The Influence of Packing at a Molecular Level: Conformation, Geometry, Spectroscopy (Ir, Raman)

Abstract

Ab initio calculations are widely applied to optimize the geometry of isolated molecules. To include intermolecular interactions is a necessary prerequisite to optimize a molecular geometry in its crystal environment. We do this using a ‘crystal field’ approach. For full details we refer to Popelier et al (J. Am. Chem. Soc. 1989, 111, 5658-5660). In our calculations we apply a 4-21G basis set. This choice is dictated by a purely practical argument; it is the only basis set for which a list of empirical corrections δ exists, which allows us to extrapolate from the calculated re geometry to the r_e geometry via r_g = r_e(4–21G)+δ(4–21G). This allows a direct comparison between the theoretical result and the experimental evidence. Analysing a series of carbonylhydrazide derivatives we noted that for 12 products the position of the NH₂ group in the solid state is practically equal to the position typical for the conformation of the isolated minimum energy structure. However, in oxalyldihydrazide (Quaeyhaegens et al, J. Mol Struct., accepted for publication) and cyanoacetohydrazide (Nanni et al.,J. Mol Struct. 1986, 147, 369-380), the NH₂ group is rotated over an additional 120°. This solid state configuration is stabilised by the formation of intermolecular H-bonds. It is gratifying to note that these discrepancies between solid state and free molecule are reproduced by our ‘crystal field’ approach, in which we always started our calculations with the minimum energy conformer derived for the isolated form.