Timothy J Norwood

An improved chemical-shift-selective filter

Two-dimensional NMR correlation experiments (1-4) are now widely used for assigning all but the simplest of NMR spectra. Paradoxically, it is the very virtue of these experiments, the completeness of the set of correlations that they provide, that can in some circumstances be a disincentive to using them. It is often the case in the spectra of small or medium sized molecules that only the assignments of relatively few of the resonances are ambiguous. If a two-dimensional correlation experiment is used in these circumstances much time will inevitably be expended in redetermining what is already known. Consequently it may be much quicker to use a one-dimensional experiment, which performs the same assignment function, but selectively for a single spin. A number of 1 D correlation experiments have been developed (5-1 I), many of which are the 1 D analogs of 2D experiments such as COSY (I-4), designed by replacing the first nonselective “hard” pulse of one of these experiments by a semiselective “soft” one (8-10). The information produced by such 1 D analogs pertains to the spin which is initially excited selectively by the sol% pulse. Unfortunately, these 1 D experiments have a number of drawbacks that restrict the extent of their application, including the condition that the multiplet of the spin under investigation be resolved from the transitions of its neighbors. Even if this criterion is met, it is often necessary to be able to precisely shape the soft pulse if the selected spin is to be excited cleanly and the necessary hardware is not yet widely available. We have recently shown (II) that both of these limitations can be overcome by using a chemical-shift-selective filter (Fig. 1A) which requires only that the chemical shift of the spin, and not its multiple& be resolved from those of its neighbors. Unfortunately the usefulness of that chemical-shift-selective filter is greatly diminished by the need to repeat the experiment many times (up to 90 or more) if it is to provide a high level of rejection. In the present communication we demonstrate that by combining a chemical-shift-selective tilter together with a semiselective pulse in one pulse sequence (Fig. 1B) it is possible to overcome the disadvantages that both techniques have when used separately, thereby making possible the rapid selection of any spin which has a uniquely resolved chemical shift, and without the necessity of shaping the soft pulse. Frequency-selective filtration (12-16) is accomplished by combining the free induction decay signals from a number of experiments in which evolution due to free precession has been allowed to occur for different integer multiples of a period A prior

Analysis of 1H NMR spectra with the combined use of zero- and double-quantum coherence

Abstract Both homonuclear zero-quantum and double-quantum coherence correlation experiments have been suggested previously as alternatives to the standard single-quantum coherence correlation experiment (COSY). However, both those experiments have a number of associated problems, including the difficulty in obtaining uniform excitation of coherence and the size of the data matrix which in both cases may be twice as large, and hence takes twice as long to acquire, as that of the corresponding COSY experiment. Both those problems are substantially alleviated in the approach demonstrated here, which combines the simultaneous acquisition of both types of correlation spectra and the most economical formatting of the data; this is shown to be particularly significant where the F 1 dimension is broadband decoupled. The method is demonstrated at 300 MHz for allyl bromide and dehydrotestosterone.

Zero-quantum-coherence correlation spectroscopy

,4lthough the practicing chemist interested in the use of NMR for structural analysis has access to a wide array of methods whereby individual resonances can be assigned to specific protons, no such methods yet exist for zero-quantum-coherence (I-3) spectra. As a result, such spectra are extremely difficult to interpret, a problem which is further exacerbated by the unfamiliarity of their spectral parameters (4). The significance of this problem is readily apparent when one considers the in vivo applications of NMR imaging and spectroscopy, where unavoidably large sample volumes make it impossible to obtain high-resolution single-quantum (SQ) spectroscopic information. Under these circumstances high-resolution spectra can still be obtained by using zeroquantum coherence (ZQC) (5-7). This is because ZQCs have the endearing property of being independent of magnetic field inhomogeneity. In this communication we describe an experiment, which for brevity we denote ZECSY (zeroquantum~herence correlation spectroscopy), which offers a solution to this problem. A spectrum obtained with the ZECSY experiment contains additional peaks to those of the conventional ZQ spectrum, at the (frequency) midpoint between pairs of ZQCs containing a common spin. This provides a facile method for tracing spin-spin coupling networks among ZQCs, and hence aids in their assignment. However, other peaks besides those halfway between ZQCs containing a common spin are produced. These are also a type of correlation peak and will be described in more detail below. The ZECSY pulse sequence, Fig. 1 B, differs from the conventional ZQC experiment, Fig. lA, in the structure of its evolution period; in the former, the evolution time t, is divided into two halves by a coherence-transfer pulse (Y. This pulse serves to effect a coherence transfer between ZQCs with a common spin, in a manner analogous to single-quantum coherence-transfer correlation experiments (I). The ZECSY experiment can be most easily understood by analogy to the singlequantum-coherence correlation experiment SECSY. The ZECSY experiment produces correlation peaks which are ZQ analogs of the SQ cross peaks produced in a SECSY experiment (8, 9). A SECSY experiment will prdUCe peaks for a given pair of spins A and X at the frequencies (WA wx)/2, (WA + w&2, WA and wx , of which the latter three are usually removed by phase cycling.

