Columnar self-assembly of N,N',N''-trihexylbenzene-1,3,5-tricarboxamides investigated by means of NMR spectroscopy and computational methods in solution and the solid state.

The columnar self-assembly resulting from units of N,N',N''-trihexylbenzene-1,3,5-tricarboxamide is investigated in solution and the solid state by means of NMR spectroscopy. A parallel computational study utilizing both semiempirical and DFT methods allows comparison between experimental results and calculated data for self-assembled and non-assembled structural hypotheses. The hybrid functional B3LYP is compared with the B3LYP-D and B97D functionals to assess contributions due to dispersion interactions. Interatomic distances are studied utilizing ROE experiments on proton spins in solution. Isotropic shifts as measured experimentally are shown to offer a method to assess the self-assemblies 'on-the-fly'. The anisotropic part of the shift interaction for carbon nuclei is probed in the solid state with specific magic-angle spinning experiments. The sensitivity of the NMR parameters for various carbon environments with respect to the orientation of the substituents and packing effects is investigated computationally. We show that all the utilized experimental techniques, in both solution and the solid state, and in combination with DFT calculations, are capable of discerning between assembled and non-assembled systems and offer a robust set of independent tools to highlight atomic details in self-organized structures.

Fluorine as a hydrogen-bond acceptor: experimental evidence and computational calculations.

Hydrogen-bonding interactions play an important role in many chemical and biological systems. Fluorine acting as a hydrogen-bond acceptor in intermolecular and intramolecular interactions has been the subject of many controversial discussions and there are different opinions about it. Recently, we have proposed a correlation between the propensity of fluorine to be involved in hydrogen bonds and its (19)F NMR chemical shift. We now provide additional experimental and computational evidence for this correlation. The strength of hydrogen-bond complexes involving the fluorine moieties CH2F, CHF2, and CF3 was measured and characterized in simple systems by using established and novel NMR methods and compared to the known hydrogen-bond complex formed between acetophenone and p-fluorophenol. Implications of these results for (19)F NMR screening are analyzed in detail. Computed values of the molecular electrostatic potential at the different fluorine atoms and the analysis of the electron density topology at bond critical points correlate well with the NMR results.