Effect of lanthanide shift reagents on conformational equilibria

Abstract

The conformational equilibrium constants for a series of 1-alkyl substituted cyclohexanols have been determined using a lanthanide shift reagent, and the results compared with other methods. In comparison to other techniques greater preference is shown by the hydroxyl group for the equatorial position. This is rationalized by the effect on the equilibrium of the bulky shift reagent.

Solid‐state NMR studies and DFT calculations of flavonoids: baicalein, baicalin and wogonoside

“…The three-dimensional (3D) structure of solid baicalein is known from XRD measurements, the crystals were grown in water-methanol [19] and in ethyl acetate, [21] whereas our sample was obtained from methanol. In the crystals, the C2-C1 bond distance is 1.476(5) Å [20] or 1.4691(7) Å, [21] which clearly indicates its double-bond character and, as a result, the two rings are almost planar. The torsional angle about O2-C2-C1 -C2 bond is only 8…”

“…Chemical shifts of C4 O of 182-183 ppm in solution and solid-state spectra of compounds 1-3 indicate the presence of the intramolecular hydrogen bond C5-OH· · ·O C4 in both phases; such an interaction exists in the crystals of baicalein (see XRD data [20,21] ). One can expect that the glucuronide carboxyl group will form hydrogen bonds in the solid state; the increase of chemical shift ( = −2.3 ppm) is observed in compound 3, but the difference of only 0.9 ppm is not informative in compound 2.…”

“…[20,21] However, skew (κ) and span ( ) parameters, which reflect anisotropic distribution of electron density, differ significantly when calculated using the experimental and theoretical data. (Skew is obtained from the equation κ = 3(δ iso −δ 22 )/ and span is given by = δ 11 −δ 33 .)…”

“…However, several sidebands separation techniques are now available, such as the phase-adjusted spinning sideband (PASS)-2D one. [21] This technique was successfully applied in the structural studies of solid flavonoids. [22,23] …”

Chemical shift anisotropy amplification

“…Of these, experiments based on the PASS technique [16] are some of the simplest to implement and most efficient in terms of acquisition time. In PASS sequences of p-pulses synchronized with the MAS result in a phase shift of magnitude MH for the sideband at a frequency Mx r from the isotropic line.…”