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Spin Systems (spin + system)
Selected AbstractsThe theory of cosy NMR experiments revisited: Application to an AX spin system of quadrupolar nucleiCONCEPTS IN MAGNETIC RESONANCE, Issue 3 2010P. Kempgens Abstract The two-dimensional nuclear magnetic resonance correlation spectroscopy (2D NMR COSY) spectrum of an AX spin system of spin-1 has been calculated by numerical density matrix calculations. The mathematical expressions found are valid for an AX spin system of any spins. These expressions should be used to calculate the 2D NMR COSY spectrum of an AX spin system of high spin nuclei as their use will significantly simplify the calculations. More precisely, one needs to calculate only one set of coefficients despite the need of two steps in the phase cycling to achieve phase modulation during t1. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 170,177, 2010. [source] Cross-correlated and conventional dipolar carbon-13 relaxation in methylene groups in small, symmetric moleculesCONCEPTS IN MAGNETIC RESONANCE, Issue 2 2007Leila Ghalebani Abstract A theory for dipolar cross-correlated relaxation processes in AMX or AX2 spin system, with special reference to 13C-methylene groups, is reviewed briefly. Simple experiments and protocols for measuring the transfer rates between the carbon-13 Zeeman order and the three-spin order, and for their analogues in the transverse plane, are discussed using a concentrated solution of the disaccharide trehalose as a model system. Experimental data sets consisting of conventional carbon-13 relaxation parameters (T1, T2, and NOE), along with the cross-correlated relaxation rates, are also presented for some small, rigid, polycyclic molecules. These data are interpreted using spectral density functions appropriate to spherical or symmetric tops reorienting according to small-step rotational diffusion model. The analysis results in a consistent picture of the auto- and cross-correlated spin relaxation processes. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 100,115, 2007. [source] Geometric algebra and transition-selective implementations of the controlled-NOT gateCONCEPTS IN MAGNETIC RESONANCE, Issue 1 2004Timothy F. Havel Geometric algebra provides a complete set of simple rules for the manipulation of product operator expressions at a symbolic level, without any explicit use of matrices. This approach can be used not only to describe the state and evolution of a spin system, but also to derive the effective Hamiltonian and associated propagator in full generality. In this article, we illustrate the use of geometric algebra via a detailed analysis of transition-selective implementations of the controlled-NOT gate, which plays a key role in NMR-based quantum information processing. In the appendices, we show how one can also use geometric algebra to derive tight bounds on the magnitudes of the errors associated with these implementations of the controlled-NOT. © 2004 Wiley Periodicals, Inc. Concepts Magn Reson Part A 23A: 49,62, 2004 [source] Syntheses and spectroscopic investigation of some cyclophosphazanes: Analysis of pseudo-triplet splittingHETEROATOM CHEMISTRY, Issue 4 2006Khodayar Gholivand Some new phosphoramidates, 1,3, and the corresponding cyclophosphazanes, 4,6, with formula Cl2P(p -NHC6H4CH3) 1, Cl2P(O)(p -NHC6H4NO2) 2, (CH3)2NP(O)Cl(p -NHC6H4CH3)3, [ClP(p -NC6H4CH3)]24, [ClP(O)(p -NC6H4NO2)]25, and [(CH3)2NP(O)(p -NC6H4CH3)]26 were synthesized and characterized by 1H, 13C, 31P NMR, IR, mass spectroscopy, and elemental analysis. A pseudo-triplet signal was observed in the 1H NMR spectrum of molecule 6 for the N(CH3)2 protons. The A6A, 6X2 spin system was suggested for the pseudo-triplet pattern of 3JPNCH coupling in this molecule. Ab initio calculations were performed at the HF and B3LYP levels of theory with 6-311G** standard basis set on the geometry of compound 6. Also, the NMR chemical shift calculations were done to compare the computed results with the experimental ones. The calculated results are in good agreement with experimental data. