Abstract
A theoretical analysis is given of the response of particular nuclear magnetic spin systems to resonant pulse sequences of the form 90 x -τ-(-θϕ-ts -) n - with nθϕ = Θϕ, where the sequence 90 x -τ-Θϕ is known to excite particular forbidden transitions in the spin system for definite values of the product Ωτ, Ω being a spin coupling giving rise to splittings in the spectrum of the spin system. The systems investigated are (i) a spin-1 with a finite, first order quadrupole coupling and (ii) an AX scalar-coupled spin-½ pair. In the spin-1 case it is shown, for example, that excitation of double quantum coherence (DQC) by the 90 x -τ-90 x sequence is made increasingly selective on the value of the quadrupole coupling ΩQ by using the sequences 90 x -τ-(-θ x -4τ-) n - or 90 x -τ-(-θ x -2τ-θ-x -2τ-) n/2>-. These sequences excite DQC for ΩQ values around ΩQτ = (2q + 1)π/2, q integer, and increasing n leads to increasing selectivity with respect to ΩQ. A similar result is obtained for excitation of a state of J order in the AX spin system. Experimental measurements are reported for the spin-1 system using the 2H resonance in an oriented soap/D2O mesophase. The results agree quantitatively with the theory and show the increasing selectivity with the number of pulses as predicted.
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