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 deriv ed. This experiment enables the determination of the strength of the d ipolar interaction of a homonuclear spin pair in a solid, rotating at the magic angle, and thus provides a way to measure internuclear dista nces. The dipolar decay of the rotational-echo amplitudes of powder sa mples, generated by a set of rr pulses, is measured together with the echo decay that is not influenced by the dipolar interaction. The latt er is measured by the transverse-echo SEDRA experiment that refocuses the SEDRA decay. The Floquet theory approach is utilized to evaluate t he effective Hamiltonians that describe the behavior of the spin syste ms. The influence of the chemical-shift anisotropy parameters of the i nteracting spins on the effective SEDRA Hamiltonian is also discussed. Results of Delta S/S-0 SEDRA experiments on the N-15 spin pair in sol id 3(5)-methyl-5(3) -phenylpyrazole-N-15(2) are shown and compared wit h exact calculations. The data suggest a nuclear distance between the nitrogen atoms of 1.385 +/- 0.025 Angstrom. (C) 1994 Academic Press, I nc.