Rotational resonance tickling: Accurate internuclear distance measurement in solids

Accurate distance measurements between pairs of nuclear spins can provide detailed information about molecular structure in the solid state. The rotational resonance (R-2) technique, in particular, has been used to measure internuclear distances in a variety of compounds, often between pairs of C-13 nuclei where the outer range is 5-6 Angstrom. Recent studies have revealed that the spin dynamics at rotational resonance are influenced by zero-quantum line shape parameters including T-2(zq) and the dispersion in isotropic chemical shift differences. Errors in the estimation of these parameters are often the limiting factor in determining the accuracy of a distance measurement. Here we present a modification of R-2, termed ''rotational resonance tickling'' ((RT)-T-2), which uses a ramped rf field to induce fast passage through the dipolar resonance condition, thereby greatly reducing the dependence of the spin dynamics on zero-quantum parameters. Extraction of distance information from the resulting exchange curves is approximately a single-parameter fit, with accuracies in model systems that appear to be on the order of +/- 0.1 Angstrom or better. An additional feature of the technique is that it does not demand the very high-power H-1 decoupling fields typically required in other recoupling experiments to limit signal loss during mixing. We demonstrate the technique in a pair of C-13(2)-labeled model compounds, tyrosine ethyl ester and glycylglycine hydrochloride, with effective internuclear distances (including intermolecular effects) of 5.05 and 4.3 Angstrom, respectively.