DFT Study on complexation behaviors of 1,2-bridged p-Tert-butylcalix[6]crown-5-ether with Alkylammonium Ions

Abstract

The structures and energies of 1,2-bridged p-Tert-butylcalix[6]crown-5-ether (1) and its alkylammonium endo-complexes were calculated using a DFT B3LYP/6-31G(d,p) method. Smaller alkylammonium cations exhibited better complexation efficiency with host 1 than bulkier alkylammonium ions; mainly, the hydrogen bonding and steric hindrance contributed to the complexation efficiency. Complex 1·NH+ 4 had the smallest HOMO-LUMO gap, resulting in the longest UV-Absorption wavelength. In a series of butylammonium guests, the calculated complexation efficiency decreased as follows: n-BA+ > iso-BA+∼sec-BA+ > tert-BA+. The DFT results are consistent with the experimentally determined association constants of branched butylammonium ions. Compared to the unrestricted frequencies, the complexes showed unusually low N-H and O-H stretching vibrational frequencies because of the strong hydrogen bonding between host 1 and alkylammonium cations. © Copyright 2016 American Scientific Publishers All rights reserved.