15N–13C Dipole Couplings in Smectic Mesophase of a Thermotropic Ionic Liquid

Abstract

Unique combination of ionic conductivity and anisotropic physical properties in ionic liquid crystals leads to new dynamic properties exploited in modern technological applications. Structural and dynamics information at atomic level for molecules and ions in mesophases can be obtained by nuclear magnetic resonance (NMR) spectroscopy through the measurements of dipole–dipole spin couplings. While 13C–1H and 15N–1H dipolar NMR spectra can be routinely acquired in samples with natural isotopic abundance, recording 15N–13C dipolar NMR spectra is challenging because of the unfavourable combination of two rare isotopes. In the present study, an approach to measure 15N–13C dipole-dipole NMR spectra in static liquid crystalline samples with natural abundance is introduced. We demonstrate that well-resolved spectra can be recorded within 10 h of experimental time using a conventional NMR probe and a moderately strong magnetic field. The technique is applied to a thermotropic smectic mesophase formed by an ionic liquid with imidazolium-based organic cation.

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

“…1c) has been previously applied to a weakly ordered ionic mesophase with relatively small splittings of B100 Hz. 19 Application of this technique to conventional highly ordered thermotropic LCs with kHz-range 13 C-15 N dipolar interactions, such as MBBA, is demonstrated here for the first time. Spin decoupling of low-g 15 N nuclei requires a decoupling field with a large amplitude B 1 = 2pn rf /g N with a nutation frequency exceeding the dipolar coupling magnitude n rf 4 D CN .…”

“…1c in a weakly ordered smectic mesophase of an ionic liquid can reveal satellites caused by carbonnitrogen combined dipolar and J-couplings of the order of 100 Hz and smaller. 19 Here, we extend this approach to thermotropic LCs with a high molecular order parameter S of 40.5 and corresponding strong dipolar interactions up to the kHz range.…”

“…We did not observe any significant 15 N NOE effect in LC samples studied in this and previous work. 18,19 Ramped Hartmann-Hahn CP (HH-CP) is commonly used to enhance 15 N polarization in samples with static dipolar couplings, such as LCs and solids. It has been shown that in static oriented samples of LCs, 13 C CP via adiabatic demagnetization in the rotating frame (ADRF-CP) leads to a higher signal gain than HH-CP.…”

“…Thus, 13 C- 15 N dipolar couplings were not exploited in studies of unlabeled LCs until very recently. 18,19 In a straightforward but long experiment on the 4-cyano-4 0 -pentylbiphenyl (5CB) nematic phase (88k scans over 48 h), 13 C satellites were observed in its NAN15 spectrum. 18 The measurement time required using this approach is impractically long for most applied studies.…”

“…In another recent experiment performed in a weakly ordered ionic smectic A phase, small 13 C-15 N dipolar splittings in 13 C spectra were observed. 19 In more conventional thermotropic LCs with high molecular order, this experiment suffers from further sensitivity losses and longer measurement time because of the heating effects induced by radio-frequency (rf) irradiation with a high power required for efficient spin decoupling. The above considerations prompted us to explore new strategies and experimental designs to increase the signal intensity of 13 C-15 N pairs and suppress signals of uncoupled spins.…”