Measurement of Dipolar Interaction of Quadrupolar Nuclei in Solution Using Multiple-Quantum NMR Spectroscopy

Relaxation resulting from the modulation of dipolar interaction is commonly used for estimating distances in molecules in solutions. However, for most nuclei with spin I > ½ the single-quantum-transition relaxation by dipolar interaction is masked by quadrupolar relaxation. In the present study, it is shown that even in systems where single-quantum relaxation times are dominated by quadrupolar interaction, dipolar relaxation can be measured by following the -m $\leftrightarrow$ m transitions. This is demonstrated for 7Li in the complex [Li-Kryptofix 211]+X- (X = Cl, Br) dissolved in glycerol at temperatures for which slow motion prevails and no 1H-7Li NOE can be observed. The relaxation times that are most important for the assessment of the dipolar interaction of 7Li are -½ $\leftrightarrow$ ½ and -${{3}\over{2}}$ $\leftrightarrow$ ${{3}\over{2}}$ and they are measured by multiple-quantum-filtration techniques. For estimating the quadrupolar interaction, the relaxation times of the populations and those of the transitions ±½ $\leftrightarrow$ ±${{3}\over{2}}$ were measured. The longitudinal and transverse relaxation times of 6Li as well as the 1H-6Li NOE were also measured and, together with the 7Li measurements, were used to obtain the strengths of dipolar (D) and quadrupolar (chi) interactions. The experimental data were analyzed using several models to describe the motion. The model that gave the best fit and resulted in parameters that were physically meaningful encompassed a whole-body isotropic motion as well as internal anisotropic motion. For this particular model, the following values for the quadrupolar and the dipolar interactions strength were obtained: D(7Li)/2pi = 6.8 kHz, chi(7Li)/2pi = 85 kHz and D(6Li)/2pi = 1.4 kHz, chi(6Li)/2pi = 2.6 kHz. From the value of D, an estimate of the average lithium-proton distance was calculated to be 3.3 Å, which is in fair agreement with crystallographic studies. The sizes of the quadrupolar and dipolar interactions were independently confirmed by the 7Li NMR powder spectra of the complexes that were used for the solution studies.