Conformational Analysis of [12]aneN4(1,4,7,10-Tetraazacyclododecane) and [14]aneN4(1,4,8,11-Tetraazacyclotetradecane) Using Molecular Mechanics and ab Initio Methods

The macrocyclic polyamines [12]aneN(4) and [14]aneN(4) have been investigated using molecular mechanics (MM3) and ab initio quantum chemical methods. Molecular mechanics conformational searching was performed using two schemes, and the lowest energy minima were submitted to HF/6-31G** ab initio calculations. Differences between molecular mechanics and ab initio results are discussed, with emphasis on intramolecular interactions, most notably intramolecular hydrogen bonding and electrostatic interactions. MM3 gives structures that agree well with the ab initio structures but is unable to reproduce subtle effects governing the ab initio energetic order of rotamers. Basis set dependence and influence of electron correlation is also investigated for [12]aneN(4).