Vibrational spectra as a probe of local and collective flexibility in chain molecules: recent aspects

Abstract

The experimental data (vibrational frequencies, vibrational intensities and band shapes) which can be used for probing local and/or collective conformational defects are presented and applied to the study of the dynamics of polymethylene systems in the solid state. The case of n-alkanes which crystallize in the orthorhombic lattice is considered in more detail. It is shown that upon heating towards the so-called α-phase ‘surface melting’ takes place as the result of longitudinal displacements of the molecular chains. It is shown that longitudinal displacements generate diffusion even in the solid. The molecular origin of such diffusion is analysed in terms of band shapes and frequency shifts. It is shown that large-amplitude librotorsional motions do not seem to be consistent with the observed spectra. The concept of a conformational soliton or twiston as a gentle twist which propagates along the molecule and carries matter is proposed, and its spectroscopic implications are discussed.