Perethylated pillar[n]arenes versus pillar[n]arenes: theoretical perspectives

Abstract

In this article we obtain the electronic structure, ¹H NMR, and the vibrational spectra of perethylated pillar[n]arene (EtPn) (n = 5-8, 10) macrocycles using the ωB97x-based density functional theory. A single cavitand structure endowed with the robust pillar-like architecture was derived for the EtPn (n = 5-7) hosts while for the EtP8 receptor, two conformers one possessing the single cavitand and other double cavitand structure having four hydroquinone monomeric units in each cavity were obtained as the local minima. A double cavitand conformer of the EtP8 was found to be ~ 50 kJ mol^-1 lower in energy than the single cavitand one which was attributed to the increased hydrogen bonding cooperativity and attractive H-H interactions. Further increase of hydroquinone monomers as in EtP10 host engenders only the double cavitand structure, composed of five hydroquinone monomeric units in each cavity. The molecular electrostatic potential investigations along the EtPn series of hosts reveal large electron-rich aromatic moieties compared to unsubstituted pillararenes which suggest the EtPn to be better artificial receptors toward cationic or neutral guests than their unsubstituted analogs. Calculated vibrational frequencies of EtPn (n = 5-8) show the cooperative stretching vibrations in a narrow span of 3241 cm^-1 to 3228 cm^-1. Besides, the most deshielded signals from aromatic protons along the EtPn (n = 5-8) series show up near 7.35 ppm in their ¹H NMR spectra. An increased cooperativity in the lowest energy of the EtP8 conformer further is evident from up-field signal corresponding to methylene protons of ethyl substituent compared with the single cavitand structure. Graphical abstractMolecular electrostatic potential for EtP5 and correlations obtained for δ_H Vs_ρBCP and ν Vs G_(r) plots.

Keywords: H-H interactions; Hydrogen bond cooperativity; Molecular electrostatic potential; Perethylated pillararenes.