Ultrahigh-Resolution NMR Spectroscopy**

Spectral resolution is vital in NMR spectroscopy, but is instrument-limited. Recent “pure shift” pulse-sequence developments greatly improve resolution, but often at a high cost in sensitivity. We introduce a new class of pure shift experiments (PSYCHE) with superior sensitivity, spectral purity, and tolerance of strong coupling. The key parameters for any spectroscopic technique are sensitivity and resolution. In the case of NMR spectroscopy, the sensitivity gains provided by the introduction of Fourier transform methods and of improved probe technologies make spectral resolution the limiting factor for most applications. From the early days of NMR spectroscopy it has been recognized that for some nuclei, in particular the proton, large gains in resolution could be achieved if the effects of homonuclear spin–spin couplings could be suppressed, but for many years all the methods proposed proved more or less unsatisfactory. Recently, much more practical methods, socalled “pure shift” or “chemical shift” methods have emerged, which partially or completely suppress the effects of homonuclear coupling, thereby generating spectra consisting of a single signal for each chemically distinct site, but generally at a high price in terms of sensitivity. Here we present a versatile new approach to pure shift NMR spectroscopy which has approximately tenfold better sensitivity than competing methods, gives clean spectra, and is tolerant of strong coupling. In conventional proton NMR spectroscopy, scalar couplings (J) between protons carry structural information but reduce resolution, with the splitting of signals into multiplets greatly increasing signal overlap and complicating analysis and the assignment of spectra. The effects of heteronuclear couplings, for example between H and C, can easily be suppressed by using appropriate pulses during evolution times and broadband decoupling during detection. Homonuclear couplings are much more challenging, but the prize is a spectrum without multiplet structure, with only a single signal per chemical shift. In H NMR spectroscopy, this can represent a resolution improvement of almost an order of magnitude over conventional NMR spectroscopy; by way of comparison, 30 years of magnet development have delivered only a factor of two improvement, from 500 MHz to 1 GHz. Recent developments have finally allowed such pure shift NMR spectra to be obtained, albeit at a significant cost in sensitivity. The elegant method of Zangger and Sterk (ZS) uses a frequencyand spatially selective 1808 pulse; it has been enhanced and adapted for 1D NMR, DOSY, and 2D experiments such as TOCSY and NOESY. The ZSmethod is effective, but its sensitivity falls rapidly as the chemical shift difference between the resonances to be decoupled decreases. The BIRD method relies on isotopically sparse heteronuclei. It typically selects protons directly bonded to C nuclei at natural abundance, so has a minimum sensitivity penalty of two orders of magnitude; it does not decouple geminal interactions, and suppresses signals of protons not bound to C nuclei. In experiments such as HSQC, however, which already rely on the presence of C nuclei, there is no additional sensitivity penalty and BIRD pure shift methods are highly effective. Both the ZS and BIRD methods are typically used to construct a pure shift interferogram, which can be Fourier transformed to yield a decoupled spectrum from a series of short chunks of data acquisition of duration 1/SW1; [3a] real-time windowed acquisition can sometimes be used to speed up experiments, but at some cost to spectral quality and resolution. A new robust, general method is presented in Figure 1 that uses low flip angle (b) swept-frequency pulses in the presence of a weak magnetic field gradient. The method is related to the anti-z-COSY experiment, but avoids the latter s long minimum acquisition times, awkward data processing, and failure to deal efficiently with unwanted coherence transfer and strong coupling. The combined effect of the two b pulses is to refocus a small proportion of spins (the active spins) in a stimulated echo, while leaving the majority (the passive spins) unaffected. Taken together with the hard 1808 pulse and the remaining field gradient pulses, the overall effect is to leave the net evolution of the active spins unchanged, but to invert the passive spins and to dephase all but the required single quantum coherences of the active spins. The distinction between passive and active spins is purely statistical, the former being a proportion sinb of the whole and the latter cosb, rather than being determined by [*] Dr. M. Foroozandeh, Dr. R. W. Adams, N. J. Meharry, Dr. M. Nilsson, Prof. G. A. Morris School of Chemistry, University of Manchester Oxford Road, Manchester M13 9PL (UK) E-mail: [email protected] Homepage: http://nmr.chemistry.manchester.ac.uk