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:337,343, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20229 [source] Metabolic profile of lettuce leaves by high-field NMR spectraMAGNETIC RESONANCE IN CHEMISTRY, Issue 8 2005Anatoli P. Sobolev Abstract A detailed analysis of the proton high-field NMR spectra of aqueous and organic extracts of lettuce leaves is reported for the first time. A combination of COSY, TOCSY, 1H,13C HSQC, 1H,13C HMBC bidimensional sequences and DOSY was used to assign each spin system and to separate the components of the complex patterns. A large number of water-soluble metabolites belonging to different classes such as carbohydrates, polyols, organic acids and amino acids were fully assigned. Moreover, the complex spectra of metabolites extracted in organic solvents belonging to sterols, fatty acids, diacylglycerophospholipids, galactosyldiacylglycerols, sulpholipids, pheophytins, carotenoids and hydrocarbons were also assigned. Copyright © 2005 John Wiley & Sons, Ltd. [source] Structure elucidation of [1,3]oxazolo[4,5- e][2,1]benzisoxazole and naphtho[1,2- d][1,3]- and phenanthro[9,10- d]oxazoles using gradient selected gHMBC, gHMQC and gHMQC-TOCSY NMR techniquesMAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2003Alan R. Katritzky Abstract Structure elucidation of compounds in the benzisoxazole series (1,6) and naphtho[1,2- d][1,3]- (7,10) and phenanthro[9,10- d][1,3]oxazole (11,14) series was accomplished using extensive 2D NMR spectroscopic studies including 1H,1H COSY, long- range 1H,1H COSY, 1H,13C COSY, gHMQC, gHMBC and gHMQC-TOCSY experiments. The distinction between oxazole and isoxazole rings was made on the basis of the magnitude of heteronuclear one-bond 1JC2, H2 (or 1JC3, H3) coupling constants. Complete analysis of the 1H NMR spectra of 11,14 was achieved by iterative calculations. Gradient selected gHMQC-TOCSY spectra of phenanthro[9,10- d][1,3]oxazoles 11,14 were obtained at different mixing times (12, 24, 36, 48 and 80 ms) to identify the spin system where the protons of phenanthrene ring at H-5, H-6 and at H-9 and H-7 and H-8 were highly overlapping. Copyright © 2003 John Wiley & Sons, Ltd. [source] Methyl TROSY: explanation and experimental verificationMAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2003Jason E. Ollerenshaw Abstract In TROSY experiments, relaxation interference effects are exploited to produce spectra with improved resolution and signal-to-noise. Such experiments cannot be explained using the standard product operator formalism, but must instead be analyzed at the level of individual density matrix elements. Herein we illustrate this point using an example from our recent work on a TROSY 1H,13C correlation experiment for methyl groups in large proteins. Methyl groups are useful spectroscopic probes of protein structure and dynamics because they are found throughout the critical core region of a folded protein and their resonances are intense and well dispersed. Additionally, it is relatively easy to produce highly deuterated protein samples that are 1H,13C labeled at selected methyl positions, facilitating studies of high molecular weight systems. Methyl groups are relaxed by a network of 1H,1H and 1H,13C dipolar interactions, and in the macromolecular limit the destructive interference of these interactions leads to unusually slow relaxation for certain density matrix elements. It is this slow relaxation that forms the basis for TROSY experiments. We present a detailed analysis of evolution and relaxation during HSQC and HMQC pulse schemes for the case of a 13C1H3 spin system attached to a macromolecule. We show that the HMQC sequence is already optimal with respect to the TROSY effect, offering a significant sensitivity enhancement over HSQC at any spectrometer field strength. The gain in sensitivity is established experimentally using samples of two large proteins, malate synthase G (81.4 kDa) and ClpP protease (305 kDa), both highly deuterated and selectively 1H,13C-labeled at isoleucine , methyl positions. Copyright © 2003 John Wiley & Sons, Ltd. [source] Proton high-field NMR study of tomato juiceMAGNETIC RESONANCE IN CHEMISTRY, Issue 4 2003Anatoli P. Sobolev Abstract A detailed analysis of the proton high-field (600 MHz) NMR spectra of tomato juice and pulp is reported for the first time. A combination of J -resolved, COSY, TOCSY, DOSY, 1H,13C HSQC and 1H,13C HMBC 2D sequences was used to assign each spin system and to separate the components of the complex patterns in the 1D overlapped proton spectra. To obtain resolved proton spectra of tomato pulps the high-resolution magic angle spinning technique was used; a comparison with the liquid-state NMR spectra of the corresponding juices was accomplished. On the basis of the assignments made, the chemical composition of tomato juices from two cultivars (Red Setter and Ciliegino) was determined. Copyright © 2003 John Wiley & Sons, Ltd. [source] A tutorial comparison of the NMRIT and LAOCOON approaches for analyses of complex solution-phase nuclear magnetic resonance spectraMAGNETIC RESONANCE IN CHEMISTRY, Issue 5 2002Stanley L. Manatt Abstract For spin ½ nuclei the two most frequently used iterative approaches for determinations of NMR chemical shifts (hi) and