Measuring multiple, weak heteronuclear 13C-2H couplings involving large 2H quadrupolar coupling constants and large 13C chemical shift anisotropy values: An experimental and theoretical REDOR study
Ryan C. Nieuwendaal
We discuss the implementation of a REDOR experiment that is capable of measuring multiple, weak 13C-2H dipolar couplings when 2H is a strong quadrupole and 13C exhibits large chemical shift anisotropy. We find that at 7.05 T in the presence of large 13C chemical shift anisotropy values and slight magic angle spinning (MAS) / pulse asynchronization heteronuclear recoupling experiments that rely on either pulses spaced by half-rotor periods or rotatory resonance can exhibit significantly shorter T2s in the S0 signal than those that include pulses spaced by full rotor periods. The shortening of T2 values is due to incomplete refocusing of the CSA with imperfect MAS timing, an effect that we show to be worse at increasing field strengths and will limit the use of REAPDOR or RESPDOR for detecting weak 13C-2H couplings at even moderate fields. We demonstrate the feasibility of a slightly adjusted REDOR experiment to measure weak couplings by systematically measuring and simulating the dephasing rates as a function of the 2H pulse flip angle and B1 field on 13C,2H-L-alanine. The maximum dephasing observed for B1 ≈ 80 kHz is slower than the ideal REDOR dephasing curve by a factor of ≈0.15. Furthermore, upon increasing the number of detected (chemically-identical) nuclei close to the dephaser nucleus, we observe concomitant (1) decreases in the amplitudes of asymptotic oscillations in the dephasing curve and (2) increases in the asymptote from 0.6 to 1. In the limit of multiple, chemically-identical nuclei being dephased from a single nucleus, we propose an empirical dephasing equation so as to extract distances when the number of detected nuclei is large. Such experiments are important for determining distances at polymer/polymer or polymer/molecule interfaces which are heterogeneous, exhibit multiple length scales, and can include > 7 Å internuclear distances depending on monomer size.
Measuring multiple, weak heteronuclear 13C-2H couplings involving large 2H quadrupolar coupling constants and large 13C chemical shift anisotropy values: An experimental and theoretical REDOR study, Solid State Nuclear Magnetic Resonance
(Accessed June 9, 2023)