Experiments on selective excitation of multiple-quantum transitions in NMR spectroscopy

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Abstract
Irradiating a spin system with an appropriate sequence of phase shifted pulses excites only certain orders of multiple-quantum transitions. excited. Using such sequences in preparation and detection produces a large signal/noise enhancement for the multiple-quantum spectra over nonselective excitation, as predicted from theory. In a previous paper the theory was presented in detail. In this paper some aspects of the theory are first outlined briefly. Experimental results are then presented demonstrating four-quantum, six-quantum, and A-symmetry selection in oriented benzene and eight-quantum selection in oriented 1-bromobutane. A six-quantum selective sequence applied to benzene with nonequilibrium initial reduced density matrix proportional to Ix produces one- and five-quantum spectra. The n-quantum signal/noise enhancements are obtained and the selectivity is measured as a function of pulse sequence parameters. The behavior is that expected if one assumes that the limit to selectivity is determined by leading nonselective terms in an average Hamiltonian expansion.