Virtual infinite-pulse radio-frequency driven dipolar recoupling in solid-state NMR.

Radio-frequency-driven dipolar recoupling (RFDR) is a classical dipolar recoupling method in solid-state nuclear magnetic resonance (NMR). However, the original infinite-pulse RFDR (ipRFDR) remains experimentally inaccessible due to finite radio-frequency (RF) power constraints. We propose virtual infinite-pulse radio-frequency-driven dipolar recoupling (vipRFDR), which employs finite RF powers to emulate ipRFDR. The key idea is to replace conventional short π pulses with full-rotor-period inversion pulses, which lead to a net flip angle of 180° without introducing additional recoupling effects. Using average Hamiltonian theory and numerical simulations, we demonstrate that vipRFDR closely reproduces the dipolar recoupling behavior of ipRFDR, recovering the homonuclear flip-flop Hamiltonian that is otherwise unattainable under practical conditions with finite RF powers. Experiments reveal that vipRFDR can effectively generate 1H-1H correlations, albeit at a slow transfer rate. The underlying principle of employing full-rotor-period pulses to mimic infinitely short pulses can be extended to other recoupling sequences that rely on the infinite-pulse approximation.