Decoupling Two‐Dimensional NMR Spectroscopy in Both Dimensions: Pure Shift NOESY and COSY

Abstract

Spectral assignment is a crucial step in the analysis of chemical structure by NMR spectroscopy, but is often limited by signal overlap. If the multiplet structure caused by spinspin coupling is suppressed, spectral resolution can be greatly increased, and the process of assignment correspondingly simplified. Collapsing a multiplet to a single "pure shift" peak can be achieved using a variety of methods, as has been demonstrated in 1D NMR spectroscopy, and in one dimension of several different homonuclear 2D experiments. [1][2][3][4][5][6][7][8][9][10] Recently, it has been shown that applying covariance processing to a TOCSY dataset decoupled in one dimension gives a fully decoupled 2D spectrum. [11] Previous routes to fully decoupled 2D spectra have used pattern recognition postprocessing applied to conventional, fully coupled 2D data. [12][13][14] The considerably increased resolution in, and simplicity of, fully decoupled 2D spectra makes them particularly attractive for automated structure elucidation.Here we compare two different decoupling methods, illustrating the very general nature, and potential for resolution gain, of pure shift covariance NMR spectroscopy, by applying different single decoupling methods to the NOESY and nQF-COSY experiments, producing doubly pure shift 2D spectra by covariance processing. Using pure shift acquisition for one spectral dimension greatly improves resolution, collapsing multiplet structure to singlets, whereas the use of covariance processing gives a further gain in interpretability by condensing the correlation information into 2D singlets.Methods for achieving broadband homonuclear decoupling in 2D NMR spectroscopy include the Zangger-Sterk (ZS) experiment and its adaptations; [1][2][3][4] the Pell-Keeler (PK) 458 projection of phase-sensitive 2D-J data; [5] the "BIRD" (bilinear rotation decoupling) family of sequence elements; [6,7] and the "constant time" (CT) approach. [8][9][10] All such methods trade sensitivity for resolution, to a greater or lesser extent. The first three methods are appropriate for experiments that generate in-phase cross-peaks, whereas CT

Simultaneously Enhancing Spectral Resolution and Sensitivity in Heteronuclear Correlation NMR Spectroscopy

“…Phase cycling: 3 1 3 (3 1 3 1) 2 1 3 1 3 3 1 3 1 (1 3 1 3) 2 3 1 3 1}, all other pulses are of phase 0 (for the explicit phase table, see Table S1). Similar results were obtained for quinine (Supporting Information, Figure S3); in this case the wider range of 13 C chemical shifts means that some degradation in performance is seen at the edges of the spectrum. Any discontinuities in the decoupled signal, such as those caused by T 2 relaxation during the BIRD sequence element, mismatch between the BIRD timing and 1 J CH , or a breakdown of the condition n @ (at J HH ), will lead to small F 2 sidebands at multiples of n/at.…”

“…It is routine to suppress heteronuclear couplings (J XH ) by broadband decoupling, [3][4][5][6][7] but only recently have experimental methods for homonuclear broadband decoupling become practical. These "pure shift" or "chemical-shift resolved" or "d-resolved" methods [8][9][10][11][12][13][14][15][16][17][18][19] can give resolution improvements approaching an order of magnitude, far in excess of any gains to be realistically expected from increases in the static magnetic field. However, all of these methods suffer to a greater or lesser extent from reduced sensitivity compared to conventional measurements.…”

“…The BIRD sequence element, [1] which, as its name suggests, was originally intended for broadband homonuclear decoupling, has, until recently, [12] been used almost exclusively for decoupling in the indirect dimension of heteronuclear 2D experiments. [27] Here, the combined effect of the BIRD sequence and the hard 1808 pulse is to invert only those protons not directly coupled to 13 C, thus refocusing the effects of couplings between the latter protons and protons that are directly coupled (bonded) to 13 C and whose signals are recorded in HSQC. The great advantage of the BIRD method here is that, in contrast to Zangger-Sterk type methods, [8,9,22,23] it incurs no extra sensitivity penalty; indeed, the sensitivity is generally increased.…”

“…Here we demonstrate how pure shift HSQC data with comparable resolution may be obtained much more quickly (to the point where a pure shift spectrum can require less time to acquire than a conventional spectrum) and without the need for any extra data processing. The one restriction is that the nucleus observed indirectly, generally 13 C, should not itself show homonuclear coupling; thus, for example, the proposed sequence is not suitable for fully 13 C-labeled compounds.…”

“…Heteronuclear couplings are suppressed as usual by broadband irradiation (denoted CPD in Figure 1); the intermittent nature of the decoupling limits the types of modulation favored. Because BIRD selects protons directly bonded to 13 C, one class of coupling is not refocused, that between geminal protons. Spectra thus show singlet signals for all 1 H sites except for nonequivalent methylene protons, for which doublet signals are seen (full details of the sequence are given in the Supporting Information).…”