Deuterium REDOR: Principles and Applications for Distance Measurements

Abstract

The application of short composite pulse schemes ([figure] and [figure]) to the rotational echo double-resonance (REDOR) spectroscopy ofX–2H (X: spin12, observed) systems with large deuterium quadrupolar interactions has been studied experimentally and theoretically and compared with simple 180° pulse schemes. The basic properties of the composite pulses on the deuterium nuclei have been elucidated, using average Hamiltonian theory, and exact simulations of the experiments have been achieved by stepwise integration of the equation of motion of the density matrix. REDOR experiments were performed on15N–2H in doubly labeled acetanilide and on13C–2H in singly2H-labeled acetanilide. The most efficient REDOR dephasing was observed when [figure] composite pulses were used. It is found that the dephasing due to simple 180° deuterium pulses is about a factor of 2 less efficient than the dephasing due to the composite pulse sequences and thus the range of couplings observable byX–2H REDOR is enlarged toward weaker couplings, i.e., larger distances. From these experiments the2H–15N dipolar coupling between the amino deuteron and the amino nitrogen and the2H–13C dipolar couplings between the amino deuteron and the α and β carbons have been elucidated and the corresponding distances have been determined. The distance data from REDOR are in good agreement with data from X-ray and neutron diffraction, showing the power of the method.