Mills-Nixon effect computations

A second category of benzannelated compounds that might express the Mills-Nixon effect are exemplified by 71 and 72. Vollhardt s 1986 synthesis of starphenylene (72) revitalized interest in the Mills-Nixon effect. The X-ray crystal structure of 72 indicates a long endo distance of 1.495 A and a short exo distance of 1.335 A—strong evidence of the Mills-Nixon effect. Computations have reproduced this large bond alternation in 72 (Table 3.28). 

Advances in spectroscopic and crystallographic techniques, and in computational methods, have allowed for detailed physical and theoretical analyses of the cycloproparenes. Central to a consideration of this interesting class of compounds is their role in debate over the Mills-Nixon effect—the concept of bond localization within the aromatic ring. 

---