Heteronuclear decoupling by multiple rotating frame technique

The paper describes the multiple rotating frame technique for designing modulated rf fields, that perform broadband heteronuclear decoupling in solution NMR spectroscopy. The decoupling method presented here is understood by performing a sequence of coordinate transformations, each of which demodulates a component of the rf field to a static component, that progressively averages the chemical shift and the dipolar interaction. We show that by increasing the number of modulations in the decoupling field, the ratio of dispersion in the chemical shift to the strength of the static component of the rf field is successively reduced in the progressive frames. The known decoupling methods like continuous wave decoupling, TPPM, etc., can be viewed as special cases of this method and their performance improves by adding additional modulations in the decoupling field. The technique is also expected to find use in design of broadband excitation, inversion and mixing sequences and broadband experiments in solid state NMR.

Depiction of magnetization trajectory of the observed spin as function of modulations in MODE sequence and comparison with other decoupling sequences.120Research highlights► Heteronuclear decoupling by a CW field, in the presence of chemical shifts, creates an effective field that is not perpendicular to the coupling interaction and part of the coupling interaction parallel to this field is not averaged. ► We show that it possible to design a multiply MODulated rf field (MODE sequence) for broadband decoupling, whose effect is best understood by performing a sequence of coordinate transformations, specific to each chemical shift. ► Each transformation reduces the ratio of chemical shift interaction to static rf field and makes the effective field perpendicular to the coupling interaction. ► We show that by increasing the number of modulations in the decoupling field, we can significantly improve the decoupling. Performance of MODE decoupling is compared with state of the art methods. ► The method is expected to find applications in design of broadband decoupling, excitation and mixing pulse sequences and broadband experiments in solid state NMR.