DISTANCE MEASUREMENTS BETWEEN HOMONUCLEAR SPINS IN ROTATING SOLIDS

The effective Hamiltonian of the ''simple excitation for the dephasing of the rotational-echo amplitudes'' (SEDRA) experiment has been derived. This experiment enables the determination of the strength of the dipolar interaction of a homonuclear spin pair in a solid, rotating at the magic angle, and thus provides a way to measure internuclear distances. The dipolar decay of the rotational-echo amplitudes of powder samples, generated by a set of rr pulses, is measured together with the echo decay that is not influenced by the dipolar interaction. The latter is measured by the transverse-echo SEDRA experiment that refocuses the SEDRA decay. The Floquet theory approach is utilized to evaluate the effective Hamiltonians that describe the behavior of the spin systems. The influence of the chemical-shift anisotropy parameters of the interacting spins on the effective SEDRA Hamiltonian is also discussed. Results of Delta S/S-0 SEDRA experiments on the N-15 spin pair in solid 3(5)-methyl-5(3) -phenylpyrazole-N-15(2) are shown and compared with exact calculations. The data suggest a nuclear distance between the nitrogen atoms of 1.385 +/- 0.025 Angstrom. (C) 1994 Academic Press, Inc.