K. Saalwächter, B. Herrero, M. A. López-Manchado.
Chemical Shift-Related Artifacts in NMR Determinations of Proton Residual Dipolar Couplings in Elastomers
Macromolecules 38, 4040-4042 (2005).
Abstract
Proton NMR experiments such as Hahn-echo relaxometry or uncompensated multiple-quantum spectroscopy, aimed at the determination of residual dipolar couplings in elastomers containing protons in different functional groups, are shown to be subject to artifacts related to a second averaging of these couplings by the chemical shift difference. This leads to field-dependent changes in the apparent coupling constants and to potential misinterpretations when for instance the transverse relaxation behavior is monitored separately for the different chemical groups when they are resolved at high field. Such experiments were recently performed on natural rubber (Steren et al., Macromolecules 37 (2004) 5624), and one of the conclusions of this work concerning a significantly weaker coupling for the methine signal is therefore proven incorrect. We present experimental evidence in terms of Carr-Purcell-Meiboom-Gill transverse relaxation and proton multiple-quantum data acquired with a shift-compensated sequence, which both agree in that methine as well as methylene and methyl groups in natural rubber experience very similar residual couplings.