Theoretical Investigations on the Structure of Poly(iminomethylenes) with Aliphatic Side Chains. Conformational Studies and Comparison with Experimental Spectroscopic Data

Abstract

The ground state energies and structures of octamers of poly(methyl)iminomethylene are investigated ab initio at the 6-31G* SCF and MP2 levels and compared with those produced by molecular mechanics (MM), using Allinger's MM3 force field. The torsional parameters used in MM3 calculations for the dihedral angle NCCN (φ) have been obtained from ab initio calculations of model diimines ((E,E), (E,Z), and (Z,Z) N,N‘-diisopropylethanediimine], in the flexible rotor approximation. At the MM level, bulkier substituents have been considered as well (R = isopropyl, 1-cyclohexylethyl). Both at the ab initio and classical level, conformations close to a 41 helix are found to be stable, but they never are the absolute minimum. The minimum energy geometry shows a regularly alternating disposition of the substituents on the iminic double bonds (syndio configuration): its backbone conformation (dihedral angles φ) shows dimeric sections which are alternatively trans-planar (E,E sections) and close to ±90° (Z,Z sections)...