Recoupling of heteronuclear dipolar interactions with rotational-echo double-resonance at high magic-angle spinning frequencies

Heteronuclear dipolar recoupling with rotational-echo double-resonance (RED OR) is investigated in the rapid magic-angle spinning regime, where radiofr equency irradiation occupies a significant fraction of the rotor period (10 -60%). We demonstrate, in two model C-13-N-15 Spin systems, [1-C-13,N-15] a nd [2-C-13, N-15]glycine, that REDOR Delta S/S-0 curves acquired at high MA S rates and relatively low recoupling fields are nearly identical to the De lta S/S-0 curve expected for REDOR with ideal delta-function pulses. The on ly noticeable effect of the finite pi pulse length on the recoupling is a m inor 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 pi pulse phasing schemes. For xy-4 and extensions thereof, finite pulses scale only the dipolar oscillation freque ncy by a well-defined factor. For other phasing schemes (e.g., xx-4 and x ( x) over bar-4) both the frequency and amplitude of the oscillation are expe cted to change. (C) 2000 Academic Press.