N.M.R. study of monofluorobenzene partially oriented in thermotropic liquid crystals

Unequivocal identification of two-bond heteronuclear correlations in natural products at nanomole scale by i-HMBC

HMBC is an essential NMR experiment for determining multiple bond heteronuclear correlations in small to medium-sized organic molecules, including natural products, yet its major limitation is the inability to differentiate two-bond from longer-range correlations. There have been several attempts to address this issue, but all reported approaches suffer various drawbacks, such as restricted utility and poor sensitivity. Here we present a sensitive and universal methodology to identify two-bond HMBC correlations using isotope shifts, referred to as i-HMBC (isotope shift detection HMBC). Experimental utility was demonstrated at the sub-milligram / nanomole scale with only a few hours of acquisition time required for structure elucidation of several complex proton-deficient natural products, which could not be fully elucidated by conventional 2D NMR experiments. Because i-HMBC overcomes the key limitation of HMBC without significant reduction in sensitivity or performance, i-HMBC can be used as a complement to HMBC when unambiguous identifications of two-bond correlations are needed.

rα Structures of partially oriented pentafluorobenzenes C6F5X (X = H, Cl, I) as determined from 19F NMR spectra with 13C satellites

The (19)F spectra with natural-abundance carbon-13 satellites of C(6)F(5)X (X = H, Cl or I) in ZLI 1695 liquid crystal were analysed. Excluding fluorine-fluorine dipolar coupling constants from the fitting, the vibrationally corrected structures of these molecules were derived and compared with those calculated at DFT/B3LYP level of theory with the aug-cc-pVTZ basis set. The results show that pentafluorobenzene did not exhibit noticeable distortion of the aromatic ring, while chloropentafluorobenzene and iodopentafluorobenzene molecules showed some deformations in their determined structures. Relative anisotropies of F-F couplings have been deduced with accuracy for C(6)F(5)H molecule. Due to uncertainties in structure determination of C(6)F(5)Cl and C(6)F(5)I molecules, it was not possible to obtain precise values for all the corresponding anisotropies. In addition, it was found that the orientation of these molecules in the solvent used can be qualitatively explained in terms of dispersion forces.