DISTRIBUTION OF INFRARED INTENSITY IN THE SPECTRA OF CONFORMATIONALLY DISORDERED CHAIN MOLECULE ASSEMBLIES

The distribution of intensity of the infrared spectrum of a simple model system of flexible chain molecules has been examined as a function of conformational disorder in order to establish a basis for the characterization of disorder by vibrational spectroscopy. A finding of practical importance is that the band intensity associated with a delocalized mode is nonlinearly related to the concentration of disorder measured in terms of the conformational state of the bonds and that the degree of the nonlinearity increases with the chain length. Therefore, to determine quantitatively the degree of conformational disorder for an assembly of flexible chain molecules, it is necessary to employ bands associated with modes that are highly localized. A second aspect of this study concerns the well-known observation that, for a given chain molecule system, the degree to which the intensity distribution is affected by a change in the average conformation of the system is dependent on the type of vibrational mode involved. There are two principal factors that determine the sensitivity of an infrared band to conformation. One is the relation, within the repeating unit of the chain, between the direction of the local dipole moment derivative associated with the mode and the direction of the skeletal bond whose internal rotational states determine the conformation of the chain. The second factor concerns how the normal coordinate of the mode is dependent on the conformation of the chain. These two factors are interrelated, and, in fact, depending on the direction of the dipole moment derivative, their effects may cancel. This leads to different modes having different sensitivites to conformational change. © 1990, American Chemical Society. All rights reserved.