Frequency filtration and the elucidation of scalar coupling networks in zero-quantum coherence spectra

The lim itations of static magnetic field homogenei ty usually confine high-resolution NMR spectroscopy to sample volumes of no more than a few cubic centimeters. One way to overcome this lim itation is to observe zero-quantum coherences (ZQCs) (I4), since they have linewidths which are independent of magnetic field homogeneity. Unfortunately, ZQC spectra are difficult to interpret, and it is this problem that we address in this communication. We have developed a method whereby it is possible to extract from a ZQC spectrum subspectra which contain only those ZQCs produced by a single spin system. Zero-quantum coherences are phase coherences between sets of coupled, but chemically inequivalent, spins which obey the transition rule AA4 = 0. Since they are, therefore, spin-forbidden they can neither be created directly by the action of a single pulse on the equilibrium magnetization of a spin system nor be detected directly as they have no net magnetization in any direction. Fortunately, both of these problems can be overcome by using an appropriate series of pulses and delays in the form of a two-dimensional experiment (I, 2). The biggest drawback to using ZQCs is the unfamiliarity of their spectral parameters (4). Both their precessional frequencies, ueff, Eq. [ 11, and their scalar couplings, Jeff, Eq. [2], are such as to preclude conventional spectroscopic analysis:

An alternative design strategy for one-dimensional correlation experiments

Abstract A new strategy for reducing two-dimensional NMR experiments to their one-dimensional analogs is proposed. The pulse sequences produced using this method are more sensitive and less parameter-dependent than those designed using previously proposed strategies. This approach can also be used to produce the two-dimensional analogs of three-dimensional experiments.

A new method for studying diffusion using a static magnetic field gradient

In this Communication we demonstrate the principle of a technique based upon zero-quantum coherence for measuring diffusion coefficients, which circumvents all of the above problems associated with pulsed magnetic field gradients by using a static magnetic field gradient, while still retaining differentiation between the individual components of multicomponent systems

High-resolution heteronuclear NMR spectroscopy in an inhomogeneous magnetic field

Abstract A family of five related pulse sequences is described, each of which produces a high-resolution 1H13C heteronuclear spectrum from an inhomogeneous magnetic field. All are based upon observation of a single-quantum coherence-transfer echo. The basic pulse sequence was tailored so that the peaks exhibited heteronuclear scalar coupling, homonuclear scalar coupling, or no couplings at all. One pulse sequence produces a spectrum which is remarkably similar, once a frequency scaling is taken into account, to the conventional 1H-decoupled 13C spectrum obtained in a homogeneous magnetic field. Semiquantitative peak integrals can also be recovered from these spectra.

Connectivity and zero-quantum coherence

Abstract When it is necessary to perform NMR measurements using an inhomogeneous magnetic field, the logical way to obtain a high-resolution spectrum is to use zero-quantum coherence. Unfortunately the usefulness of zero-quantum coherence is limited by the unfamiliarity of its spectral parameters, which precludes the type of spectral analysis which is familiarly associated with single-quantum coherence. We have developed two approaches to overcome this problem: one utilizes the patterns inherent within the zero-quantum coherence spectrum to deduce spectral connectivities, the other achieves this objective by generating correlation peaks. Both techniques are assessed and compared using allyl bromide as an example.

31P NMR spectral editing technique for blood with short T2 values

Since the work of Moon and Richards (I), the 3’P chemical shift of inorganic phosphate (Pi) has been widely used as an indicator of the pH of biological systems. In blood, the Pi resonance often overlaps the doublet resonance of 2,3-diphosphoglycerate (2,3-DPG) and several spectral editing techniques to separate them have been suggested (2-4). However, all of those techniques require a delay of 1/2J,n(n in this method the delay between excitation and acquisition can be much less than 1/2n the 180” pulse refocuses any dephasing due to magnetic field inhomogeneity and the evolution due to chemical shift. Any evolution due to heteronuclear scalar coupling is also refocused. Consequently, both the components of the DPG doublet and Pi will have the same phase. In the second sequence, a 180” pulse is applied at the 31P and ‘H frequencies. Since the 180” proton pulse inverts the spin states of the proton spins, the evolution due to the heteronuclear scalar coupling of the DPG doublet is not refocused. Therefore at the end of the sequence, one of the resonances of the doublet would have evolved through an angle of hJpH, and the other through -rJpH (= kq5). However, the magnetization of the Pi singlet will have the same phase as before. The positions of the magnetization vectors after the two sequences are illustrated in Fig. 2 as EXP 1 a and EXP lb. The phase of the FID acquired with the sequence in Fig. 1 b is now rotated through k,

A chemical-shift-selective filter

. When this FID is subtracted from that resulting from the sequence in Fig. lA, one peak of the DPG doublet cancels out, leaving the singlet from the Pi, plus the