Attenuating Multispin Contributions during Selective Proton–Proton Distance Measurements in Magic Angle Spinning NMR

Solid-state nuclear magnetic resonance (NMR) enables atomic-level structural insights, but accurate ¹H-¹H distance measurements in fully protonated solids remain challenging due to multispin effects. Spin diffusion combined with modeling offers one approach, while selective recoupling of protons (SERP) allows targeted dipolar measurements; however, both methods are hindered by interference from homogeneously coupled spins. We introduce isotropic chemical shift-amplified SERP (iCSA-SERP), which enhances spin-pair selectivity by amplifying isotropic chemical shift evolution in the interaction frame. The effective Hamiltonian is derived using bimodal Floquet theory, providing analytical insight into the recoupling mechanism. Simulations at high magnetic fields (e.g., 1200 MHz) demonstrate improved distance extraction in the 2-4 Å range with reduced spin-bath interference. Experimental validation in protonated model systems, including NAV and f-MLF, confirms improved measurement accuracy in agreement with theory and simulations. iCSA-SERP offers a robust approach for precise interproton distance measurements in dense spin networks, especially at high fields.