Squaramide as a binding unit in molecular recognition

Urea and squaramide are used as building blocks of powerful hosts which are involved in the molecular recognition of quaternary ammonium guests. Complexes of urea and squaramide with ammonium cations were studied using both high level MP2/6-311 + G** ab initio and molecular interaction potential with polarization and MEP calculations, in order to explore the physical nature of the interactions. The same procedure was applied to ammonium complexes with water and benzene as prototype models of crown ethers and calixarenes. Our findings reveal that either urea or squaramide are better binding units for the recognition of ammonium cations than water or benzene. (C) 2000 Published by Elsevier Science B.V.