Recoupling of Heteronuclear Dipolar Interactions with Rotational-Echo Double-Resonance at High Magic-Angle Spinning Frequencies

摘要

Heteronuclear dipolar recoupling with rotational-echo double-resonance (REDOR) is investigated in the rapid magic-angle spinning regime, where radiofrequency irradiation occupies a significant fraction of the rotor period (10-60%). We demonstrate, in two model ~(13)C-~(15)N spin systems, [1-~(13)C, ~(15)N] and [2-~(13)C, ~(15)N]glycine, that REDOR ΔS/S_0 curves acquired at high MAS rates and relatively low recoupling fields are nearly identical to the ΔS/S_0 curve expected for REDOR with ideal δ-function pulses. The only noticeable effect of the finite π pulse length on the recoupling is a minor scaling of the dipolar oscillation frequency. Experimental results are explained using both numerical calculations and average Hamiltonian theory, which is used to derive analytical expressions for evolution under REDOR recoupling sequences with different π pulse phasing schemes. For xy-4 and extensions thereof, finite pulses scale only the dipolar oscillation frequency by a well-defined factor. For other phasing schemes (e.g., xx-4 and xx-bar-4) both the frequency and amplitude of the oscillation are expected to change.