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)im inomethylene are investigated ab initio at the 6-31G SCF and MP2 leve ls and compared with those produced by molecular mechanics (MM), using Allinger's MM3 force field. The torsional parameters used in MM3 calc ulations 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 th e MM level, bulkier substituents have been considered as well (R=isopr opyl, 1-cyclohexylethyl). Both at the ab initio and classical level, c onformations close to a 4(1) helix are found to be stable, but they ne ver are the absolute minimum. The minimum energy geometry shows a regu larly alternating disposition of the substituents on the iminic double bonds (syndio configuration): its backbone conformation (dihedral ang les phi) shows dimeric sections which are alternatively trans-planar ( E,E sections) and close to +/-90 degrees (Z,Z sections). The energy ga p between the syndio geometry and the 41 helix amounts to similar to 3 0 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 signi ficantly passing from methyl to isopropyl and 1-cyclohexylethyl. Also, conformations having an inversion of the helical sense along the chai n (wormlike chains) are found to be stable according to both calculati on methods, and their energies are comparable to those of helical conf ormers. CD calculations were carried out employing an independent syst ems approach. The calculated intensities of the CD bands of a model 4( 1) helix are much higher than what experimentally found, while those c alculated for the syndio structure (considered as composed of a series of diiminic units) are comparable to experiment. A negative CD band i s predicted to be associated with the n-pi transition of the iminic c hromophore in a 4(1) P-helix, as previously found in the literature. T he preference for nonhelical conformers could account for various feat ures observed in the UV, CD, and NMR spectra of some poly(iminomethyle nes), as reported in literature (absorption at long wavelengths, low i ntensity of CD bands, chemical shift dispersion of backbone carbons re sonances).