Abstract
A new scheme for low-power broadband heteronuclear decoupling is described, based on the use of a composite radiofrequency pulse sequence 90°(+ X) 180°(− X) 270°(+ X), incorporated into a repeated cycle or supercycle. Its principal attribute is that the residual splittings on the observed resonances (usually carbon-13) are very small (less than 0.1 Hz) for a wide range of decoupler offsets (approximately − B 2 < ΔB < + B 2). Existing theories of broadband decoupling are used to calculate the effects of various possible instrumental imperfections on decoupling performance. It is concluded that spatial inhomogeneity of the B 2 field has a perceptible influence near the extremes of the decoupler bandwidth. Only 180° shifts of the radiofrequency phase are used, and the performance is remarkably insensitive to the exact setting of this phase shift. Any decoupler which employs a systematic modulation scheme runs the risk of introducing “cycling sidebands” into the observed spectrum; it is demonstrated that with the proposed sequence these sidebands are very weak, particularly when the decoupling cycle and the signal acquisition processes are not synchronized. As an illustration, the broadband-decoupled carbon-13 spectrum of an aniline derivative is recorded showing natural-abundance carbon-13 satellite signals but no appreciable cycling sidebands. The circuit for a practical implementation of this decoupling sequence is described.
Published Version
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