coupling constants (Ji), NMRIT and LAOCOON, are discussed. When multiple pulse techniques for extraction of these parameters fail or lead to complicated spectra in the cases of very strongly coupled spin systems and systems involving magnetically nonequivalent, chemical shift equivalent nuclei, recourse to these iterative methods is necessary. The former approach employs the energy levels derived from the observed transition frequencies, whereas the latter approach uses the observed transition frequencies. Derivations of the general iterative equations for both approaches are given, along with the specific equations for the ABC spin system. The matrix nature of the derivation of these equations is stressed. Also discussed is the modified NMRIT program, NMRENIT, which has major advantages over the former in cases with symmetry and in cases where not enough lines can be assigned to link all the energy levels. The advantages of the latter program over LAOCOON in certain cases are discussed. Some general advice on the use of both approaches is presented. The Hoffman energy level approach is also discussed and it is shown that it yields the same iterative equations as the LAOCOON approach. Copyright © 2002 John Wiley & Sons, Ltd. [source] Connectivities in molecules by INADEQUATE: recent developments,MAGNETIC RESONANCE IN CHEMISTRY, Issue 1 2002J. Buddrus Abstract The INADEQUATE (Incredible Natural Abundance DoublE QUAntum Transfer Experiment) consists of a pulse sequence which eliminates the NMR signals from isolated spins (spin system A) displaying signals from coupled spins (spin system AX or higher spin systems). It is of great importance when applied to molecules with skeleton elements such as carbon, silicon or tungsten, all of which contain a small percentage of spin-½ isotopes embedded in magnetically inactive isotopes. Analysis of the AX type spectra gives one-bond and long-range coupling constants (see compound 6) and, most important, the connectivity pattern of the skeleton atoms in molecules of unknown structure such as the carbon compounds 1 or 2, the silicon compound 3 or the lithium compound 4b. Unfortunately, INADEQUATE is rather insensitive, in the case of carbon only one out of 104 molecules gives the desired response. Efforts to reduce this drawback are described; recently, a remarkable step forward has been made by concentrating the four lines of an AX spin system to just two signals (see Fig. 2). Copyright © 2001 John Wiley & Sons, Ltd. [source] In vivo differentiation of N-acetyl aspartyl glutamate from N-acetyl aspartate at 3 TeslaMAGNETIC RESONANCE IN MEDICINE, Issue 6 2007Richard A.E. Edden Abstract A method is described that allows the in vivo differentiation of N-acetyl aspartate (NAA) from N-acetyl aspartyl glutamate (NAAG) by in vivo MR spectroscopy (MRS) at 3 Tesla (3T). The method, which is based on MEGA-point-resolved spectroscopy (PRESS) editing, selectively targets the aspartyl spin system of one species while deliberately removing the other species from the spectrum. This allows quantitative measurements of NAA and NAAG without the need for fitting of unresolved peaks. White matter concentrations of NAA (6.7 ± 0.3 mM) and NAAG (2.2 ± 0.3 mM) were measured in 10 healthy volunteers to demonstrate the method. Magn Reson Med 57:977,982, 2007. © 2007 Wiley-Liss, Inc. [source] Dynamics of spin interactions in diluted magnetic semiconductor heterostructuresPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2007*Article first published online: 19 DEC 200, D. R. Yakovlev Abstract This paper gives an overview of the recent studies of spin dynamics in diluted magnetic semiconductor heterostructures based on (Zn,Mn)Se and (Cd,Mn)Te. The spin dynamics is controlled by energy and spin transfer between systems of magnetic ions, lattice (phonon system) and free carriers. Spin,lattice relaxation time of the Mn spin system is a very strong function of the Mn content, it decreases by five orders of magnitude when the Mn content changes from 0.4 to 11%. Additionally this time can be tuned by the varying free carrier concentration and by the growth of heteromagnetic structures with inhomogeneous profile of Mn ions. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Synthesis, X-ray Single-Crystal Structure Determination, and Magnetic Properties of [Rb(benzo[18]crown-6)]+ Salts Containing Well-Ordered Fulleride Trianions C603,,CHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2009Bele Boeddinghaus Dipl.