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(a) Bifluorenylidenes have a “fjord” region, defined as a five-sided pocket within a polyaromatic hydrocarbon. Fjords cause steric clashing between internally directed positions and lead to dihedral twisting. (b) X-ray crystallography of 2 shows a twist of 33.6°. (c) Twisting reduces p orbital overlap and weakens the π interaction from its theoretical nontwisted strength ΔE.
