Dipolar couplings and internuclear distances by double-quantum nuclear magnetic resonance spectroscopy of solids

The analysis of high-resolution double-quantum nuclear magnetic resonance spinning sidebands for measuring dipolar couplings and internuclear distances in dipolar solids is described. For this purpose, the response of a dipolar-coupled spin system in a rigid solid is investigated with respect to high-resolution multiple-quantum experiments using rotor-frequency synchronized pulse sequences. For isolated, magnetically equivalent spin-1/2 pairs, exact expressions for the double-quantum spinning-sideband patterns are derived. These patterns show spinning sidebands only at odd numbers of the rotor frequency. For longer excitation/reconversion cycles, the double-quantum spinning sidebands are sensitive to changes in the internuclear distances. Using this technique, the dipolar couplings for C-13 spin pairs in double-labeled polyethylene were measured in crystalline and amorphous domains, respectively. In the former the dipolar coupling reflects the carbon-carbon distance, in the latter it is reduced due to molecular dynamics. The possibility to use multiple-quantum pulse sequences as a dipolar filter for the rigid domains is also shown. (C) 1997 American Institute of Physics.