-Chem. Abstract Finally, a structure with well-resolved C603,ions and S=1/2 spin system: The two novel title compounds have clearly S=1/2 and not S=3/2 electronic states, as expected for the occupation of the triply degenerate LUMO of C60. These structures with well-ordered fullerene trianions show that the expected Jahn,Teller distortion is not observable in X-ray diffraction experiments. Crystals of the composition [Rb(benzo[18]crown-6)]3[,3 -C60](C7H8)(C3H7NO)4.5 (1) and [Rb(benzo[18]crown-6)]3[C60](C3H7NO)5.33(C4H8O)1.66 (2) were synthesized by reduction of C60 with elemental rubidium in a mixture of dimethylformamide/tetrahydrofuran in the presence of benzo[18]crown-6. X-ray single-crystal structure determinations reveal well-ordered C60 trianions in both compounds. In contrast to previously reported structures of compounds containing C603, ions the present structure refinements allow the description of the buckyballs with good resolution (R1/wR2 for I>2,(I) (all data) are 0.030/0.069 (0.051/0.071) and 0.042/0.101 (0.076/0.108) for 1 and 2, respectively). In 1 the C603, unit coordinates in an ,3 -fashion to one Rb atom, whose coordination sphere is completed by one benzo[18]crown-6 and one dimethylformamide molecule. In 2 no coordination of the alkali metal atom to the C60 trianion is observed. Magnetic measurements for 1 reveal a magnetic moment of 1.74,,B, as expected for an isolated S=1/2 spin system. The high-quality data of the present studies allowed a comparison of structural details of C603, ions with other C60n, units (n=0, 2, 3) and a discussion of the structural Jahn,Teller distortions. [source] Cyano-bridged Ln3+ -Cr3+ Binuclear Complexes [Ln(L)x(H2O)yCr(CN)6]·mL·nH2O (Ln=La,Nd, x=5, y=2, m=1 or 2, n=2 or 2.5; Ln=Sm,Dy, Er, x=4, y=3, m=0, n=1.5 or 2.0; L=2-pyrrolidinone): Structure, Magnetism and Spin Density MapCHINESE JOURNAL OF CHEMISTRY, Issue 3 2007Xian-Ru Sun Abstract In order to shed light upon the nature and mechanism of 4f-3d magnetic exchange interactions, a series of binuclear complexes of lanthanide(3+) and chromium(3+) with the general formula [Ln(L)5(H2O)2Cr(CN)6]·mL·nH2O (Ln=La (1), Ce (2), Pr (3), Nd (4); x=5, y=2, m=1 or 2, n=2 or 2.5; L=2-pyrrolidinone) and [Ln(L)4(H2O)3Cr(CN)6] ·nH2O (Ln=Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Er (10); x=4, y=3, m=0, n=1.5 or 2.0; L2-pyrrolidinone) were prepared and the X-ray crystal structures of complexes 2, 6 and 7 were determined. All the compounds consist of a Ln-CN-Cr unit, in which Ln3+ in a square antiprism environment is bridged to an octahedral coordinated Cr3+ ion through a cyano group. The magnetic properties of the complexes 3 and 6,10 show an overall antiferromagnetic behavior. The fitting to the experimental magnetic susceptibilities of 7 give g=1.98, J=0.40 cm,1, zJ,=,0.21 cm,1 on the basis of a binuclear spin system (SGd=7/2, SCr=3/2), revealing an intra-molecular Gd3+ -Cr3+ ferromagnetic interaction and an inter-molecular antiferromagnetic interaction. For 7 the calculation of quantum chemical density functional theory (DFT), combined with the broken symmetry approach, showed that the calculated spin coupling constant was 20.3 cm,1, supporting the observation of weak ferromagnetic intra-molecular interaction in 7. The spin density distributions of 7 in both the high spin ground state and the broken symmetry state were obtained, and the spin coupling mechanism between Gd3+ and Cr3+ was discussed. [source] Lie Theory for Quantum ControlGAMM - MITTEILUNGEN, Issue 1 2008G. Dirr Abstract One of the main theoretical challenges in quantum computing is the design of explicit schemes that enable one to effectively factorize a given final unitary operator into a product of basic unitary operators. As this is equivalent to a constructive controllability task on a Lie group of special unitary operators, one faces interesting classes of bilinear optimal control problems for which efficient numerical solution algorithms are sought for. In this paper we give a review on recent Lie-theoretical developments in finite-dimensional quantum control that play a key role for solving such factorization problems on a compact Lie group. After a brief introduction to basic terms and concepts from quantum mechanics, we address the fundamental control theoretic issues for bilinear control systems and survey standard techniques fromLie theory relevant for quantum control. Questions of controllability, accessibility and time optimal control of spin systems are in the center of our interest. Some remarks on computational aspects are included as well. The idea is to enable the potential reader to understand the problems in clear mathematical terms, to assess the current state of the art and get an overview on recent developments in quantum control-an emerging interdisciplinary field between physics, control and computation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Semiautomatic