restricted to an angular amplitude of ±22.5° for each of the 1′, 3′, 4′, 2, and 8 positions in the purine nucleotides in the entire duplex.  相似文献   

16.
In reported low-field nuclear magnetic resonance (NMR) measurements using Superconducting Quantum Interference Device (SQUID) detection, the pre-polarizing magnetic field has been usually oriented orthogonal to the measuring field, Bp  Bm. Melton et al. systematically analyzed the consequences of Bp decay in time after turnoff and showed that this decay should be nonadiabatic. We evaluated our measuring procedure in the light of that analysis, and found good quantitative agreement. It was showed that, when the decay time constant is comparable to the precession period of the magnetization of the sample, M, the optimum procedure is to orient Bp parallel to Bm and to apply a π/2 pulse to flip M, similar as in the case of conventional NMR.  相似文献   

17.
The accurate temperature measurement of solid samples under magic-angle spinning (MAS) is difficult in the cryogenic regime. It has been demonstrated by Thurber et al. (J. Magn. Reson., 196 (2009) 84-87) [10] that the temperature dependent spin-lattice relaxation time constant of 79Br in KBr powder can be useful for measuring sample temperature under MAS over a wide temperature range (20–296 K). However the value of T1 exceeds 3 min at temperatures below 20 K, which is inconveniently long. In this communication, we show that the spin-lattice relaxation time constant of 127I in CsI powder can be used to accurately measure sample temperature under MAS within a reasonable experimental time down to 10 K.  相似文献   

18.
Quantum effects in theM w -M z relation are studied in the framework of the standard SU(2)×U(1) electroweak theory. After reviewing this relation in detail, numerical analysis is carried out by using recent data of ALEPH, DELPHI, L3, OPAL and MARK II collaborations onM z , and those of CDF and UA2 collaborations onM w . For the weighted averageM z exp =91.157±0.032 GeV, theW mass is calculated asM w (0) =80.90±0.04 GeV,M w (l.l) =79.75±0.04±0.02 GeV andM w =80.21±0.04±0.04 GeV at the tree, leading-log and full correction level (form top=140 GeV andm Higgs=100 GeV) respectively. Remarkable is that only the fully correctedM w is consistent withM w exp =80.24±0.37 GeV, which clearly demands the heavy top-quark correction, and therefore gives a strong phenomenological support, independent of the neutral current data, to the electroweak theory as a renormalizable quantum field theory with the spontaneous symmetry breakdown.  相似文献   

19.
EPR of Gd3+ doped in Ce2M3(NO3)12.24H2O (M″ = Mg, Zn, Co) single crystals has been studied at various temperatures from room temperature to 77 K using ∼ 9.45 GHz EPR spectrometer. The observation of resolved Gd3+ spectra at room temperature in Ce3Co2(NO3)12.24H2O has been interpreted in terms of a random modulation of the interaction between the Gd3+ and the Co2+ ions by the rapid spin-lattice relaxation of Co2+ ions. It is found that the effective spin-lattice relaxation time T1T−n where n = 1.85 (Bz axis) and n = 1.75 (Bz axis) if 103 < T < 283 K.  相似文献   

20.
Electron paramagnetic resonance (EPR) study of Cu2+-doped sodium zinc sulfate tetrahydrate is done at liquid nitrogen temperature. Two magnetically equivalent sites for Cu2+ are observed. The spin-Hamiltonian parameters determined by fitting the EPR spectra to the rhombic-symmetry crystalline field are g x  = 2.2356, g y  = 2.0267, g z  = 2.3472, A x  = 27 × 10−4 cm−1, A y  = 54 × 10−4 cm−1and A z  = 88 × 10−4 cm−1. The ground state wave function is also determined. The g-anisotropy is evaluated and compared with the experimental value. With the help of optical study, the nature of bonding in the complex is discussed.  相似文献   

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