James Keeler

An Improved Sequence for Broadband Decoupling: WALTZ-16

Progressive improvements in broadband decoupling performance have recently been achieved with the pulse sequences known as MLEV-4, MLEV-16, MLEV-64, etc. (Z-5). Applied to carbon-13 spectroscopy, such sequences permit operation with a much lower radiofrequency power. A common feature of these and related experiments (6) is that their effectiveness can be improved by combining different versions of the primitive cycle into extended “supercycles” in which some of the residual pulse imperfections are compensated in a manner reminiscent of the folklore of solid state NMR. The original treatment of these experiments was based on average Hamiltonian theory (7) which, although it provides insight into the mechanism of error compensation, can be rather cumbersome in its application (5). An elegant new theory has recently been proposed (6, 8) which represents the effects of the proton irradiation sequence by means of a train of spin rotation operators, the overall effect at the end of the cycle being calculated by explicit matrix multiplication. The offset dependence of this proton response then determines the residual splitting of the carbon-13 resonance and hence the effectiveness of the decoupling. A particular virtue of this treatment is that it provides a simple mechanism for testing new decoupling sequences by computer simulation, and it acts as a guide to the intuitive approach. The principal criteria for decoupling performance are (a) wide effective proton bandwidth for a given power dissipation, (b) residual splittings of carbon-13 small compared with the line width, (c) insensitivity to pulse length error or

Evaluation of a new broadband decoupling sequence: WALTZ-16

Abstract A new scheme for low-power broadband heteronuclear decoupling is described, based on the use of a composite radiofrequency pulse sequence 90°(+ X ) 180°(− X ) 270°(+ X ), incorporated into a repeated cycle or supercycle. Its principal attribute is that the residual splittings on the observed resonances (usually carbon-13) are very small (less than 0.1 Hz) for a wide range of decoupler offsets (approximately − B 2 ΔB B 2 ). Existing theories of broadband decoupling are used to calculate the effects of various possible instrumental imperfections on decoupling performance. It is concluded that spatial inhomogeneity of the B 2 field has a perceptible influence near the extremes of the decoupler bandwidth. Only 180° shifts of the radiofrequency phase are used, and the performance is remarkably insensitive to the exact setting of this phase shift. Any decoupler which employs a systematic modulation scheme runs the risk of introducing “cycling sidebands” into the observed spectrum; it is demonstrated that with the proposed sequence these sidebands are very weak, particularly when the decoupling cycle and the signal acquisition processes are not synchronized. As an illustration, the broadband-decoupled carbon-13 spectrum of an aniline derivative is recorded showing natural-abundance carbon-13 satellite signals but no appreciable cycling sidebands. The circuit for a practical implementation of this decoupling sequence is described.

Comparison and evaluation of methods for two-dimensional NMR spectra with absorption-mode lineshapes

Abstract Two new methods of discriminating between positive and negative ω1 frequencies in two-dimensional correlation spectra were recently developed which allow spectra with pure absorption lineshapes to be obtained. These represent a very important advance since they avoid the highly undesirable phase-twist lineshape associated with echo or anti-echo selection, the techniques previously used for ω1 sign discrimination. In this paper the theoretical and conceptual basis for these new methods is examined, and the two are shown to be very closely related. Both are applicable to any experiment in which signals are amplitude modulated during t1. It is also shown that these methods offer some improvement in the signal-to-noise ratio relative to spectra obtained with echo selection. The phase properties of spectra obtained with the new methods are described, and ways of obtaining the desired double-absorption lineshape are considered. Finally, it is shown that the methods generate lineshapes with pure phases even in the presence of coherence transfer echoes formed in an inhomogeneous field.

Separation of chemical shifts and spin coupling in proton NMR. Elimination of dispersion signals from two-dimensional spectra

Abstract A practical procedure is described for eliminating unwanted dispersion-mode components from two-dimensional proton NMR spectra obtained by Fourier transformation of modulated spin echoes. The experimental spectra exhibit a complex lineshape comprising both dispersion- and absorption-mode contributions, the so-called “phase-twist” lineshape. This has the undesirable property that its 45° projection vanishes. Using the known form of the phase-twist response, an iterative computer program locates all the peaks in the two-dimensional spectrum and records their frequency coordinates and intensities. The second stage of the program processes the experimental data to remove the dispersion-mode contributions, leaving a two-dimensional spectrum with pure absorption signals. This permits a 45° projection to be calculated which contains only chemical shift information, while suitable 45° sections through the data matrix give well-resolved spin multiplets for each individual proton site. Provided that the spin system is only weakly coupled, chemical shift and spin coupling effects are completely separated.

