Calculations on the Electronic Excited States of Ureas and Oligoureas

Abstract

We report CASPT2 calculations on the electronic excited states of several ureas. For monoureas, we find an electric dipole forbidden n --> pi* transition between 180 and 210 nm, dependent on the geometry and substituents of the urea. We find two intense pinb --> pi* transitions between 150 and 210 nm, which account for the absorptions seen in the experimental spectra. The n' --> pi* and pib --> pi* transitions are at wavelengths below 125 nm, which is below the lower limit of the experimental spectra. Parameter sets modeling the charge densities of the electronic transitions have been derived and permit calculations on larger oligoureas, using the exciton matrix method. For glycouril, a urea dimer, both the CASPT2 method and the matrix method yield similar results. Calculations of the electronic circular dichroism spectrum of an oligourea containing eight urea groups indicate that the experimental spectrum cannot be reproduced without the inclusion of electronic excitations involving the side chains. These calculations are one of the first attempts to understand the relationship between the structure and excited states of this class of macromolecule.

Folding and self-assembly of aromatic and aliphatic urea oligomers: Towards connecting structure and function

“…[133][134][135] Recently, the exciton matrix method was applied to the calculation of the circular dichroism spectrum of an oligourea containing eight urea groups. 136 The finding that the experimental spectrum could not be reproduced without the inclusion of electronic excitations involving the side chains tends to suggest, however, that additional studies are still needed to increase our understanding of the electronic structure of oligoureas.…”

Solid state NMR studies of oligourea foldamers: Interaction of 15N-labelled amphiphilic helices with oriented lipid membranes

“…2.5 residues per turn), by sequestration of cationic residues on one face of the helix. The 2.5-helical structure of aliphatic urea foldamers has been thoroughly investigated for oligomers of various lengths in solution by diverse spectroscopic methods (NMR, 25-27 electronic circular dichroism (ECD), 28 FT-IR 29 ) and in the crystal state by X-ray diffraction. 30, 31 The helix is characterized by a (+)-synclinal arrangement around the main chain C-C bonds and is stabilized by a collection of three centered H-bonds closing 12-and 14-membered pseudo-rings.…”

Solution structure determination of oligoureas using methylene spin state selective NMR at 13C natural abundance

“…Similarly, the contribution of the urea chromophore dominates the far-UV CD spectra of N,N -linked oligoureas and the first attempt to correlate far-UV chiroptical properties and conformational preferences of oligoureas has been reported recently. [2,8] However, as shown for β-peptides, CD patterns are not necessarily conformationspecific and interpretations of spectra may be tricky. [5,9] Finally, CD spectroscopy is not suitable for specific conformations that lack intense bands arising from coupling of closely interacting amides (e.g.…”