Benzene crystal packing, calculation

Highlights in the chemistry of cyclopentadienyl compounds have been reviewed.65 Trends in the metallation energies of the gas-phase cyclopentadienyl and methyl compounds of the alkali metals have been studied by ab initio pseudopotential calculations. Whereas there is a smooth increase in polarity of M-(C5H5) bonds from Li to Cs, lithium appears to be less electronegative than sodium in methyl derivatives. The difference between C5H5 and CH3 derivatives is attributed to differences in covalent contributions to the M-C bonds. In solution or in the solid state these trends may be masked by the effects of solvation or crystal packing.66 The interaction between the alkali metal ions Li+-K+ and benzene has also been discussed.67... 

Intetmolecular potentials between aromatic molecules, which involve n electron interactions, are different than those in aliphatic compounds. Thus, when calculating heats of sublimation and crystal packing values, special parameters for aromatics are needed. Nonbonded parameters, which include van der Waals s and R) and a Csp -H bond moment, should be included in a FF to reproduce crystal packing. In addition, accounting for electronic energy in a n system is needed to accurately calculate the heats of sublimation. For example, in the benzene dimer, molecules prefer to interact edge-to-face rather than parallel to each other or n stacked. With the appropriate nonbonded parameters in place and treatment of the electronic energy (see Section 2.2), MM3 can fit benzene and biphenyl crystal data. 

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