Para-substituted pyridines: Effects on the interaction with 2-perfluoropropyl iodide and vibrations

Abstract

Vibrational spectroscopy and quantum chemical calculations were employed to investigate the impact of electron donating and electron withdrawing para-substituents on pyridine in terms of molecular and electronic structure, electrostatic properties and the seven in-plane, ring-related vibrational normal modes (RNMs), along with the capability of pyridinyl nitrogen atom to get involved in halogen bonding (XB) with 2-perfluoropropyl iodide (iso-C3F7I). A descriptor was proposed to quantitatively describe pyridine's para-substituents’ electron donating/withdrawing ability. Some quantitative correlations were then identified for the geometric and topological characteristics on bond critical points of XB complexes, the frequency shifts of some RNMs upon XB and thermodynamic quantities of XB formation versus the aforementioned substitution descriptors. Natural bond orbital (NBO) analysis established the formation of XB is beyond a mere electrostatic interaction. Such a XB occurring between the electron lone pair of pyridinyl N and I on iso-C3F7I indicated a strong charge transfer (CT) character. The comparison between the FT-IR/ Raman spectra of 6 binary systems and the calculated spectra for monomers and XB dimer revealed the potential halogen bond between the substituents and iso-C3F7I as well as the intermolecular hydrogen bond (HB) between two same molecules of some substituted pyridines.