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Issue 45, 2010
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The origin of the conformational preference of N,N′-diaryl-N,N′-dimethyl ureas

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Poly aromatic ureas and poly aromatic amides are important classes of foldamers—oligomers with well defined conformations. We have explored the origins of the conformational preference of some N,N′-diaryl-N,N′-dimethyl ureas by a combination of NMR spectroscopy and electronic structure calculations using both a recently developed density functional (M06-2X) and a DFT approach (DFT-D) having empirical corrections for dispersive interactions. We have validated the DFT-D approach for structures of this type using high level wavefunction calculations, (CCSD(T)), of the unsubstituted N,N′-diphenyl-N,N′-dimethyl urea. For the N,N′-diaryl-N,N′-dimethyl ureas we have identified a number of ‘endo’ conformers (i.e. having an E,E geometrical conformation about the two urea C–N bonds), both π- and tert-butyl-stacked, as well as ‘exo’ structures (having a Z geometrical conformation about at least one of the C–N bonds), and have computed the relative energies of these conformers as well as the barriers for their interconversion. We find that the relative energies of the ‘endo’ structures closely follow the relative values of the dispersive interactions. The calculations have allowed us to associate different conformers with the various peaks in the NMR spectra, which point to relatively small differences in energy between the conformers. Somewhat larger energy differences are predicted by the two computational approaches, with the M06-2X functional performing the better of the two. It is suggested that the continuum model employed may not be sufficiently accurate to reflect the solvation of the various conformers.

Graphical abstract: The origin of the conformational preference of N,N′-diaryl-N,N′-dimethyl ureas

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The article was received on 14 May 2010, accepted on 15 Sep 2010 and first published on 18 Oct 2010

Article type: Paper
DOI: 10.1039/C0CP00571A
Citation: Phys. Chem. Chem. Phys., 2010,12, 15056-15064
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    The origin of the conformational preference of N,N′-diaryl-N,N′-dimethyl ureas

    J. Clayden, U. Hennecke, M. A. Vincent, I. H. Hillier and M. Helliwell, Phys. Chem. Chem. Phys., 2010, 12, 15056
    DOI: 10.1039/C0CP00571A

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