Theoretical investigations on the structure of poly(iminomethylenes) with aliphatic side chains. Conformational studies and comparison with experimental spectroscopic data

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 N=C-C=N (phi) 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 4(1) 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 phi) shows dimeric sections which are alternatively trans-planar (E,E sections) and close to +/-90 degrees (Z,Z sections). The energy gap between the syndio geometry and the 41 helix amounts to similar to 30 kcal/mol in the methyl oligomer at the ab initio level vs similar to 11 kcal/mol at the MM3 level. This latter value does not change significantly passing from methyl to isopropyl and 1-cyclohexylethyl. Also, conformations having an inversion of the helical sense along the chain (wormlike chains) are found to be stable according to both calculation methods, and their energies are comparable to those of helical conformers. CD calculations were carried out employing an independent systems approach. The calculated intensities of the CD bands of a model 4(1) helix are much higher than what experimentally found, while those calculated for the syndio structure (considered as composed of a series of diiminic units) are comparable to experiment. A negative CD band is predicted to be associated with the n-pi* transition of the iminic chromophore in a 4(1) P-helix, as previously found in the literature. The preference for nonhelical conformers could account for various features observed in the UV, CD, and NMR spectra of some poly(iminomethylenes), as reported in literature (absorption at long wavelengths, low intensity of CD bands, chemical shift dispersion of backbone carbons resonances).