EFFECT OF CRYSTAL LATTICE FORCES ON MOLECULAR GEOMETRY
Zofia Urbanczyk-Lipkowska
Institute of
Organic Chemistry, Polish Academy of Sciences, 01-224 Warsaw,
Poland
E-mail: OCRYST@alfa.ichf.edu.pl
It is generally acepted that weak interactions are driving force of chemical, physical and biological processess. Solid state studies on molecular interactions gave experimental base for their classification and evaluation of geometric requirements on molecular level.
Crystal lattice forces are believed to influence molecular conformation but not electronic structure of the molecule. Classic studies performed in the solid state by Domenicano and Vaciago [1] showed that substituent effects can be manifested in molecular geometry mostly by deformation of torsion and valence angles. In crystal lattice however, effect of substitution overlaps with multiple interactions generally described as "crystal lattice forces". Recent interest in identification and characterization of specificity of various types of intermolecular interactions allows for model studies on influence of collective, and particular types of interactions on conformation and electronic structure of the molecules.
Few compounds were found to form in the solid state numerous molecular complexes with vast variety of organic and inorganic molecules. Their superior complexing properties are due to the fact that they can form ions, act both as hydrogen bond donors and acceptors or interact with -electron systems. This creates an opportunity to investigate changes in geometry of single molecule induced entirely by crystal lattice forces or even detect geometric changes introduced by particular types of interactions. An interesting example is picric acid, used for over a century as an agent improving crystallinity of organic solids. Recent studies of its co-crystals revealed that correlations between geometric parameters found in numerous co-crystals of picric acid suggest substantial influence of non-bonding interactions on resonance effect of the nitro groups. Regarded as very weak CH ... O2N interactions change torsion angle of the NO2 group by at least 6o[2].
It will be shown on several other examples (TCNE, TCNQ,
quinone) that global effects of multiple non-bonding interactions
like ionic forces, hydrogen bonds, both of classic (strong) and
non-classic (weak) character, and iteractions between aromatic
systems, on molecular geometry can be comparable with effects of
substitution.