Molecular crystals geometrical model

Since the unit in molecular crystals is a whole single molecule, it appears appropriate to consider molecule-molecule interactions. This is done in the geometrical model, with the assumption of totally incompressible molecules (see Figure 2). But, since the forces between molecules cannot be considered central, it... 

The geometrical model allowed Kitaigorodskii to make predictions of the structure of organic crystals in numerous cases, knowing only the cell parameters and, obviously, the size of the molecule itself [80], In the age of fully automated, computerized diffractometers, this may not seem to be so important, but it indeed is for our understanding the packing principles in molecular crystals. 

The development of experimental techniques and the appearance of more sophisticated models have pushed the frontiers of molecular crystal chemistry much beyond the original geometrical model. Some of the limitations of this model will be mentioned later. However, its simplicity and the facility of visualization ensure this model a lasting place in the history of molecular crystallography. It has also exceptional didactic value. 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

The basic theories of physics - classical mechanics and electromagnetism, relativity theory, quantum mechanics, statistical mechanics, quantum electrodynamics - support the theoretical apparatus which is used in molecular sciences. Quantum mechanics plays a particular role in theoretical chemistry, providing the basis for the valence theories which allow to interpret the structure of molecules and for the spectroscopic models employed in the determination of structural information from spectral patterns. Indeed, Quantum Chemistry often appears synonymous with Theoretical Chemistry it will, therefore, constitute a major part of this book series. However, the scope of the series will also include other areas of theoretical chemistry, such as mathematical chemistry (which involves the use of algebra and topology in the analysis of molecular structures and reactions) molecular mechanics, molecular dynamics and chemical thermodynamics, which play an important role in rationalizing the geometric and electronic structures of molecular assemblies and polymers, clusters and crystals surface, interface, solvent and solid-state effects excited-state dynamics, reactive collisions, and chemical reactions. 

The same geometric and mathematical principles lie at the root of all types of diffraction experiments, whether the samples are powders, solutions, fibers, or crystals, and whether the experiments involve electromagnetic radiation (X rays, visible light) or subatomic particles (electrons, neutrons). My aim in this chapter was to show the common ground shared by all of these probes of molecular structure. Note in particular how the methods complement each other and can be used in conjunction with each other to produce more inclusive models of macromolecules. For example, phases from X-ray work can serve as starting phase estimates for neutron work, and the resulting accurate... 

---