Coupling, intramolecular, classification

In Chapter 1 we briefly described an interface as a layer with uncompensated intermolecular forces. The thermodynamics of a liquid interfaces covered with a soluble or insoluble monolayer layer has been describe in detail by many other competent authors and we want to present only the thermodynamic basis needed for the subsequent chapters of this book. Let us consider the interface between water and air. The specific properties of the bulk water, e.g. the freezing point, boiling point, vapour pressure, viscosity, cluster formation and hydrophobic bonds, are well described by long and short-range intermolecular forces and strong and weak intramolecular forces. Israelachvili recently (1992) remarked in a short note on the usefulness of this classification, although it is not clear whether the same interaction is counted twice or two normally distinct interactions are strongly coupled. 

There are two extreme approaches to the description of vibroiiic states in crystals. The crystal states may be regarded as products of molecular vibronic states, or as linear combinations of vibronic site states. In the former case the electrons and nuclei in the unit cell couple independently, while for the latter description the intramolecular vibronic interactions are assumed to dominate. These two approaches are somewhat analogous to y and LS coupling of spin and orbital angular momentum in atoms and are called strong and weak coupling, respectively. The approach to the classification of states is different for each case. 

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