SCF-MO study of the preferred conformations of nylons 1, n

Abstract

Semiempirical SCF-MO calculations have been carried out to study the effect of packing interactions on the conformations of nylon 1, n. For this purpose the conformational preferences in vacuo and with the crystal environment of the model molecule bis(acetoamido)methane (BAAM) were investigated. Only two minima with the same energy were found when in-vacuo calculations were performed. These were validated by comparison with ab initio SCF-MO calculations using both 3–21 G and 6–31G ∗ basis sets. To mimic the crystal environment, hydrogen bonding sites of the model molecule were saturated with two molecules of ammonia and two of formaldehyde. The results show a change of approximately 15° in the dihedral angles when the molecule is treated including environmental effects relative to when it is considered isolated, which are in remarkable agreement with the experimental results available. In summary, the BAAM molecule tends to adopt helical conformations with a single direction of hydrogen bonds. These preferences are the consequence of intra-chain interactions, although the conformations are also modified by inter-chain interactions.