Density Functional Theory Study of p-tert-Butylcalix[4]crown-7-ether Ester Complexed with Alkylammonium Ions

Abstract

In this study, we describe the optimization of the three conformers (cone, partial cone, and 1,3-alternate) of 1,3-bridged p-tert-butylcalix[4]crown-7-ether ester (1) using the density functional theory (DFT) B3LYP/6-31G (d,p) method. 1(CONE) was calculated to be the most stable of the three conformations of 1. The structures, energies, and frontier orbitals (HOMO and LUMO) of the exo-complexes of 1(CONE) with alkylammonium ions were calculated using the DFT method. It was found that small alkylammonium cations exhibit a higher complexation efficiency with 1 than with larger alkylammonium ions. However, the branched alkylammonium ions exhibited stronger binding properties with 1 than the linear n-propyl and n-butyl ammonium ions. The DFT-calculated energies of 1 complexed with bulky alkylammonium ions exhibited stronger binding efficiencies than the calix[5]crown-6-ether (2). Hydrogen bonding and steric hindrance of the alkylammonium cations were found to be the primary factors governing the complexation efficiency of 1 with various cations. The DFT B3LYP/6-31G(d,p)-calculated IR spectra of the conformers of 1 and its cone-type exo-complexes with various alkylammonium ions were subsequently analyzed. The hydrogen-bonded N-H and O-H stretching vibrational frequencies were compared to the unrestricted vibrational frequencies in detail, and the unusually low N-H stretching frequency (3054 cm(-1)) of the 1 center dot iso-Bu+ complex and the unusually low O-H stretching frequency (3387 cm(-1)) of the 1 center dot n-Bu+ complex were determined to be caused by strong hydrogen bonding.