Etude de la conformation des acides n-nonanoïque, Z et E-9 octadécénoïques à 90 K par spectromètrie infrarouge et Raman. Partie 1. Etude par spectromètrie de vibration de la conformation de l'acide n-nonanoïque à 90 K

Abstract

The vibrational study of nonanoic acid at 90 K is proposed for explaining the molecular interactions of aliphatic chains involved in several biological systems constituted in part by the oleyl group. In fact, the spectra of the C 9 acid should have similarities with the spectrum of the portion between the carboxyl group and the double bond in Z- and E-9-octadecenoic acids. After slow crystallization, nonanoic acid appears as a polymorphic form very close to the C form of saturated even-numbered fatty acids. Splitting of the carboxylic vibrations is characteristic of a strong dimeric interaction and also indicates the presence of two dimers in the crystal. The polymethylene vibrational entity is caused by a “pseudo-paraffin” with a number of carbon atoms twice that of the monomeric species CH 3(CH 2) 7COOH. Moreover, an intense band at 109 cm −1 is attributed to the LAM1 of a C 18 segment. The carboxyl group of the dimer appears equivalent to a CC bond, the inversion centre being that of the crystal. The subcell structure of the alkyl chains is of the orthorhombic polyethylene type O⊥ with deformations particularly near the carboxyl group. We conclude that the acid crystallizes in the C′ form, the hydrogen bond having a length close to 2.60 Å.