Quantitative comparison of REAPDOR and TRAPDOR experiments by numerical simulations and determination of H-Al distances in zeolites

Quantitative H-Al distances in acid sites of two zeolites with MFI and IFR framework topology were obtained by numerical simulation of H-1{Al-27} rotational echo adiabatic passage double resonance (REAPDOR) experiments. A Al-27 offset-dependent data set yields for each resolved H-1 NMR line a corresponding nuclear electric quadrupole coupling constant of the neighboring Al-27 site. This information is used for analyzing a second data set for on-resonance irradiation, where the dipolar evolution time (number of rotor cycles) was varied, to yield the H-1-Al-27 dipolar coupling constant. Numerical simulations indicate that the REAPDOR method does not depend significantly on the polar angles, defining the orientation of the electric field gradient tensor of Al-27 with respect to the Al-H dipolar vector. In contrast, the transfer of populations in double resonance sequence is sensitive to these angles, and it can be thus used to measure them. (C) 2002 Elsevier Science (USA).