High-resolution NMR spectra in inhomogeneous magnetic fields: application of total spin coherence transfer echoes (original) (raw)
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High-resolution NMR in inhomogeneous magnetic fields
Journal of Magnetic Resonance (1969), 1976
High-Resolution NMR in Iuhomogeneous Magnetic Fields It has become the accepted custom to record carbon-13 spectra under conditions of complete proton decoupling to avoid the problem of overlap of spin multiplets; considerable simplification is thereby achieved, but only at the expense of protoncarbon spin coupling information. Structural and conformational investigations often hinge on the measurement of these very coupling constants and thus require alternative approaches to the problem of simplification. In these applications long-range couplings are particularly important, and since they tend to be weak, spectrometer resolution can be the critical factor. The purpose of this note is to demonstrate a technique for circumventing the problem by breaking down the spectrum into separate responses from individual carbon sites in the molecule. Unlike earlier solutions (I, 2) it has the advantage that magnet field inhomogeneity is no longer a fundamental limitation on the resolving power since spin-echo refocusing methods (3) are used to counteract the dispersal of signal components from different regions of the sample. The experiment extends the principle of J spectroscopy (4,5) to the case of heteronuclear spin-spin coupling (6). Under normal conditions the 180" pulse of a Carr-Purcell spin-echo experiment (7) refocuses the components of a heteronuclear spin multiplet in the same way as for chemically shifted lines, and the echoes are not modulated (8). The idea is to restore this modulation by one of two possible methods. Suppose that in the interval 0 to z while the carbon spins are defocusing, the protons are irradiated, making the carbon-13 multiplet components degenerate in frequency, but that in the interval T to 22 while the carbon spins are refocusing, the decoupler is switched off, leaving the carbon-13 multiplet components free to precess at their individual frequencies. At the time of the echo (2~) each multiplet component has accumulated a different phase angle. If the echo is followed as a function of the time t1 = 2~ the result is a phase modulation of the echo components at a frequency J/4 Hz (9). There is an alternative experiment which generates echo modulation at a frequency J/2 Hz. It employs no continuous-wave irradiation of the protons in the interval t, but a 180" pulse on the protons synchronized with the 180" refocusing pulse applied to carbon-13 at time r. This "proton flip" experiment has the effect of inverting the proton spin states, interchanging the identities of the carbon-13 multiplet components and generating a continued phase divergence throughout the interval t,. This behavior is analogous to the echo modulation observed in homonuclear coupled spin systems (5). Both the "gated decoupler" and the "proton flip" methods employ proton noise irradiation before the spin-echo experiment is initiated, in order to establish a nuclear Overhauser enhancement of the carbon-13 signal. The noise decoupler is also switched on again during acquisition of the second half of the echo, causing the frequencies of the multiplet components to coalesce into a single response and thereby converting their individual phase modulations into a pure amplitude modulation. This is important for subsequent phase adjustment of the spectra. Echo modulation as a function of tl is converted into the frequency domain (F1)
Journal of Magnetic Resonance, 2008
In general, the proton NMR spectra of chiral molecules aligned in the chiral liquid crystalline media are broad and featureless. The analyses of such intricate NMR spectra and their routine use for spectral discrimination of R and S optical enantiomers are hindered. A method is developed in the present study which involves spin state selective two dimensional correlation of higher quantum coherence to its single quantum coherence of a chemically isolated group of coupled protons. This enables the spin state selective detection of proton single quantum transitions based on the spin states of the passive nuclei. The technique provides the relative signs and magnitudes of the couplings by overcoming the problems of enantiomer discrimination, spectral complexity and poor resolution, permitting the complete analyses of the otherwise broad and featureless spectra. A non-selective 180°pulse in the middle of MQ dimension retains all the remote passive couplings. This accompanied by spin selective MQ-SQ conversion leads to spin state selective coherence transfer. The removal of field inhomogeneity contributes to dramatically enhanced resolution. The difference in the cumulative additive values of chemical shift anisotropies and the passive couplings, between the enantiomers, achieved by detecting Nth quantum coherence of N magnetically equivalent spins provides enhanced separation of enantiomer peaks. The developed methodology has been demonstrated on four different chiral molecules with varied number of interacting spins, each having a chiral centre.
Asymmetric Spin Echo Sequences A Simple New Method for Obtaining NMR !H Spectral Images
Investigative Radiology, 1985
The nuclear magnetic resonance (NMR) signal decay produced by reversible tissue-induced dcphasing of the magnetization compo nents in the transverse plane (reversible tissue-induced dephasing) was measured and expressed as a function of a new transverse relaxation time Ti (T2 prime) for samples of rat liver, retroperito neal fat, inflated lung, and corn oil. Simple exponentials did not adequately describe the observed NMR signal decay. Inflated lung demonstrated the most rapid signal decay (T2 = 4.8 ms) followed by retroperitoneal fat (T2 = 16 ms). No reversible tissue-induced dephasing was observed in liver (T2 immeasurably long). In tissues which contain both fat and water, the chemically shifted 'H reso nance peaks from-OH and-CH-are in phase with symmetric spin echo sequences but out of phase with asymmetric sequences. The interference of these two peaks produces a beat pattern with asymmetric sequences. Subtraction images obtained from paired symmetric-and asymmetric-sequence images accurately (r = .96) reflect T2 ' and can be used to indicate the presence of fat. In vivo subtraction images of ethionine-induced fatty rat livers were signif icantly different from similar in vivo images of normal rat livers (P < .0005). Since for each pixel of a subtraction image, the magni tude of the difference signal should be approximately proportional to the ratio of hydroxyl and alkyl protons, this simple spin echo sequence modification may obviate the need for more time consuming 3-dimensional Fourier transform proton chemical shift images.
