Understanding the planar conformations in diarylsubstituted heteroarenes: structural and theoretical insights

Abstract

To access the applications of a conformationally flexible molecule, thorough understanding of even the weak contributors to a particular conformation is necessary. We have synthesized and X-ray characterized a series of six diaryl-substituted heteroarenes in order to analyze the role of unconventional intramolecular C_Ar–H⋯N/O vs. conventional H-bonds on the preferred planar geometry of such molecules. The distance well below the sum of van der Waals radii in their solid state structures indicated the existence of intramolecular C_Ar–H⋯N/O interactions. Consequently, the theoretical study was carried out to analyze the energies associated to each of these bonds using the quantum theory of “atoms-in-molecules” (QTAIM) and also the NCI plot. The results indicate that the unconventional C–H⋯O H-bonds involving aromatic H-atoms and the O-atoms of o-methoxy or o-hydroxy substituents in 2, 3a and 3b despite being weaker (dissociation energies around 3–5 kcal mol⁻¹) are relevant and contributing to their planar geometry along with various other factors. However, the electronic repulsion between the lone pair of the nitrogen/oxygen atoms and the neighboring π cloud in these molecules likely contributes more to their preferred planar conformation.