Helical structures of N-methylated aromatic oligoamides: A density functional study

To clarify the effect of N-methylation to aromatic oligoamides on their conformations, we investigated Nmethylated aromatic amides theoretically, using the density functional theory calculations. It is well-known that Nmethylbenzanilide adopts a cis-conformation rather than a trans-conformation, in contrast to benzanilide. This is ascribed to large destabilization of trans-N-methylbenzanilide and large stabilization by the orbital interaction from nitrogen lone pair to anti-bondingπ* orbital of carbonyl group in cis-N-methylbenzanilide. N-methylated para-linked aromatic diamide has two preferable conformations and two of the conformations of the N-methylated para-linked aromatic oligoamidesexhibit two distinct helical structures. The potential for helix control by solvent is presented.