Royal Society of Chemistry Dielectric and Related Molecular Processes: Volume 3 (Specialist Periodical Reports - Dielectric and Related Molecular Processes)

Excerpt. © Reprinted by permission. All rights reserved. Dielectric and Related Molecular Processes Volume 3Reviews of Recent Developments up to December 1976By Mansel DaviesThe Royal Society of ChemistryCopyright © 1977 The Chemical SocietyAll rights reserved.ISBN: 978-0-85186-525-6ContentsChapter 1 Correlation and Memory Function Analysis of Molecular Motion in Fluids By M. W. Evans, 1, Chapter 2 Quasielastic Neutron Scattering Studies of Molecular Reorientations By J. A. Janik, 45, Chapter 3 Studies of Molecular Characteristics and Interactions using Hyperpolarizabilities as a Probe By B. F. Levine, 73, Chapter 4 Some Dielectric Studies of Molecular Association By E. Jakusek and L. Sobczyk, 108, Chapter 5 Dielectric and Related Properties of Polymers in the Solid State By Y. Wada, 143, Chapter 6 Dielectric Studies of Adsorbed Molecules By G. Jones, 176, Chapter 7 The Dielectric Behaviour of Non-crystalline Solids By T. J. Lewis, 186, Chapter 8 Some Dielectric and Electronic Properties of Biomacro-molecules By R. Pethig, 219, CHAPTER 1Correlation and Memory Function Analysis of Molecular Motion in FluidsBY M. W. EVANS1 IntroductionThe foundations and framework of this review have been established in two previous articles of this series, the first by Wyllie, who discussed the broad theoretical concepts necessary for the interpretation of far i.r. and lower frequency spectroscopic data; and the other by Brot, whose main contribution lay in annotating and evaluating the recent attempts at solving the problem of the dynamic internal field. In both these articles several models of fluid dynamics were discussed on the molecular scale, and related to bulk properties such as the dielectric loss and dispersion with classical fluctuation-dissipation theory. This third report on advances within six years aims to be selective and complementary rather than comprehensive. The natural complement is found in fields of work covering the dynamical processes in fluids that give rise to absorption in the far i.r. region of the electromagnetic spectrum and which represent different kinds of experimentally observable phenomena.The past twenty years have seen the emergence of Kubo's generalizations of classical fluctuation-dissipation theory, enabling a coherent analysis of superficially different bulk transport properties to be made in terms of the motions of vectors defined in the molecular frame. Zwanzig, Gordon, and Berne have made significant advances in isolating some fundamental statistical theorems pertaining to the fluid state, as well as in emphasizing the role of the most important concept of the correlation function. This is fonnally the ratio of the covariance of stationary (time independent) random process to its variance. Here 'correlation' takes on the narrow statistical sense of being the relationship between two or more measurable random events occurring in a temporal sequence. The frequencies of such events brings us into the domain of spectroscopy, since a spectral function, being a distribution of probabilities of events occurring with given frequencies, is itself statistical in nature. Fourier1 2 provided the link between temporal and frequency domains in his integral theorem, so that the correlation and spectra) functions, C(t) and [??](iω) respectively, are Fourier transform pairs.Molecular fluctuations in phase space may be correlated statistically and related to many different kinds9 of absorptions and dispersions, the former being the real and the latter the imaginary parts of the Fourier transform of C(t). The correlated random variable is conveniently a vector, or tensor trace, whose magnitude and direction may be evolving in time. To discuss the absorption of energy by molecules from an electromagnetic field (a stream of photons), it is sufficient8 to define such a vector u in the molecular frame (x,y,z). The direction may be that of the permanent dipole (µ)if this exists, so that u may be normalized to unity (u