Probing Spatial Distribution of Alignment by Deuterium NMR Imaging

Abstract

Deuterium NMR imaging was used to evaluate the spatial distribution of the degree of alignment in different types of alignment media by monitoring the deuterium quadrupolar splitting using spatially resolved NMR techniques in conventional liquid state NMR instruments. These images allow the unambiguous distinction of magnetic field and alignment inhomogeneities present in partially aligned samples, revealing the underlying reasons for linebroadening within an alignment medium that cannot be explained by the sole analysis of 1D (2)H NMR spectra. For example, alignment inhomogeneities due to broken gels or the presence of concentration gradients in liquid crystalline solutions are clearly detected by the imaging methods proposed in this work.

Biphasic Liquid Crystal and the Simultaneous Measurement of Isotropic and Anisotropic Parameters by Spatially Resolved NMR Spectroscopy

“…In the course of these experiments to determine the critical concentration, we observed the formation of a biphasic system when the concentration is slightly below this point (11 % (m m −1 )). To learn more about this system, we applied the deuterium imaging experiment published by two of the authors in 2013 (pulse sequence in Figure A) . This way, we identified the upper layer as being anisotropic, displaying a doublet, split by the quadrupolar coupling (Δ ν Q (300 K)=322 Hz) and the lower layer as being nearly isotropic with a very small quadrupolar splitting (Δ ν Q (300 K) ≈1 Hz).…”

“…Two corresponding approaches for the measurement of spectra for the extraction of one‐bond couplings have been designed and will be introduced in the following. Both experiments are based on the different, already mentioned deuterium imaging experiments, which nowadays are frequently used to control the homogeneity of alignment in partially aligned samples …”

“…The first technique uses phase encoding for spectral resolution in a separate dimension and the basic principle is best understood on the original spatially resolved 2 H‐experiment . Based on a well‐known phase encoding scheme, the strength of a pulsed field gradient of constant length is incremented to add a space‐dependent frequency to the Larmor frequency of the signals (see Figure A) …”

CLIP–ASAP‐HSQC for fast and accurate extraction of one‐bond couplings from isotropic and partially aligned molecules

“…A 100 mM sucrose solution was put on top of the gel to diffuse into it, and the readily swollen gel was placed into a silicone tube‐based stretching device . The gel was stretched to an extension factor of Ξ = 0.5 calculated according to Ξ = ( L / L 0 ) − 1 with L being the length of the stretched gel and L 0 the length of the unstretched gel corresponding to a quadrupolar splitting of the deuterium signal of the solvent of Δ ν Q = 3.4 Hz at 600 MHz and 300 K. Gel homogeneity was checked using 2 H‐imaging along the z ‐axis . The scalar one‐bond CH couplings were obtained from corresponding spectra using a 100 mM isotropic solution of sucrose dissolved in D 2 O.…”

Perspectives in the application of residual dipolar couplings in the structure elucidation of weakly aligned small molecules

“…The quadrupolar splitting Δν Q of the deuterated solvent is generally used as an experimental indicator of both the degree of orientation as well as the homogeneity of the aligned sample . While a simple 1D 2 H spectrum may be sufficient in some cases, a crucial extension is the use of 2 H chemical shift imaging . These experiments usually take less than a minute to acquire and should therefore generally be applied to ensure spatially homogenous alignment conditions and high‐quality RDC data.…”