sequence-specific assignment of proteins based on the tertiary structure,The program st2nmrJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2002Primo, Pristov Abstract The sequence-specific assignment of resonances is still the most time-consuming procedure that is necessary as the first step in high-resolution NMR studies of proteins. In many cases a reliable three-dimensional (3D) structure of the protein is available, for example, from X-ray spectroscopy or homology modeling. Here we introduce the st2nmr program that uses the 3D structure and Nuclear Overhauser Effect spectroscopy (NOESY) peak list(s) to evaluate and optimize trial sequence-specific assignments of spin systems derived from correlation spectra to residues of the protein. A distance-dependent target function that scores trial assignments based on the presence of expected NOESY crosspeaks is optimized in a Monte Carlo fashion. The performance of the program st2nmr is tested on real NMR data of an ,-helical (cytochrome c) and ,-sheet (lipocalin) protein using homology models and/or X-ray structures; it succeeded in completely reproducing the correct sequence-specific assignments in most cases using 2D and/or 15N/13C Nuclear Overhauser Effect (NOE) data. Additionally to amino acid residues the program can also handle ligands that are bound to the protein, such as heme, and can be used as a complementary tool to fully automated assignment procedures. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 335,340, 2002 [source] 1H and 13C NMR study of a series of C-9-substituted 19-norsteroidsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 1 2002Laurence Dallery Abstract The 1H and 13C NMR spectra of a series of 9-substituted 19-norsteroids were completely assigned using a series of 2D NMR experiments, which included 1H,1H COSY, NOESY and 1H,13C heteronuclear HETCOR and HMQC. For second-order spin systems, chemical shifts and coupling constants were obtained by simulation of the experimental spectrum. Criteria were deduced to characterize the stereochemistry of these different compounds (multiplet pattern of H-8, and variation of chemical shifts). The results allow the easy determination of the configuration at C-9. Conformational changes resulting from the substitution were studied by NMR and molecular modeling calculations (AM1). Copyright © 2001 John Wiley & Sons, Ltd. [source] Precision 1H,1H distance measurement via 13C NMR signals: utilization of 1H,1H double-quantum dipolar interactions recoupled under magic angle spinning conditionsMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2004Yoh Matsuki Abstract We applied the POST-C7 DQ-dipolar recoupling pulse sequence to the measurement of 1H,1H distances with high precision. The spectral resolution is enhanced by detecting the 1H magnetization via 13C signals. A least-squares fitting of the build-up curve of the transferred magnetization to the exact numerical simulations yielded a 1H,,1H, distance of 248 ± 4 pm for fully 13C-labeled L -valine. This distance agrees with the neutron diffraction study. The negative transferred magnetization clearly indicates that the direct DQ 1H,1H dipolar couplings have the largest effect. The signal for the magnetization transfer builds up rapidly by the direct 1H,1H dipolar coupling, and decreases to zero at longer mixing time when the relayed magnetization transfer becomes significant. This large intensity change of the signal leads to the high precision in the distance measurement. We inspected factors that limit the effective bandwidth of the POST-C7 recoupling for the 1H and 13C homonuclear spin systems. The spin interactions at times shorter than the cycle time of the C7 sequence were also evaluated to measure the distances. The carbon-detected 2D 1H DQ mixing experiment was demonstrated for the measurement of multiple 1H,1H distances. Copyright © 2004 John Wiley & Sons, Ltd. [source] A tutorial comparison of the NMRIT and LAOCOON approaches for analyses of complex solution-phase nuclear magnetic resonance spectraMAGNETIC RESONANCE IN CHEMISTRY, Issue 5 2002Stanley L. Manatt Abstract For spin ½ nuclei the two most frequently used iterative approaches for determinations of NMR chemical shifts (hi) and coupling constants (Ji), NMRIT and LAOCOON, are discussed. When multiple pulse techniques for extraction of these parameters fail or lead to complicated spectra in the cases of very strongly coupled spin systems and systems involving magnetically nonequivalent, chemical shift equivalent nuclei, recourse to these iterative methods is necessary. The former approach employs the energy levels derived from the observed transition