Molecular Structures of Gaseous Hexachloro- and 1,2,4,5-Tetrachlorobenzene as Determined by Electron Diffraction

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Abstract
The molecular structures of hexachloro- and 1,2,4,5-tetrachlorobenzene have been determined by least-squares refinements based on gaseous electron-diffraction intensity data. The equilibrium configuration of hexachlorobenzene is found to be planar with D6h symmetry. The observed shrinkage for this molecule is compared with the shrinkage calculated from spectroscopic data for hexafluorobenzene. 1,2,4,5-Tetrachlorobenzene is planar with D2h symmetry; the repulsion between the neighboring chlorine atoms leads to an angle of 62.8°±0.4° between the ortho carbon—chlorine bonds. The most important molecular parameters and estimated standard deviations are listed. Hartley's modification to the least-squares method was applied in refinements of the structure of 1,2,4,5-tetrachlorobenzene. It was found that the least-squares method modified in this fashion has a wider circle of convergence than the regular Gauss—Newton method, and that the regular method diverged even when the final parameters from the modified method were applied as a starting vector for the regular method.