Xi-Li Wu

Selective excitation with the DANTE sequence. The baseline syndrome

The simplest type of selective radiofrequency pulse is the single long weak pulse (I, 2) which we shall refer to as a soft pulse. If it has a rectangular envelope, the frequency-domain excitation spectrum contains a set of side-lobe responses flanking the ma in response, approximating a sine function. For practical reasons it is often convenient to replace this soft pulse with a repetitive sequence of hard pulses separated by free precession delays, the so-called DANTE sequence (3, 4). If all the hard pulses are identical, the response approximates that of the rectangular soft pulse, except that there are s ideband responses at intervals of the pulse repetition rate 1 /(At). Alternatively the hard pulses can be amp litude modu lated (either in intensity or in pulse width) to tailor the effective pulse envelope according to some suitable shape (4). Gaussian (5)) hyperbolic secant (6)) sine function ( 7)) and half-Gaussian (8) shaping functions have been used for various purposes. The soft pulse and the corresponding DANTE sequence have very similar properties provided that they both have the same total duration and overall flip angle. In a sense, DANTE is merely a digitized version of the soft pulse. When the corresponding magnetization trajectories are calculated from the Bloch equations, starting on the 3-Z axis, they terminate very close to the same point. The soft pulse trajectory is a smooth curve and the DANTE trajectory follows a zigzag path (representing the effects of alternate pulsing and free precession) which crosses and recrosses the smooth curve. When the DANTE sequence contains a large number of hard pulses (fine digitization) then the results obtained with the two kinds of selective pulse are virtually indistinguishable. However, for a DANTE sequence with a lim ited number of pulses (coarse digitization) there are two important discrepancies. In the frequency doma in, they take the form of a positive displacement of the baseline and an oscillatory contribution. Both effects were evident in the first simulations of DANTE excitation spectra (3). For selective excitation experiments, baseline distortions of this kind are particularly unfortunate, leading to weak excitation of the NMR spectrum across the entire frequency range. The purpose of this communicat ion is to point out that these baseline artifacts may be corrected by halving the intensity of the first and last pulses of the DANTE sequence. We take as an example a rectangular soft pulse (Fig. 1 a) compared with a IO-pulse DANTE sequence (Fig. 1 b) of the same total duration ( T = 9At) and the

User-friendly selective pulses

Abstract The “spin-pinging” selective excitation scheme is described and analyzed with particular attention to its tolerance of phase errors, pulse-width misadjustment, and spatial inhomogeneity of the radiofrequency field. Through difference spectroscopy, this scheme generates pure-absorption-mode spectra at all offsets from resonance. The selectivity may be varied from very high (line-selective) to low (band-selective) simply by adjusting the duration of the soft pulse. When a rectangular soft pulse is employed, the excitation profile has only very weak side-lobe responses, and the profile may be considerably improved by shaping the pulse with an envelope that is symmetrical in time. An operator-guided evolutionary algorithm has been used to design a soft-pulse shape that gives uniform excitation over a broad central band of frequencies, flanked by narrow transition regions and negligible excitation elsewhere. In addition to its use as an excitation pulse, spin pinging can be employed for coherence transfer in a COSY experiment. This suggests an application for band-selective excitation and coherence transfer in multidimensional NMR with a view to limiting the size of the data table and the duration of the experiment.

Fine structure of cross peaks in truncated COSY experiments

We demonstrate that N can be reduced much further than is generally realized without obscuring crosspeak fine structure. We call this a truncated COSY experiment since t 1 (max) is far shorter than normal

Broadband-decoupled proton spectroscopy

Abstract Proton NMR spectra are presented in a form that incorporates chemical-shift effects but excludes spin-spin splittings, as if homonuclear broadband decoupling had been employed. By Fourier transformation of spin echoes modulated by proton-proton coupling, a two-dimensional J spectrum is obtained. A 50 ms purging pulse at the end of the evolution period suppresses antiphase product-operator terms; this changes the character of the two-dimensional spin-multiplet patterns and ensures that the signals are in the pure absorption mode. In the limit of weak coupling, each multiplet pattern possesses C 4 symmetry and a software “symmetry filter” separates overlapping multiplets and rejects all other signal components. By the appropriate projection of the processed J spectrum, a one-dimensional spectrum is obtained with narrow singlet responses at the chemical-shift frequencies and no spin-spin splittings. The peak heights are proportional to the numbers of equivalent protons at each site. A separate spin-multiplet pattern from each site is available in the other frequency dimension ( F 1 ). The technique can be incorporated into more complicated experiments such as spin-lattice relaxation studies.

