Probing Binding of Ethylated Pillar[5]arene with Pentene and Chlorobutane Positional Isomers

Host guest binding from alkyl-modified pillararene macrocycles has been of significant interest in a variety of applications in the domains of supramolecular chemistry. In this work, we analyze the selectivity in binding of the ethylated pillar[5]arene (EtP5) macrocycle with 1-pentene, cis and trans 2-pentene, and the 1- and 2-chlorobutane isomer guests employing the ωB97x-based density functional theory. EtP5 reveals stronger binding with 1-pentene the accompanying change of energy upon the complexation being 85.5 kJ mol^-1 compared to 71.3 and 75.8 kJ mol^-1 for the cis and trans-2-pentene isomers, respectively. The complexation of EtP5 with pentene isomers is governed by the interplay of CH···π, H-H, and O···H noncovalent interactions. The inferences on the guest binding rationalized through the quantum theory of atoms in molecules are in consonance with data on relative uptake of pentene isomers observed from the gas chromatography experiments reported earlier. A stronger binding of EtP5 with 1-pentene is borne out from a large C-H···π and H-H interactions. The chlorobutane isomers are held together within the EtP5 cavity via C-H···π as well as Cl···H interactions those prevail over the O-H···O hydrogen bonding in such complexes. The host-guest binding emerges with the signature in "frequency shifts" of the characteristic alkyl vibrations of the host and corroborated with the natural bond orbital analyses.