Molecular modeling of complexation of alkyl ammonium ions by p-tert-butylcalix[4]crown-6-ether

The conformations and energies of p-tert-butylcalix[4]crown-6-ether (1) and its alkyl ammonium complexes have been simulated by AM1 semi-empirical qua ntum mechanics and molecular mechanics calculations using a variety of forc efields (MM2, MM+, CVFF). We performed molecular dynamics calculations to s imulate the behavior of these complexes primarily focusing on the three rep resentative conformations (cone, partial cone, 1,3-alternate) of host molec ule 1. When we performed AMI semi-empirical and molecular mechanics calcula tions, the cone conformation was generally found to be most stable for all the employed calculation methods. The primary binding site of host 1 for th e recognition of alkyl ammonium guests was confirmed to be the central part of the crown moiety. The complexation enthalpy calculations revealed that the alkyl ammonium cations having smaller and linear alkyl group showed the better complexation efficiencies when combined with p-tert-butylcalix[4]cr own-6-ether, that is in satisfactory agreement with the experimental result s.