Echoes in spin systems with dipolar interactions

The effects of molecular motions on spin-echo signals (2n + 1)90°y—τ—90°x:—Acq(t) in nuclear spin systems with dipolar interactions have been investigated. It was found that in the case of a rigid lattice (M2τ2 c: >> 1) and a motionally narrowed NMR line (M2τ2 c,<< 1) a maximum of echo signals is observed at te = 2τ + t2—t1/2, where t1 and t2 are the widths of the RF pulses. It also was found that in the slow motion region (M2τ2 c ≊ 1) an amplitude of the dipolar echo signal is reduced and the maximum of the echo signal is shifted to the end of the second pulse. The theory developed is confirmed by the experimental results obtained for C6H6.