Spatial localization of NMR signals in an inhomogeneous radiofrequency field

NMR methods are now having a dramatic impact on medical science, through spin imaging and the high-resolution spectroscopy of particular organs observed in viva In almost all cases, the necessary spatial localization has been achieved through the imposition of strong gradients of the static field Bo, in effect converting the spatial dimension into an NMR frequency dimension. This has produced impressive results in the field of imaging, although the tailoring of the Bo gradients to produce a localized region where high-resolution spectra can be recorded has proved a more difficult goal. It is no simple matter to create a volume of highly homogeneous field which abruptly changes into a region of strong gradients. Furthermore, the signal from this latter region is not destroyed but merely broadened, and may interfere with the desired high-resolution spectrum. Such limitations have prompted a search for methods which exploit the inhomogeneity of the radiofrequency field B1 to define spatial coordinates (1-6). By the very nature of the NMR experiment, BI is several orders of magnitude less intense than Bo. It therefore seems unlikely that gradients of BI could have any deleterious effect if BI itself is weak enough to avoid radiofrequency heating effects. (This contrasts with conventional NMR imaging methods where the use of intense switched Bo gradients gives some cause for concern.) There is the further advantage that gradients in the BI field do not interfere with the resolution of chemical-shift effects, whereas Bo gradients act in a very similar fashion to chemical shielding, making it difficult to combine imaging with chemical-shift discrimination. Finally the nature of the sample itself and the usual requirement for noninvasive methods of generating the transmitter field (surface coils) both impose appreciable B1 gradients anyway, and it is tempting to make a virtue out of a necessity. Briefly the idea is to excite transverse nuclear magnetization and then invert this with one or more composite 180 pulses designed to be particularly sensitive to the actual value of the BI field (7). Then, by employing the EXORCYCLE scheme (8), signals not properly inverted by the 180 pulses are canceled, leaving only signals from sample regions which experience a radiofrequency field close to the nominal value @. The composite 180 pulses may be cascaded to reinforce the spatial discrimination, the radiofrequency phases being cycled independently. n

Investigation of individual proton spin multiplets by C → H correlation spectroscopy

On montre que des correlations de deplacements heteronucleaires peuvent etre realisees par le transfert «inverse» de polarisation, du carbone 13 au proton

Elimination of truncation artifacts from NMR spectra. Application to carbon-13 multiplicity determination by two-dimensional spectroscopy

Abstract A method is described for the removal of “sinc wiggles” from spectra obtained by Fourier transforming truncated time-domain signals. The procedure is especially useful when the signal is severely truncated, as in such a case, the use of a conventional apodization function would seriously degrade the resolution. The method is used to remove “sinc wiggles” from cross sections of a two-dimensional carbon-13 proton J spectrum. It is shown that such a spectrum recorded with only five t1 increments has sufficient resolution to determine the multiplicities of the carbon lines. However, since the t1 sampling is very restricted, the cross sections are confused by “sinc wiggles” but the multiplicities can be determined once the procedure described has been used to remove these truncation artifacts. By keeping the maximum value of t1 to the minimum necessary to achieve the desired resolution the sensitivity of the spectrum is maximized. This approach to determining multiplicities is compared with the existing methods and is shown to have several practical advantages.

Real-time J scaling in carbon-13 NMR. I. A simple technique for multiplicity determination

Presentation dune methode simple de decouplage «en dehors de la resonance», pour determiner des multiplicites, basee sur une sequence dimpulsions melangees, la sequence WALTZ-8, donnant une echelle de J a «temp reel», cest-a-dire durant lacquisition dun declin dinduction libre

Real-time J scaling in carbon-13 NMR. II. An analysis of instrumental imperfections

Letude montre que les experiences de determination des multiplicites des carbones 13, par une nouvelle methode utilisant la sequence WALTZ-8, sont plus sensibles aux erreurs dues a lappareillage, quaux sequences de decouplage correspondantes