Deuterium REDOR: Principles and applications for distance measurements

The application of short composite pulse schemes (90(x)degrees-90(x)degrees -90(x)degrees and 90(x)degrees-180(x)degrees-90(x)degrees) to the rotationa l echo double-resonance (REDOR) spectroscopy of X-H-2 (X: spin 1/2 observed ) systems with large deuterium quadrupolar interactions has been studied ex perimentally and theoretically and compared with simple 180 degrees pulse s chemes. The basic properties of the composite pulses on the deuterium nucle i have been elucidated, using average Hamiltonian theory, and exact simulat ions of the experiments have been achieved by stepwise integration of the e quation of motion of the density matrix, REDOR experiments were performed o n N-15-H-2 in doubly labeled acetanilide and on C-13-H-2 in singly H-2-labe led acetanilide. The most efficient REDOR dephasing was observed when 90(x) degrees-180(x)degrees-90(x)degrees composite pulses were used. It is found that the dephasing due to simple 180 degrees deuterium pulses is about a fa ctor of 2 less efficient than the dephasing due to the composite pulse sequ ences and thus the range of couplings observable by X-H-2 REDOR is enlarged toward weaker couplings, i.e., larger distances. From these experiments th e H-2-N-15 dipolar coupling between the amino deuteron and the amino nitrog en and the H-2-C-13 dipolar couplings between the amino deuteron and the al pha and beta carbons have been elucidated and the corresponding distances h ave been determined, The distance data from REDOR are in good agreement wit h data from X-ray and neutron diffraction, showing the power of the method. (C) 1999 Academic Press.