Approximate Molecular Orbital Theory: The Hückel/Tight-binding Model

Huckel molecular orbital (HMO) theory is a simple approximate parameterized molecular orbital (MO) theory that has been very successful in organic chemistry and other fields. This chapter introduces the approximations made in HMO theory, and then treats as examples ethane, hetratriene and other linear polyenes, and benzene and other cyclic polyenes. The pi binding energy of benzene is particularly large according to HMO theory, rationalizing the special ‘aromatic’ behaviour of benzene. But there is a lot more to benzene than that. It is shown that the pi bond framework of benzene would rather prefer a structure with alternating single and double C-C bonds, rather than the actually observed 6-fold symmetric structure where all C-C bonds are equivalent. The observed benzene structure is a result of a delicate balance between the tendencies of the pi framework to create bond length alternation, and the sigma framework to resist bond length alternation.

The chemistry of bowtiene (tricyclo[5.3.0.02,6]decapentaene): a computational study

Reactions of bowtiene (tricyclo[5.3.0.02,6]decapentaene, cyclobuta[1,2:3,4]dicyclopentene, bicyclopentadienylene, a tricyclic analogue of [10]annulene), in the sphere of unimolecular decomposition and dimerization were studied computationally to gauge its stability and reactivity, which are relevant to its feasability as a synthetic goal. The intuitively likeliest rearrangement mode to give species of lower energy begins with rearrangement to the 1,5-dehydronaphthalene diradical; this was calculated to have a barrier of approximately 220 kJ mol−1, too high for reaction at room temperature. Nevertheless, the energetics of rearrangement of the 1,5-diradical to the monocyclic 1,6-didehydro[10]annulene, a species of interest regarding its relationship to bowtiene and the diradical, were studied. The energy of bowtiene was compared with that of several other tricyclic 10π C10H6 cyclic polyenes.