- R. Kurz, M. F. Cobo, E. R. deAzevedo, H. Schneider, M. Sommer, A. Wicklein, M. Thelakkat, G. Hempel, K. Saalwächter.
Avoiding Bias Effects in NMR Experiments for
Heteronuclear Dipole–Dipole Coupling Determinations:
Principles and Application to Organic Semiconductor
Chem. Phys. Chem. 14, 3146-3155(2013).
Carbon–proton dipole–dipole couplings between bonded
atoms represent a popular probe of molecular dynamics in
soft materials or biomolecules. Their site-resolved determination,
for example, by using the popular DIPSHIFT experiment,
can be challenged by spectral overlap with nonbonded carbon
atoms. The problem can be solved by using very short crosspolarization
(CP) contact times, however, the measured modulation
curves then deviate strongly from the theoretically predicted
shape, which is caused by the dependence of the CP efficiency
on the orientation of the CH vector, leading to an anisotropic
magnetization distribution even for isotropic samples.
Herein, we present a detailed demonstration and explanation
of this problem, as well as providing a solution. We combine
DIPSHIFT experiments with the rotor-directed exchange of orientations
(RODEO) method, and modifications of it, to redistribute
the magnetization and obtain undistorted modulation
curves. Our strategy is general in that it can also be applied to
other types of experiments for heteronuclear dipole–dipole
coupling determinations that rely on dipolar polarization transfer.
It is demonstrated with perylene-bisimide-based organic
semiconductor materials, as an example, in which measurements
of dynamic order parameters reveal correlations of the
molecular dynamics with the phase structure and functional