Comparison with linear 1,3,5-hexatriene

In order to put the structure of benzene into better perspective, we give a similar calculation of 1,3,5-hexatriene for comparison. Table 15.5 shows the energies for a set of calculations parallel to those in Table 15.2 for benzene. The most obvious difference is the smaller total spread in the energies, about 3. 3 eV rather than the 5.5 eV for benzene, and the SCVB energy is closer to the full n than in benzene. Standard arguments say that there is only a rather small amormt of resonance 

There are three inequivalent SCVB orbitals for hexatriene, and these are given in Figs. 15.2, 15.3, and 15.4. The first of these shows a principal peak at the first orbital and a small satellite at the adjacent position. The second is more interesting with the principal peak at the second C atom, but showing a larger satellite at position 1 than at position 3. This is consistent with having essentially a double bond between atoms 1 and 2 or 5 and 6, with single bonds between 2 and 3 and 4 and 5. 

Fignre 15.4. Altitnde plot of the third SCVB orbital for the n system of 1,3,5-hexatriene. There are two symmetrically eqnivalent versions of this at each end of the molecule. The amplitude is given in a plane 0.5 A in the positive z-direction from the plane of the nuclei. 

The third orbital has a similar interpretation with a larger satellite at position 4 than at position 2. 

The overall conclusion is that there is considerably less resonance in hexatriene than in benzene, and the bond lengths and types alternate along the chain unlike the equivalence in benzene. 

---