Intermolecular interactions and energetics

On the intermolecular level, in molecular crystals one is dealing potentially with a variety of interactions, most of them quite weak compared to those involved in chemical bonding. A crystal structure corresponds to a free energy minimum that is not necessarily the global minimum. The existence of a number of energetically closely spaced local minima is the thermodynamic rationale for the possible appearance of 

For each entry the first number is the distortion from the nominal equihbrium value, and the second 

At any rate, a minimum does represent a situation in which attractions and repulsions are balanced (Brehmer et al. 2000). The nomenclature of these intermolecular interactions is quite variegated and the terms are not always clearly defined or distinguished from one another. Some in common usage include van der Waals interactions, London forces, dipole-dipole interactions (and higher terms), dispersion forces, steric repulsion, hydrogen bonds, charge-transfer interactions (also called donor-acceptor interactions), electrostatic interactions, exchange repulsion forces, etc. 

Conformational polymorphism is the existence of different conformers of the same molecule in different polymorphic modifications, as represented in Fig. 5.1(a),(b). Additionally, Corradini has specified fhaf fhe conformers are nearly isoenergefic, and also includes as an example, racemic and optically active crysfals for the case of chiral molecules. While both the general definition of polymorphism and chemists general understanding of conformation are fraught with nuances and special complicating circumstances, we prefer the general definition of conformational polymorphism here, without Corradini s additional qualifications. Specific questionable or unusual cases can be dealt with as encountered. 

Corradini also defined conformational isomorphism as the existence of different conformers of a molecule in the same crystal structure, a situation which can arise when there is more than one molecule in the asymmetric unit (Z 1). In such a case, each crystallographic site in the unit cell is always occupied by the same conformer and the ratio of conformers is defined by whole numbers as in Fig. 5.1(c). 

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