Conformational analysis of [12]aneN(4) (1,4,7,10-tetraazacyclododecane) and [14]aneN(4) (1,4,8,11-tetraazacyclotetradecane) using molecular mechanicsand ab initio methods

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