Time evolution of multiple quantum coherences in NMR

In multiple quantum NMR, individual spins become correlated with one another over time through their dipolar couplings. In this way, the usual Zeeman selection rule can be overcome and forbidden transitions can be excited. Experimentally, these multiple quantum coherences (MQC) are formed by the application of appropriate sequences of radio frequency pulses that force the spins to act collectively. 1H spin coherences of even order up to 16 were excited in a polycrystalline sample of ferrocene (C5H5)2Fe and up to 32 in adamantane (C10H16) and their evolutions studied in different conditions: (a) under the natural dipolar Hamiltonian, HZZ (free evolution) and with HZZ canceled out by (b) time reversion or (c) with the MREV8 sequence. The results show that when canceling HZZ the coherences decay with characteristic times (τc≈200 μs), which are more than one order of magnitude longer than those under free evolution (τc≈10 μs). In addition, it is observed that with both MREV8 and time reversion sequences, the higher the order of the coherence (larger number of correlated spins) the faster the speed of degradation, as it happens during the evolution with HZZ. In both systems, it is observed that the sequence of time reversion of the dipolar Hamiltonian preserves coherences for longer times than MREV8.