Two-Dimensional Double-Quantum NMR Spectroscopy of Isolated Spin ³/₂Systems: ²³Na Examples

Of the four possible types of NMR spectral behavior for a system consisting of isolated quadrupolar I = ³/2 nuclei, double-quantum coherence can be excited by multipulse techniques in three. Two-dimensional ²³Na NMR spectra have been obtained from samples exhibiting each of these three. These are the following: (a) single crystal (represented here by an oriented lyotropic liquid crystal), (b) powder pattern (represented by an unoriented lyotropic liquid crystal), and (c) homogeneous biexponential relaxation (represented by a sufficiently dense solution of reasonably globular macromolecules). The fourth type, for which double quantum coherence generation by pulsed NMR is unknown, is the extreme narrowed situation. The pulse sequence (90_x-τ/2-180_y-τ/2-90_x-t₁-90_x-t₂) generates a two-dimensional spectrum when the response is sequentially Fourier transformed from the t₂and t_xtime domains. The projection onto v₂is the single-quantum spectrum while the projection onto v₁is the double-quantum spectrum (given appropriate phase cycling of the pulse sequence). The double-quantum spectrum is quite distinct for all three spectral types. This is also true for the single-quantum spectrum. However, the latter is less diagnostic and is subject to distortions from an insufficiently short receiver dead time, while the former is not. In addition, all of the double-quantum spectral information derives from modulation of the strong, sharp central single-quantum signal. Thus, this experiment can be useful in cases where the broad satellite resonance(s) is (are) not detectable in the single-quantum spectrum because of electric field gradient effects on the quadrupolar nucleus. This is often the case in tissue-containing samples. The theory of this experiment is briefly discussed, as are the differing requirements for the excitation of double quantum coherence for type a and type b spectra on the one hand and type c spectra on the other. Drawbacks of this approach, particularly the trade-off between total acquisition time and digital resolution in the v₁dimension, are also considered.