Double-quantum excitation in the NMR of spinning solids by pulse-assisted rotational resonance

We describe a new technique for double-quantum excitation in magic-angle-sp inning NMR of powdered solids. The technique is designed to efficiently exc ite double-quantum coherence in the vicinity of a rotational resonance cond ition, The offset from rotational resonance allows the double-quantum filte red signals to be observed with high resolution and sensitivity. The method uses rotational excitation of zero-quantum coherence, assisted by radiofre quency pulse cycles. The zero-quantum coherence is converted into double-qu antum coherence by a frequency-selective inversion sequence. Experiments on [C-13(2),N-15]-glycine demonstrate a double-quantum filtering efficiency o f similar or equal to 41% at a sample rotation frequency of 8.300 kHz, whic h is 1.600 kHz away from the n = 1 rotational resonance. We achieve 32% dou ble-quantum filtering efficiency at a spinning frequency of 9.250 kHz, whic h is 2.550 kHz away from rotational resonance, (C) 2001 Academic Press.