Étude par CNDO/2 des mouvements de rotation des unités moléculaires constituant le squelette des polymères du type: PEK, PEEK et PEEKK

Abstract

RCsumC : Cette ttude a pour objectif une analyse conformationnelle d'unitts moltculaires telles que le diphtnyl tther (DPE) et la diphtnyl &tone (DPC) constitutives du squelette de polymbres tels que PEK, PEEK et le PEEKK, par des calculs semi empiriques en CND012. Deux types de modtlisations ont t t t rtalists : une premibre strie de calculs a t t t effectute sans prendre en compte les interactions tlectroniques et de zones d'imptnttrabilitt de van der Waals et une deuxibme en considtrant ces diverses interactions. La conformation d'tnergie minimale absolue pour le diphtnyl tther correspond j. la position dtfinie par les angles de torsion (01,02) = (30°,1500) avec et sans interaction, et j. la position (01,02) = (90°,900) pour la diphtnyl cttone sans prendre en compte les interactions. Dans les regions sous vitreuses et i contrainte sttrique tgale au sein d'une chaine de PEK, PEEK ou PEEKK, on peut stipuler que I'unitC moltculaire DPE sera sensiblement plus facilement mise en mouvement que I'unitt DPC.Mots cl6s : analyse conformationnelle, diphtnyl tther, diphtnyl cCtone, CNDOI2, barribres de rotation. Abstract: Diphenyl ether (DPE) and diphenyl ketone (DPK) are the molecular constituents of polymers such as PEK, poly(ether-ketone); PEEK, poly(ether-ether-ketone), and PEEKK, poly(ether-ether-ketone-ketone).A conforrnational analysis of DPE and DPK is examined by means of semiempirical calculations using CND012 (Complete Neglect of Differential Overlap). The modelization is first run disregarding electronic interactions and steric effects, then introducing them in the calculations. We found that the absolute minimum energy conformer has torsional angles (0,,€12) = (30°,1500) for the DPE (with and without interactions) and (0,,0,) = (90°,900) for the DPK (without interactions). Considering the polymer chains of PEK, PEEK, and PEEKK, the motion of DPE is expected to be easier than that of DPK in regions of local motions and for equivalent steric effects.

Importance of Substituent Intramolecular Charge-Transfer Effect on the Molecular Conformation of Diphenyl Ethers

“…This would undoubtedly arise from the steric hindrance between the ortho hydrogen atoms . Present calculations suggest that diphenyl ethers easily internally rotate around the C−O bonds, keeping the dihedral angle of the two phenyl rings at about 90°, and provide good agreement with recent experiments and calculations. ,−

2 Nuclear repulsion and electronic energies of diphenyl ether calculated by the HF/6-31G(d) method pertinent to change in the θ 1 and θ 2 angles from 0° to 90° and from 90° to 0°, respectively.

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“…We have demonstrated that the conformation of diphenyl ether is nonrigid with a dihedral angle of the two phenyl rings of about 90° at room temperature . Recent proposal of the nonrigid form is also provided by Verot et al with the aid of semiempirical MO calculations . These conclusions are synonymous, and the nonrigid form of diphenyl ether has been well documented at present.…”

“…Minimum Energy Conformations of Diphenyl Ether and the 4-Substituted Derivatives. Although the minimum energy conformation of diphenyl ether was discussed mainly within a framework of HF calculations using the ab initio method with minimal basis sets and semiempirical methods at a single point on the potential energy surface up to the present, ,− important concepts have been proposed in terms of the conformation, such as the so-called one-ring flip mechanism 4 and the disrotatory model 3,5 via the skew form for the internal motion. These results were derived from discussion with regard to a ravine on the conformational energy map of diphenyl ether, as well as from spectroscopic experiments. ,− We have easily found that the change in the θ 1 and θ 2 angles under the condition that θ 1 + θ 2 ≈ 90° gives rise to a ravine on the present 6-31G(d) energy map showing the relative total energies calculated at 15° intervals of θ 1 and θ 2 .…”

“…The conformation of two aromatic rings bridged by an oxygen atom, such as diphenyl ether, has been a subject of continuing interest in regard to critical oxygen aromatic properties − and the structure−activity relationships of isodityrosines and thyroid hormones. − It is well known that the diaryl ether bond plays an important role in keeping the three-dimensional structure of natural products biologically active. The conformation is traditionally classified into four groups, that is, planar, skew, butterfly, and twist, as shown in Chart .…”

“…The nonrigid conformation of diphenyl ether is brought about by the balance between the electronic stabilization (conjugation) and the steric crowding. ,,− The ortho steric hindrance therefore easily governs the configuration of the diaryl ether bond. ,,, Montaudo et al have prospectively wrestled with the NMR and dipole moment studies on the relation between the number of ortho substituents and control of internal rotation in diphenyl ether, with the results showing that the ortho substituents allow it to be rigid in the skew and butterfly forms. In a series of our conformational investigations on oxygen-bridged aromatic rings, the disubstituted product by the bulky Me group at the ortho positions of a benzene ring, that is, 1,3-dimethyl-2-phenoxybenzene, predominantly occupies a symmetric skew form due to steric crowding .…”