Multipole NMR. VI. application to coherence transfer and 2-D spectroscopy in a spin-1 system
Journal of Magnetic Resonance (1969), 1983
Using the multipole formulation the recent experiments of H. Hatanaka and C. S. Yannoni (J. Magn. Resort. 42, 330 (1981)) are interpreted. It is shown how transfer of nuclear coherence between levels in a spin-1 system corresponds to a transfer of polarization between single quantum dipole character and single quantum quadntpole character. Also discumed in the multipole formulation are the recent experiments of L. Milller (J. Magn. Reson. 42, 324 (198 1)) in which 2-D spectroscopy is used to extract the double quantum spectrum. Some comments are made regarding a spin I = '/*.
Chemical Physics Letters, 2008
Internuclear couplings between selected homonuclear spin pairs in a multiplylabelled spin system are determined by NMR spin echo experiments in the solid state. The spin echoes are induced by an amplitude-modulated shaped pulse. The time shift in the echo modulation curve is treated by average Hamiltonian theory and verified by numerical simulation. The J-couplings may be estimated by experiments on samples spinning at the magic angle, while the direct dipoledipole couplings may be estimated by off-magic-angle spinning. The concept is tested on a uniformly 13 C-enriched sample of L-histidine hydrochloride monohydrate.
Spin-State-Selective Excitation in Selective 1D Inverse NMR Experiments
Journal of Magnetic Resonance, 2001
A general and very simple strategy for achieving clean spinstate-selective excitation with full sensitivity in carbon-selective gradient-enhanced 1D HMQC and HSQC pulse schemes is presented. The incorporation of an additional hard 90°1 3 C pulse applied along a specific orthogonal axis just prior to acquisition into the conventional sequences allows us to select a simultaneous coherence transfer pathway which usually is not detected. The superimposition of this resulting antiphase magnetization to the conventional in-phase magnetization gives the exclusive excitation of the directly attached proton showing only the ␣ or  spin state of the passive 13 C nucleus. The propagation of this particular spin state to other protons can be accomplished by adding any homonuclear mixing process just after this supplementary pulse. Such an approach affords a suite of powerful selective 1D 13 C-edited NMR experiments which are helpful for resonance assignment purposes in overcrowded proton spin systems and also for the accurate determination of the magnitude and sign of long-range proton-carbon coupling constants in CH spin sytems for samples at natural abundance. Such measurements are performed by measuring the relative displacement of relayed signals in the corresponding ␣ and  1D subspectra.
Band-selective spin echoes for invivo localized1H NMR spectroscopy
Magnetic Resonance in Medicine, 1994
This study describes a new single spin-echo spatial localization sequence, BASSALE or BAnd-Selective Spin echo Acquisition for Localized Editing, that overcomes a number of the limitations of the STEAM and PRESS volume selection pulse sequences. It achieves conformal volume localization in a single shot by spatially tailored suppression of all magnetization outside a 2D region of interest followed by selection of a single orthogonal slice. This separation of spatial localization from the echo formation process has permitted use of a spectrally selective cosine-modulated sinc refocusing pulse to acquire localized 1H spectra with the water suppression efficiency of STEAM and the sensitivity of PRESS. Echoes formed by such spectrally selective pulses have been termed bandselective spin echoes. The BASSALE sequence attains shorter echo times than PRESS, inhibits scalar spin-spin interactions to permit localized editing and T2 relaxometry of metabolites with J-coupled spins (e.g., lactate), is insensitive to homonuclear multiple-quantum and polarization transfer effects, and can be made sensitive or insensitive to spin displacement effects. Applications are shown both with phantoms and in situ in the rat brain.
Theoretical study of a simple rotational-echo double-resonance NMR for homonuclear spin-1/2 pairs
Magnetic Resonance in Chemistry, 2015
We investigate theoretically intriguing aspects of a simple rotational-echo double-resonance (REDOR) NMR technique for homonuclear spin-1/2 pairs undergoing MAS. The simple technique sets Gaussian soft π pulses at every half MAS rotational period in the pulse sequence. The reduction in rotational echo amplitude (the REDOR echo reduction) is observed at the end of the evolution period t e = (n + 1)T r , where T r is a MAS rotational period. The exact average Hamiltonians for the homonuclear REDOR (hm-REDOR) technique are calculated by dividing the evolution period into four periods. We show theoretically and experimentally that the hm-REDOR technique produces the REDOR echo reductions for homonuclear spin-1/2 pairs. In addition, the theoretical results reveal that the REDOR echo reductions are independent of the chemical-shift difference, δ, under a simple condition of κ = δ/ω r ≥ 6 and t e < 10 Á (1/d′), where ω r is the sample spinning frequency and d′ is the dipolar coupling constant expressed in Hz. We call this simple condition the master condition. This means that the REDOR echo reductions for a homonuclear spin-1/2 pair can be calculated under the master condition by considering only d′ and ω r , which is the case for a heteronuclear spin pair. Finally, we demonstrate that four-phase cycling yields the multiple-quantum filtered hm-REDOR experiment, where the appearance of the REDOR echo reductions shows that the echo reductions are definitely attributable to the homonuclear dipolar interaction even if there is a slight unwanted effect from the recovered chemical-shift anisotropy in these reductions.