Separation of overlapping cross peaks in NMR correlation spectroscopy

Correlation spectroscopy (COSY) has proved very successful for mapping out NMR coupling networks (1-5) and has recently been extended into a third frequency dimension (6-8) to handle situations where the two-dimensional spectrum is too crowded for straightforward analysis. An alternative approach is to modify the basic two-dimensional COSY experiment by multiple-quantum filtration (9, IO), spin topology filtration (1 Z-13), or some form of editing procedure (24) with a view to simplifying the overcrowded region. We propose here a selective excitation method for separating overlapping COSY cross peaks. It may be used as an adjunct to conventional COSY to clarify cluttered spectra. The method relies on finding a suitable line-selective excitation frequency in the F, dimension which picks out a transition belonging to one cross peak without affecting any transitions of the other. For example, we might slice through the edge of one overlapping cross peak.

A simple selective pulse sequence for coherence transfer

Of all the two-dimensional NMR experiments, homonuclear correlation spectroscopy (COSY) has probably enjoyed the most success in recent years (l-4). It provides clear and direct evidence of scalar coupling and can also be used to measure coupling constants in situations where the conventional NMR spectrum might be too complex to be properly analyzed. More recently the same kind of information has been obtained from a one-dimensional coherence transfer experiment initiated by a selective radiofrequency pulse. This has been called “pseudo-correlation spectroscopy” or +COSY (5-9). The key to the +COSY experiment is the use of a suitably shaped soft pulse, usually a DANTE sequence where the pulse widths follow a half-Gaussian envelope ( 10). Not all NMR spectrometers have facilities for shaping soft radiofrequency pulses in this manner; where they do, it can still be difficult to implement the very narrow DANTE pulses required for the tail of the time-domain envelope. We exploit here a new pulse sequence (II) which does not require any shaping to achieve a suitable frequency-domain excitation pattern, and which greatly attenuates the undesirable diagonal peaks in the correlation spectrum. The method involves the action of a rectangular soft 180” pulse on magnetization initially aligned along the + Y axis of the rotating reference frame. A free induction signal is acquired first with the soft pulse applied about the + Y axis and then with the soft pulse about the +X axis; the difference mode is a selectively excited pure absorption signal with no dispersion component. The frequency profile approximates a sinc2 function, which has much weaker side-lobe responses than the sine function normally associated with a rectangular soft pulse. For most applications the vestigial side lobes of the sinc2 response can be safely neglected-shaping in the frequency domain is achieved without shaping in the time domain. A detailed analysis of this selective excitation sequence has been presented elsewhere ( 11). A simple extension of this sequence cancels all magnetization components not generated by coherence transfer, thus suppressing the undesirable diagonal peaks of the two-dimensional correlation spectrum. This greatly facilitates the observation of cross peaks between groups with close chemical shifts. The sequence may be written

Diagonal-peak patterns in double-quantum-filtered COSY

Two-dimensional correlation spectroscopy with double-quantum filtration (DQCOSY) is now widely used by NMR spectroscopists because it facilitates examination of cros peaks close to the diagonal (1, 2). For a coupled two-spin system, the double-quantum-filtered spectrum resembles the conventional COSY spectrum except that the diagonal peaks are in the (antiphase) absorption mode and responses from uncoupled spins have been suppressed

Edited correlation spectroscopy (ϵ-COSY)

We describe a new technique (edited COSY or e-COSY) which extracts a coherence transfer subspectrum from the full COSY spectrum by selective excitation of a single, arbitrarily chosen I transition