frequencies, whereas the latter approach uses the observed transition frequencies. Derivations of the general iterative equations for both approaches are given, along with the specific equations for the ABC spin system. The matrix nature of the derivation of these equations is stressed. Also discussed is the modified NMRIT program, NMRENIT, which has major advantages over the former in cases with symmetry and in cases where not enough lines can be assigned to link all the energy levels. The advantages of the latter program over LAOCOON in certain cases are discussed. Some general advice on the use of both approaches is presented. The Hoffman energy level approach is also discussed and it is shown that it yields the same iterative equations as the LAOCOON approach. Copyright © 2002 John Wiley & Sons, Ltd. [source] 187Os subspectra in 1H and 31P{1H} spectra of triosmium carbonyl clustersMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2002Marc J. Stchedroff Abstract A survey of the use of 187Os satellite subspectra in 1H and 31P{1H} spectra of triosmium carbonyl clusters is reported. By varying evolution delays in HMQC spectra of [Os3(µ-H)2(CO)10] we have selectively extracted the values for 1J(Os,H) and 2J(Os,H), respectively. An analysis of the principal modes of phosphine coordination in triosmium clusters demonstrates that 31P{1H}187Os satellite subspectra are diagnostic for equatorial coordination [1J(Os,P) = 211,223 Hz] or for axial coordination (perpendicular to the plane of the cluster) [1J(Os,P) , 147 Hz]. Chelating and bridging diphosphines yield 187Os satellite subspectra which are the sum of A2X and AA,X spin systems. If significant P,P coupling is present, the AA,X component requires simulation. All observed 2J(Os,P) trans -equatorial couplings fall in the range 38,65 Hz. Copyright © 2001 John Wiley & Sons, Ltd. [source] Optimized detection of lactate at high fields using inner volume saturationMAGNETIC RESONANCE IN MEDICINE, Issue 4 2006Richard A.E. Edden Abstract In localized proton MR spectroscopy (1H-MRS) in vivo, the detection of lactate (Lac) is affected by modulation of its resonances due to homonuclear scalar couplings (J). A simple and convenient way to distinguish Lac from lipids is to set the TE to 1/J so that the Lac signal is inverted while other resonances (such as lipid) remain in-phase. However, at high field strengths, such as 3 Tesla or above, the modulation of the Lac signal is complicated by chemical shift effects that cause modulation patterns to vary within different subregions of the localized volume. Under some conditions the Lac signal may even disappear completely. In this note we introduce the concept of inner volume saturation (IVS), which makes use of high bandwidth spatial pulses to remove the signal corresponding to the regions of the localized volume that contribute unwanted modulation patterns. The method is described theoretically and demonstrated experimentally at 3 Tesla in a phantom and a patient with acute stroke. The phantom measurements indicate that virtually 100% of the Lac signal can be recovered using this method. The method should be feasible at magnetic fields above 3 Tesla, and may also be applied to other coupled spin systems in which modulation effects are important. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] Fast CT-PRESS-based spiral chemical shift imaging at 3 TeslaMAGNETIC RESONANCE IN MEDICINE, Issue 5 2006Dirk Mayer Abstract A new sequence is presented that combines constant-time point-resolved spectroscopy (CT-PRESS) with fast spiral chemical shift imaging. It allows the acquisition of multivoxel spectra without line splitting with a minimum total measurement time of less than 5 min for a field of view of 24 cm and a nominal 1.5 × 1.5-cm2 in-plane resolution. Measurements were performed with 17 CS encoding steps in t1 (,t1 = 12.8 ms) and an average echo time of 151 ms, which was determined by simulating the CT-PRESS experiment for the spin systems of glutamate (Glu) and myo -inositol (mI). Signals from N-acetyl-aspartate, total creatine, choline-containing compounds (Cho), Glu, and mI were detected in a healthy volunteer with no or only minor baseline distortions within 14 min on a 3 T MR scanner. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] Polarization transfer for sensitivity-enhanced MRS using a single radio frequency transmit channelNMR IN BIOMEDICINE, Issue 5 2008D. W. J. Klomp Abstract Polarization transfer techniques are used to enhance sensitivity and improve localization in multinuclear MRS, by transferring polarization from highly polarized or even hyperpolarized nuclei to less sensitive spin systems. Clinical MR scanners are in general not equipped with a second radio frequency (RF) transmit channel, making the conventional implementation of polarization transfer techniques such as distortionless enhanced polarization transfer (DEPT) impossible. Here we present a DEPT sequence using pulses sequentially that can be used on a single RF transmit channel (SC-DEPT). Theoretical simulations, phantom measurements, and in vivo results from human brain at 3,T show that the SC-DEPT method performs as well as the conventional DEPT method. The results indicate that an independent second RF transmit channel for simultaneous pulsing at different nuclear frequencies is not needed for polarization transfer, facilitating the use of these methods with common clinical systems with minor modifications in the RF architecture. Copyright © 2007 John Wiley & Sons, Ltd. [source] The design of excitation pulses for spin systems using optimal control theory: With application to NMR spectroscopyOPTIMAL CONTROL APPLICATIONS AND METHODS, Issue 5 2009Naum I. Gershenzon Abstract This paper considers the use of optimal control theory in designing radio frequency excitation pulses for magnetic spin systems satisfying Bloch dynamics. Such pulses are required in applications of nuclear magnetic resonance to initially transfer sample magnetization vectors to the transverse plane. Once transferred, signals released by nuclei as they respond to a static magnetic field normal to the transverse plane are then analyzed and interpreted. Continuous time deterministic optimal control theory is employed to determine time-dependent pulse amplitudes and frequencies that minimize the distance between final magnetization vectors and a chosen target vector. Pulses are designed to excite a range of resonant frequencies and to tolerate miscalibration errors in applied fields. The model presented permits a unified treatment of the control problem as considered by a variety of authors, and a thorough mathematical analysis of the existence, and characteristics of, optimal excitation pulses. Practical numerical algorithms for designing optimal pulses are given, and the effectiveness of the algorithms is illustrated by comparing the pulses that they generate with those commonly used in high-resolution spectroscopy. Copyright © 2008 John Wiley & Sons, Ltd. [source] Effects of dilution and disorder on magnetism in diluted spin systemsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2007Guixin Tang Abstract The influence of configurational disorder on the magnetic properties of diluted Heisenberg spin systems is studied with regard to the ferromagnetic stability of diluted magnetic semiconductors. The equation of motion of the magnon Green's function is decoupled by Tyablikov approximation. With supercell approach, the concentrations of magnetic ions are determined by the size of the supercell in which there is only one magnetic ion per supercell in our method. In order to distinguish the influence of dilution and disorder, there are two kinds of supercells being used: the diluted and ordered case and the diluted and disordered case. The configurational averaging of magnon Green function due to disorder is treated in the augmented space formalism. The random exchange integrals between two supercells are treated as a matrix. The obtained magnon spectral densities are used to calculate the temperature dependence of magnetization and Curie temperature. The results are shown as following: (i) dilution leads to increasing the averaged distance of two magnetic ions, further decreases the effective exchange integrals and is main reason to reduce Curie temperature; (ii) spatial position disorder of magnetic ions results in the dispersions of the exchange integrals between two supercells and slightly changes ferromagnetic transition temperature; (iii) the exponential damping of distance dependence obviously reduces Curie temperature and should be set carefully in any phenomenological model. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Field-induced effects in the S = 1 two-dimensional Heisenberg antiferromagnetPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2005M. E. Gouvêa Abstract We use the self-consistent harmonic approximation to study the spin S = 1 Heisenberg antiferromagnet on a two-dimensional lattice in a uniform applied magnetic field. The magnetic field destroys the ,,(3) symmetry and an XY -like behavior, with a Berezinskii,Kosterlitz,Thouless (BKT) transition, is expected. We obtain the field dependence of the transition temperature for fields varying in a wide range. There has been experimental evidence that, for low dimensional spin systems, a gap can be induced by the applied magnetic field. We also investigate the behavior of this field-induced gap. