Natural Abundance Nitrogen-15 NMR in Thermotropic Liquid Crystals With Cyano-Group

Abstract

We present natural abundance nitrogen-15 (NAN15) NMR spectroscopy in thermotropic liquid crystals. It is demonstrated that high resolution NAN15 NMR spectra in mesophases can be accurately recorded in non-spinning samples with a high orientational molecular order and strong anisotropic spin interactions. In this technique, due to low demand on radio-frequency decoupling power, standard solution-state probes can be used, which generally provide superior sensitivity and spectroscopic resolution in comparison to solid-state probes. We show that

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

“…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.…”

“…The first NAN15 spectrum with 13 C dipolar satellites was recently measured in nematic 5CB. 18 However, the experiment required about 2 days of instrumental time to accumulate a sufficient number of scans.…”

“…16,17 It was also recently demonstrated that natural abundance nitrogen-15 (NAN15) NMR spectra can be routinely recorded in static highly ordered samples of bulk LCs with strong anisotropic spin interactions. 18 Heteronuclear 13 C- 15 N dipolar couplings can provide valuable information on short-and long-range structural parameters. With a carbon/nitrogen linewidth of the order of 10 Hz, the dipolar couplings for spin pairs separated by distances as large as 5 Å in thermotropic LCs with high molecular order can in principle be measured, thus providing structural and dynamic constraints over several chemical bonds.…”

“…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.…”

“…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. A further complication arises from the presence of the ordersof-magnitude stronger line from uncoupled 15 N spins in the spectrum.…”