Separated-local-field measurements using low-power multipulse decoupling

Abstract

The separated-local-field experiment in the NMR of solids is used to measure the strength of the static dipolar interaction between isolated heteronuclear spin pairs (Z3). In the case of static solids, the strength of the dipolar interaction can be used to calculate the heteronuclear interatomic distance (4). Alternatively, in the presence of some molecular motion and a known interatomic distance, the reduction of the strength of the heteronuclear dipolar interaction due to that motion can be used to obtain information about the amplitudes of those molecular motions (3). Techniques to obtain this local-field information are two-dimensional experiments which contain the usual preparation, evolution, and detection periods. During the Preparation period, the spin system comes to equilibrium; then observe spin magnetization is generated (usually by means of Hartmann-Hahn cross polarization). In the second period, the magnetization is allowed to evolve for a time t, while the coupled heteronuclei are subjected to a homonuclear decoupling, allowing heteronuclear interactions to survive. In the final period, t2, the observe nuclei are detected under full heteronuclear decoupling. For this two-dimensional experiment, the t, increment is a cycle of some appropriate homonuclear decoupling scheme, usually WAHUHA (5) or MREV-8 (68). These “windowed” sequences, however, require very high power levels in order to obtain reasonable results. Typically, RF fields greater than 2 mT, cortespomimg to a 90” pulse length of less than three microseconds, are required to homodecouple protons. This demanding requirement is generally not available to the routine or casual solids NMR spectroscopist. This report describes a version of the separated-locall the usual WAHUHA homondear decoupling pulse train has been replaced by the more efficient windowless BLEW12 (9) sequence. Also, the time interval between cross polarization and the refocusing pulse was not constrained to be an integer number of multipulse cycles. For this work, a half-cycle of BLEW-12 is used as the tl increment time. The half-cycle represents the minimum pulse sequence necessary to cause zero-order dipolar interactions to vanish (9).