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 NCCN (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). The energy gap between the syndio geometry and the 41 helix amounts to ∼30 kcal/mol in the methyl oligomer at the ab initio level vs ∼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 41 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−π* transition of the iminic chromophore in a 41 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).

Polymorphism and electronic structure of polyimine and its potential significance for prebiotic chemistry on Titan

“…It has been established that the most common conformation of these polymers are fourfold sized helices (14)(15)(16). This contrasts with our results, which suggests that a wave conformation and the possibility of sheet structures become competitive when R=H.…”

“…These polymers may be present on Titan and escaped detection thanks to observational conditions that make it difficult to identify them spectroscopically (5). Not much is known about them; the only theoretical studies, conducted decades ago, were of isolated polymers and dimers, and used methodologies whose accuracy is now greatly surpassed (13,14). This contrasts with substituted polyimines/ poly-isocyanides, whose synthesis and conformation have been extensively studied experimentally, in part because their helical structures gives them practical and potential importance in electronics, biosensing, and tailored catalysis (14)(15)(16).…”

Polyisocyanides Derived from Tripeptides of Alanine

“…This suggests the formation of a more conjugated polymer backbone when the stabilizing influence of the hydrogen-bonding network is removed. [16] The differences in stability under acidic conditions can be tentatively explained by the fact that in LLL-PIAAA, the peptidic side arms are firmly connected with each other at two points, which makes the polymer, going from the polymer backbone to the periphery, a relatively ordered entity with well-defined grooves at its surface (Figure 5a). These grooves enable acid to freely and easily attack the hydrogen-bonding network.…”

Helical Superstructures from Charged Poly(styrene)-Poly(isocyanodipeptide) Block Copolymers

“…Polymers of isocyanides can adopt helical structures depending on their side chains, in particular when these are bulky. The helix sense of the poly(isocyanide) part can be controlled by the use of an optically active isocyanide monomer or a chiral Ni(II) catalyst (7)(8)(9)(10). The charged block copolymers we discuss here have a helical poly(isocyanide) headgroup of one particular type, either left handed (M) or right handed (P) (Fig.…”