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Mixing in time and space for lattice spin systems: A combinatorial viewRANDOM STRUCTURES AND ALGORITHMS, Issue 4 2004Martin Dyer The paper considers spin systems on the d -dimensional integer lattice ,d with nearest-neighbor interactions. A sharp equivalence is proved between decay with distance of spin correlations (a spatial property of the equilibrium state) and rapid mixing of the Glauber dynamics (a temporal property of a Markov chain Monte Carlo algorithm). Specifically, we show that if the mixing time of the Glauber dynamics is O(n log n) then spin correlations decay exponentially fast with distance. We also prove the converse implication for monotone systems, and for general systems we prove that exponential decay of correlations implies O(n log n) mixing time of a dynamics that updates sufficiently large blocks (rather than single sites). While the above equivalence was already known to hold in various forms, we give proofs that are purely combinatorial and avoid the functional analysis machinery employed in previous proofs. © 2004 Wiley Periodicals, Inc. Random Struct. Alg., 2004 [source] Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro- and antiferromagnetsANNALEN DER PHYSIK, Issue 10-11 2009B. Vogt Abstract The physical properties of quantum systems, which are described by the anisotropic Heisenberg model, are influenced by thermal as well as by quantum fluctuations. Such a quantum Heisenberg system can be profoundly changed towards a classical system by tuning two parameters, namely the total spin and the anisotropy field: Large easy-axis anisotropy fields, which drive the system towards the classical Ising model, as well as large spin quantum numbers suppress the quantum fluctuations and lead to a classical limit. We elucidate the incipience of this reduction of quantum fluctuations. In order to illustrate the resulting effects we determine the critical temperatures for ferro- and antiferromagnets and the ground state sublattice magnetization for antiferromagnets. The outcome depends on the dimension, the spin quantum number and the anisotropy field and is studied for a widespread range of these parameters. We compare the results obtained by: Classical Mean Field, Quantum Mean Field, Linear Spin Wave and Random Phase Approximation. Our findings are confirmed and quantitatively improved by numerical Quantum Monte Carlo simulations. The differences between the ferromagnet and antiferromagnet are investigated. We finally find a comprehensive picture of the classical trends and elucidate the suppression of quantum fluctuations in anisotropic spin systems. In particular, we find that the quantum fluctuations are extraordinarily sensitive to the presence of small anisotropy fields. This sensitivity can be quantified by introducing an "anisotropy susceptibility". [source] Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro- and antiferromagnetsANNALEN DER PHYSIK, Issue 10-11 2009B. Vogt Abstract The physical properties of quantum systems, which are described by the anisotropic Heisenberg model, are influenced by thermal as well as by quantum fluctuations. Such a quantum Heisenberg system can be profoundly changed towards a classical system by tuning two parameters, namely the total spin and the anisotropy field: Large easy-axis anisotropy fields, which drive the system towards the classical Ising model, as well as large spin quantum numbers suppress the quantum fluctuations and lead to a classical limit. We elucidate the incipience of this reduction of quantum fluctuations. In order to illustrate the resulting effects we determine the critical temperatures for ferro- and antiferromagnets and the ground state sublattice magnetization for antiferromagnets. The outcome depends on the dimension, the spin quantum number and the anisotropy field and is studied for a widespread range of these parameters. We compare the results obtained by: Classical Mean Field, Quantum Mean Field, Linear Spin Wave and Random Phase Approximation. Our findings are confirmed and quantitatively improved by numerical Quantum Monte Carlo simulations. The differences between the ferromagnet and antiferromagnet are investigated. We finally find a comprehensive picture of the classical trends and elucidate the suppression of quantum fluctuations in anisotropic spin systems. In particular, we find that the quantum fluctuations are extraordinarily sensitive to the presence of small anisotropy fields. This sensitivity can be quantified by introducing an "anisotropy susceptibility". [source] | ||||||||||||||||||||||||||||