Huckel Theory 2009

Huckel Theory 2009 - 6 Hckel Theory This theory was...

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6.1 6 Hückel Theory This theory was originally introduced to permit qualitative study of the π -electron systems in planar, conjugated hydrocarbon molecules (i.e. in "flat" hydrocarbon molecules which possess a mirror plane of symmetry containing all the carbon atoms, and in which the atoms of the carbon skeleton are linked by alternating double and single carbon-carbon bonds when the bonding is represented in a localised fashion). It is thus most appropriate for molecules such as benzene or butadiene, but the approach and concepts have wider applicability. Basic Assumptions 1. the atomic orbitals contributing to the π -bonding in a planar molecule (e.g. the so-called p π orbitals in a molecule such as benzene) are antisymmetric with respect to reflection in the molecular plane; they are therefore of a different symmetry to the atomic orbitals contributing to the σ -bonding and may be treated independently. 2. the Coulomb integrals for all the carbon atoms are assumed to be identical. i.e. small differences in α -values due to the different chemical environment of C atoms in a molecule such as are neglected. 3. all resonance integrals between directly-bonded atoms are assumed to be the same; whilst those between atoms that are not directly bonded are neglected. i.e. τ φ d H j i . ˆ = β : if atoms i and j are directly σ -bonded. = 0 : if atoms i and j are non-bonded. 4. all overlap integrals representing the overlap of atomic orbitals centred on different atoms are neglected. i.e. d j i . = 0 : if i j (note - if i = j then d j i . = 1 since it is assumed that the atomic orbitals are normalized) Mirror plane (of molecule) p π orbital s or p σ orbital
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6.2 A Closer Look at the Secular Determinant The basic form of the secular determinant for the bonding arising from the overlap of two orbitals (from 4.9) is reproduced below. 2 12 12 1 E E α β For three overlapping orbitals the approach outlined in Chapter 4 leads to a secular determinant of the form: E E E 3 23 13 23 2 12 13 12 1 From a comparison of the two secular determinants given above, it is becoming clear that all such secular determinants have a characteristic structure: 1. each row and column may be associated with one of the atomic orbitals; thus the first row and first column contain information about the nature of orbital 1 and its interactions with the other orbitals, the second row and second column contain information about the nature of orbital 2 and its interactions with the other orbitals. 2. The diagonal set of elements (comprised of those elements where row 1 intersects column 1, row 2 intersects column 2, …. . and so on) include the values of the relevant Coulomb integrals ( α 1 , α 2 etc.). 3. The off-diagonal elements (comprised of those elements having different row numbers and column numbers) are equal to the relevant resonance integrals (e.g.
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Huckel Theory 2009 - 6 Hckel Theory This theory was...

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