Experimental strategies for ¹³C–¹⁵N dipolar NMR spectroscopy in liquid crystals at the natural isotopic abundance

Abstract

Direct dipolar spin couplings are informative and sensitive probes for a wide range of dynamic processes and structural properties at atomic, molecular and supramolecular levels in liquid crystals and other anisotropic materials. Usually, heteronuclear 13C-1H dipolar couplings in liquid crystals with natural 13C abundance are measured. Recording 13C-15N NMR dipolar spectra in unlabeled materials is challenging because of the unfavorable combination of two rare isotopes. Here we design and compare various experimental strategies to measure short- and long-range heteronuclear 13C-15N dipolar couplings in liquid crystalline samples with high molecular orientational order. New techniques were developed to record 13C and 15N spectra of naturally occurring 13C-15N spin pairs with increased signal intensity and spectral resolution while suppressing the signals of the uncoupled isotopes. Highly resolved 13C-15N dipolar spectra were recorded within an experimental time of a few hours. Coupling constants in a broad range of 10-1000 Hz between spins separated by up to five chemical bonds and distances of up to 5 Å were measured. Because of their relatively low demands on radio-frequency power levels, the experiments were easy to implement using conventional high-resolution solution-state NMR hardware. Experimental data were compared to the results of density functional theory and molecular dynamics computational analyses. The presented experimental methods to characterize the dipolar couplings in unlabeled materials provide novel routes to investigate molecular structure and dynamics in mesophases.

Understanding ionic mesophase stabilization by hydration: a solid-state NMR study

“…22,56 15 N- 13 C dipolar couplings were obtained by a recently developed approach for the 15 N- 13 C dipolar spectroscopy at natural isotopic abundance (Section S5, ESI †). 57,58 Natural abundance deuterium (NAD) NMR spectra were recorded in the presence of the proton heteronuclear decoupling (Section S6, ESI †). The diffusion measurements using pulsed field gradient (PFG) 1 H NMR were performed with a Bruker microimaging probe MIC5 with 3-directional orthogonal magnetic field gradients of maximum strength 2.8 T m À1 .…”

“…S5a and b (ESI †). 57,58 For structure #1, the vector N(1)-C2 is along the chain backbone. Because it is nearly collinear with the C n -C n+2 vectors of the chain, similar values of the order parameters S N(1)-C2 E S C1-C3 E S C2-C4 E S C3-C5 were expected in this case.…”

Experimentally Established 15N NMR Absolute Shielding Scale for Theoretical Calculations

“…Nitrogen centers of many organic and inorganic molecules are active or potentially active for various non-covalent − and coordinative interactions. , The low natural abundance and the gyromagnetic ratio of the 15 N isotope make its detection difficult. However, this problem can be solved either by isotopic labeling , or by specially designed NMR methods. − In this work, the experimentally measured 15 N NMR chemical shift tensors (CSTs) will be compared with the calculated nitrogen absolute shielding tensors in order to identify the molecules for which these tensors are close up to a constant. This constant is σ ref ( 15 N).…”

Attached Nitrogen Test by 13C–14N Solid-State NMR Spectroscopy for the Structure Determination of Heterocyclic Isomers

“…Nitrogen has two NMR-active isotopes, namely, 14 N and 15 N, with 15 N being the preferred nucleus to probe in NMR spectroscopy because it is a spin I = 1/2 nucleus, whereas 14 N is a spin I = 1 quadrupolar nucleus. Unfortunately, 13 C– 15 N NMR experiments are challenging at natural isotopic abundance (0.004% probability of having a 13 C– 15 N spin pair) and are sometimes only feasible in concentrated systems and/or with sensitivity-enhancement techniques, such as dynamic nuclear polarization (DNP). − 13 C– 14 N NMR experiments are attractive because 14 N is 99.6% abundant. However, the quadrupolar nature of 14 N means that one-bond 13 C– 14 N J -couplings ( 1 J ≈ 10–15 Hz) often cannot be observed in solution NMR spectra due to the self-decoupling of 14 N that occurs because of the continuous alternation of the 14 N spin states by rapid longitudinal ( T 1 ) relaxation.…”