Steric hindrances and conformation of molecules

Summary

• 1. The factors determining the conformation of hexahalobenzene molecules in the vapor state and in crystale are considered. A survey of the literature reveals the contradictory nature of experimental data and theoretical calculations.

• 2. The crystal structure of hexachlorobenzene has been refined. To an accuracy of ±0.02 A the molecule is planar in crystals: C−C=1.40±0.02; C−Cl=1.71±0.02 A.

• 3. A new method is proposed for estimation of equilibrium intermolecular distances in crystals, based on the use of curves for the energy of interaction of atoms not linked by valences and on minimalization of the effective lattice energy.

• 4. The equilibrium distances are found to depend distinctly on the direction relative to the plane of the molecule. The equilibrium Cl...Cl distance is 3.64 A in the molecular plane and 3.94 in the normal to it.

• 5. The effective lattice energies have been calculated for the real hexachlorobenzene structure (−17.0 kcal/mole) and for a hypothetical structure with nonplanar molecules (−13.9 kcal/mole).

• 6. The values of the force constant for removal of the C−Cl bond from the plane of the benzene nucleus with which the hexachlorobenzene molecule must be planar in crystals and in vapor, were estimated.

• 7. The results suggest that molecules of all hexahalobenzenes must be planar not only incrystals but also in the vapor state.