title: Intermolecular modulation of IR intensities in the solid state. The role of weak interactions in polyethylene crystal: A computational DFT study.
abstract: Density functional theory calculations with periodic boundary conditions are exploited to study the infrared spectrum of crystalline polyethylene. Spectral changes lead by the intermolecular packing in the orthorhombic three-dimensional crystal are discussed by means of a careful comparison with calculations carried out for an isolated polymer chain in the all-trans conformation, described as an ideal one-dimensional crystal. The results are analyzed in the framework of the "oligomer approach" through the modelling of the IR spectrum of n-alkanes of different lengths. The study demonstrates that a relevant absorption intensity modulation of CH2deformation transitions takes place in the solid state. This finding suggests a new interpretation for the experimental evidences collected in the past by means of IR intensity measurement during thermal treatment. Moreover, the comparison between calculations for 3-D crystal and for the isolated polyethylene chain (1-D crystal) allows to put in evidence the effect of the local electric field on the computed infrared intensities. This observation provides guidelines for the comparison between infrared absorption intensities predicted for an isolated unit and for a molecule belonging to a crystal, through the introduction of suitable correction factors based on the refraction index of the material and depending on the dimensionality of such units (0D-molecule; 1D-polymer; 2D-slab).
authors: LAMBE CNRS UMR8587, Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement, Université d'Evry val d'Essonne, Boulevard F. Mitterrand, Bat Maupertuis, 91025 Evry, France.
authors: Daria
authors: D
authors: Galimberti
authors: Dipartimento di Chimica, Materiali e Ingegneria Chimica Giulio Natta, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy.
authors: Alberto
authors: A
authors: Milani
authors: Dipartimento di Chimica, Università di Torino and NIS (Nanostructured Interfaces and Surfaces) Centre, Via P. Giuria 5, 10125 Torino, Italy.
authors: Lorenzo
authors: L
authors: Maschio
authors: Dipartimento di Chimica, Materiali e Ingegneria Chimica Giulio Natta, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy.
authors: Chiara
authors: C
authors: Castiglioni