A double-component Anderson-Weiss approach for describing NMR signals of mobile SIn units: application to constant-time DIPSHIFT experiments.

A composed Gaussian local field is proposed to describe the effect of molecular motions on NMR signals of SIn units (e.g., CHn or NHn), based upon the well-know Anderson-Weiss (AW) approximation. The approach is exemplified on constant-time recoupled dipolar chemical-shift correlation (tC-recDIPSHIFT) experiments, providing an analytical formula that can be used as a fitting function in studies of intermediate-regime motions. By comparison of analytical tC-recDIPSHIFT curves and dynamic spin dynamics simulations, we show that for heteronuclear spin pairs (SI system), the AW treatment assuming the usual Gaussian local field is accurate. However, the approximation fails for the case of SIn spin systems for motional rates higher than a few kHz. Based on earlier work of Terao et al., who proposed a decomposition of CHn dipolar powder patterns into to 2(n) spin-pair-type patterns, we propose an AW approach based upon a double-Gaussian local field. We derive an analytical formula for tC-recDIPSHIFT signals, and demonstrate its accuracy by comparison with simulations of several motional geometries and rates, and with experimental results for a model sample. The approach is not limited to the tC-recDIPSHIFT experiment and should be of general use in dipolar-coupling based experiments probing (partially) mobile SIn molecular moieties.