van Remoortere, et al.148 mm, 20X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals

The crystal and molecular structure of 2,3,7,8-tetrachlorodibenzo-p-dioxin has been determined by a three-dimensional single-crystal X-ray diffraction study. The lattice parameters of the triclinic unit cell: a=3.783+0.003, b=9.975+0.009, c=15-639+0.015 /~, ~=94.14+0.02, fl=95.20+0.04, 7=92.77+ 0.04 °, give a calculated density of 1.827 g.cm -a for ClzH402C14 and Z=2. The intensities of 2666 independent reflections were measured using the 0-20 scan mode of a Picker four-circle diffractometer and Mo K~ radiation. The space group P1 was assigned on a statistical basis and subsequently confirmed by the excellent behavior of metric and thermal parameters upon refinement. The structure was solved from the Patterson function, and the coordinates of all atoms in the molecule, the isotropic temperature factors of the hydrogens, and the anisotropic temperature factors for the other atoms were refined by full-matrix least squares. The final R value was 3.6% for the 2381 reflections above background. The structure consists of two independent molecules situated on inversion centers at (0,0,0) and ~7,tl ~-,1 7J1~ and stacked along [100]. The molecules are essentially planar; no non-hydrogen atom deviates more than 0.018 A from its least-squares molecular plane. Thus the approximate molecular symmetry is mmm (D2h). The four unique C-CI distances range from 1.

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Model SCF MO calculations for small molecules have been employed to establish methods and parameters for obtaining Hamiltonian matrix elements of larger related molecules, with all electrons included. Overlap and kinetic energy elements are calculated exactly, while the SCF models yield values of diagonal Hamiltonian elements (as), and parameters which are used in a modified Mulliken approximation to generate the off-diagonal potential-energy parts of the Hamiltonian matrix. A general discussion of the Mulliken approximation is given. SCF a's are found not to vary linearly with net atomic charge, nor are they adequately approximated by free-atom parameters. It is shown that the assumption that Hamiltonian elements are proportional to overlap integrals neglects elements several electron volts in magnitude. A new formula is presented for calculating these zero-overlap elements.Molecular orbitals (MO), expressed as linear combinations of atomic orbitals (LCAO), are the basis of a large body of theoretical molecular structural discussion, including predictions, correlations, and interpretations. The LCAO MO self-consistent-field (SCF) method3 is becoming increasingly more useful as multi-centered integrals become manageable upon large computers, but the present limits of exact applications of this method are molecules of about the size of ethane and diborane. It is therefore no wonder that chemists have turned to more approximate methods4 for complex molecules. Many of these latter methods have failed to consider the explicit form of the oneelectron Hamiltonian, and nearly all of them have required some evaluation of parameters from experiment. The extended Hiickel theory,5 which goes (1) Paper I in a series of four papers.(2) National Science Foundation Predoctoral Fellow, 1964-1966 (3) Current applications of Hartree-Fock theory use the LCAO formulation given by C. C. J. Roothaan, Rev. Mod. Phys., 23, 69 (1951).(4) (a) For a recent review of these methods, see "The Quantum

Oxocarbonium ions are postulated intermediates in Friedel-Crafts acylations. Crystals of methyloxocarbonium hexafluoroantimonate, [CH3CO]+[SbF6]-, are monoclinic, space group P2i/m, with unit cell dimensions a = 5.758, b = 7.783, c = 7.879 A, and ß = 101 °3', Z = 2. Three-dimensional X-ray data were collected by the precession method (Mo Ka radiation) and estimated visually. The structure was refined by full-matrix least squares to a reliability index R¡ = 5.6% for 799 observed reflections. The crystal structure provides direct proof of an ionic structure in the solid state, and confirms the hypothesis that the oxocarbonium ion is linear. Some structural evidence that the positive charge in the cation is largely localized on the central carbon is discussed, and a molecular orbital calculation supporting this hypothesis is described.