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

Jonathan Clayden*, Ulrich Hennecke, Mark A. Vincent, Ian H. Hillier, Madeleine Helliwell

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)

25 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)15056-15064
Number of pages9
JournalPhysical Chemistry Chemical Physics
Volume12
Issue number45
Early online date18 Oct 2010
DOIs
Publication statusPublished - 7 Dec 2010

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