Per Jt electron

Figure 9. Resonance energies per jt electron in units of electronvolts for the conjugated monocyclic hydrocarbons using the Chung—Dewar reference structure and the Pariser—Parr—Pople computational method.

Electronically intermediate between the polyacetylene example with one Jt electron per center and the planar analog of the sulfur chain with two such electrons per center is the (SN) , polymer with three tt electrons per SN atom pair [10]. Let us approach this problem from the Hiickel perspective. The band structure of the trans isomer 13.32 is shown in Figure 13.12 where we have chosen a unit cell containing... 

This leaves one jt electron per carbon atom (as in ethene or benzene). Imagine an infinite jt orbital covering the whole sheet, formed from the 2p. atomic orbitals, one on each carbon (perpendicular to the sheets). But since there are three bonds to carbon in the graphite sheet, the jt bond order in each is about one-third. The total bond order is thus about 1, intermediate between a single and a double bond. 

Figure 6. Distance dependence of the interactions between two equivalent jt-type orbitals. The magnitude of the interaction, A (jt), at any given distance is approximately equal to twice the electronic coupling, Vci, for hole transfer between the two orbitals, (a) Through-space (TS) coupling. (b) Through-bond (TB) coupling. Using HF/ 3-21G theory, j8 3.0 A"1, for TS interactions, and 0.8 per bond for TB interactions in the diene series 1(n).

The first consequence of JT interaction one can think of is that it introduces a dissymmetry with respect to hopping. In a lattice of molecules with an average number of n electrons per C60, it is obvious from Fig. 3 that the gain of energy will be larger if n is even rather than odd. Therefore, if one electron hops to the next ball leaving a and a C%, the new states will be stabilized by JTD... 

Figure 17 The electronic structure of the P450 Compound I intermediate from density function theory (DFT) calculations. The two unpaired electrons on the Fe(lV) center are in the 3d it orbitals while the other unpaired electron may be in the porphyrin ei2u Jt orbital (a) or sulfur per orbital (b). The solid arrows in the 2u or per orbital show the electron spin parallel to those in the 3d n orbital (quartet state) while the dotted arrows indicate the doublet state...

---