REDOR Dephasing by Multiple Spins in the Presence of Molecular Motion

Closed-form, numerical algorithms are presented for calculating REDOR dephasing for three general cases: (i) collections of isolatedI–Sspin pairs; (ii) manySspins coupled to anIspin; and (iii) anI–Sspin pair in relative motion. For the case when more than oneSspin is dipolar coupled to anIspin, the calculation assumes that theS–Shomonuclear interaction does not affect REDOR dephasing. Full numerical simulations show that this assumption is true if theS-spin lineshapes are inhomogeneously broadened, theS-spin chemical shifts are far from rotational resonance, and a version of REDOR is used which minimizes the number ofS-spin π pulses. For the rapidly rotating –CF₃group of poly(trifluoroethyl methacrylate), the formalisms of (ii) and (iii) are combined to calculate the dephasing. The experimentally measured dephasing matches theory when the wiggling motion of the –OCH₂CF₃moiety of the polymer is taken into account.