Amplification of the Mills-Nixon Effect

Anisotropy of alternating CC bonds of the benzene fragment in some trisannelated Mills— Nixon systems as mirrored by IBDs (in A).  

Biphenylene and [3]phenylenes. Structural parameters, s-characters (in %) and Lowdin 7r-bond orders as calculated by the HF/6-31G model (distances in A and angles in degrees). 

Bond/Angle HF/6-31G Exptl. s-character 7T-bond order 

The picture of the angular [3]phenylene offered by both HF/6-31G model and the X ray structure is just the opposite to that in its linear counterpart the central benzene ring is more localized than in biphenylene, whereas the terminal ones are slightly more delocalized. This is reflected in their Lf,(dcc) indices which are 0.32 and 0.12, respectively. It should be mentioned that the double bond C(4b)-C(4c) bridging two four-membered fragments is the shortest and most localized one, which is compatible with its high average s-character of 41 %. 

In concluding remarks we would like to emphasize that the agreement between the theoretical and experimental geometries is reasonably good (Table 9). The structural features of higher phenylenes are similar to those of linear and bent [3]phenylenes [78]. The HOMO LUMO gap rapidly decreases as N increases in agreement with experimental 

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