Can 2-acylpyrroles form an intramolecular hydrogen bond?

The formation of intramolecular hydrogen bonding by certain N-substituted 2-acylpyrroles has been demonstrated by B3LYP/aug-cc-pVDZ calculations, the quantum theory of atoms in molecules, and the natural bond orbital method. Total electron energy densities HBCP at the bond critical point of the H⋯O bond were applied to analyze the strength of these interactions. The relations between quantum theory of atoms in molecules, carbonyl stretching vibrational modes νC = O, and natural bond orbital parameters associated with the formation of the C–H⋯O interaction have been established. The short contacts were found experimentally in the crystal structure of a new 2-acylpyrrole derivative 5-chloro-2-oxopentyl-1-(5-chloro-2-oxopentyl)pyrrolo-2-carboxylate. The influence of 2- and N-substitution of 2-acylpyrroles on C-H⋯O interaction energy is discussed. It was found that the methylene group may act as a proton donor leading to a red-shift or blue-shift phenomenon of the νC–H stretching mode. Copyright © 2015 John Wiley & Sons, Ltd.