Exchange-selective excitation pulses for dynamic magnetic resonance.

Coherent evolution is punctuated by dynamical processes such as chemical exchange, conformational transformation, or site hopping in many important problems ranging from biomolecular function to ion trap quantum computation. One well-explored example is nuclear magnetic resonance (NMR) spectroscopy, where experimental development is grounded in decades-old theory, but structural dynamics are not easily integrated into this picture. Here, we introduce an approach that selectively excites NMR resonances that undergo chemical exchange while suppressing the signal arising from nondynamic components of the system. We show that for exchange rates spanning more than four orders of magnitude, one can still selectively excite spins undergoing exchange while suppressing static resonances. Generalizing this approach, to selectively excite (or selectively preserve) only members of an ensemble that have undergone exchange or rearrangement, has the potential to improve the analytical power of many spectroscopic techniques.