In this paper, we report on the use and limitations of the popular double-quantum recoupling sequence back-to-back in studies of (31)P-(31)P spatial proximities in Pd-phosphine complexes at medium field (9.38 T) under the conditions of fast MAS. The effects of internuclear distances in different spin systems and the impact of isotropic chemical shift and chemical shift anisotropy (CSA) offsets on the detectibility of SQ-DQ correlations were of particular interest to us. Selected model compounds with these requirements in mind were synthesized. By optimization of the excitation times of SQ-DQ correlations up to 4.6 Å could be obtained even in four-spin systems; however, certain long-range correlations may be weak or missing. Although under fast MAS, CSA values up to 270 ppm are well tolerated, in multi-spin systems in cases of isotropic chemical shift offsets larger than 50 ppm, the DQ coherences cannot be properly excited.
Copyright © 2011 John Wiley & Sons, Ltd.