Elimination of zero-quantum interference in two-dimensional NMR spectra

High-resolution NMR experiments often contain periods during which the magnetization is placed along the z-axis. For example, the magnetization must be along the z-axis during the mixing time in a NOESY experiment so that crossrelaxation can take place. Either phase cycling or field gradient pulses are used to ensure that only the wanted zmagnetization ends up contributing to the spectrum. However, neither of these methods can distinguish between zmagnetization and homonuclear zero-quantum coherence, which is invariably present. The zero-quantum coherence gives rise to anti-phase dispersive components in the spectra, thereby reducing the effective resolution, introducing misleading correlations, and obscuring wanted features. Over the years a number of methods have been devised to suppress these zero-quantum contributions, but it is fair to say that none of these methods have proved entirely satisfactory. Herein we present a new method for suppressing zeroquantum coherence; the method is widely applicable, does not extend the duration of the experiment significantly, and can be implemented easily on any modern spectrometer. Our new method of zero-quantum suppression involves applying simultaneously a swept-frequency 1808 pulse and a gradient. Figure 1a shows how this combination can be introduced into the NOESY pulse sequence. The way in which this swept-pulse/gradient pair works can be envisaged in the following way. The application of the gradient (along the z-axis) results in the Larmor frequency becoming a function of position in the NMR tube. The swept-frequency 1808 pulse will therefore flip the spins at different positions in the sample at different times. Thus, the top of the sample might experience the 1808 pulse at the start of the sweep, the middle of the sample at time tf/2, and the bottom at time tf, where tf is the duration of the sweep. In general, the 1808 pulse occurs at time atf, where a is 0 at the top of the sample and 1 at the bottom. The 1808 pulse forms a spin echo which refocuses the evolution of the zero-quantum coherence over a time 2atf ; however, for the remainder of the time, (1–2a)tf, the zeroquantum continues to evolve. The result is that in different parts of the sample the zero-quantum has evolved for different times, and so has acquired a different phase. If the range of these phases across the sample is large enough, the net result will be cancelation of the zero-quantum coherence. A simple calculation (see Supporting Information) shows that the degree of attenuation A of the zero-quantum depends on both its frequency, WZQ (in rad s ), and the length of the swept-pulse/gradient pair, tf, [Eq. (1)]