Vibrational spectra and the structure of long-chain aliphatic compounds

Abstract

Polymorphism and the structure of mesomorphous states of long-chain aliphatic compounds (LAC) have been studied by vibrational spectroscopy (theory, experiment) using calorimetry, polarizing microscopy (PM), and X-ray diffraction (XRD) analysis as auxiliary methods. LAC are represented by homologous series of alkyl- and alkoxybenzoic acids, alkylcyclohexanecarboxylic acids, their completely or partially fluoroalkyl-substituted analogs, and 4-cyano-4′-(n-alkoxy)biphenyls. The studies were performed at 77–500 K. IR absorption and Raman spectra were measured at 30–4000 cm−1. The compounds are characterized by conformational polymorphism. The molecules and their H-complexes undergo structural rearrangements during phase and conformational transitions. In the course of the rearrangements, alkyl and fluoroalkyl radicals (AR and FAR) are twisted, and carboxyl groups are disordered. The rearrangements dictate the structure of mesophases, which are mostly formed of H-complexes consisting of dimers, monomers, and open associates.