Efficient double-quantum excitation in rotational resonance NMR

We present a new technique for double-quantum excitation in magic-angle-spi nning solid-state NMR, The method involves (i) preparation of nonequilibriu m longitudinal magnetization; (ii) mechanical excitation of zero-quantum co herence by spinning the sample at rotational resonance, and (iii) phase-coh erent conversion of the zero-quantum coherence into double-quantum coherenc e by frequency-selective spin inversion. The double-quantum coherence is co nverted into observable magnetization by reversing the excitation process, followed by a pi/2 pulse. The method is technically simple, does not requir e strong RF fields, and is feasible at high spinning frequencies. In [C-13( 2),N-15]-glycine, with an internuclear C-13-C-13 distance of 0.153 nm, we a chieve a double-quantum filtering efficiency of similar or equal to 56%. In [11,20-C-13(2)]-all-E-retinal, with an internuclear C-13-C-13 distance of 0.296 mn, we obtain similar or equal to 45% double-quantum filtering effici ency. (C) 2000 Academic Press.