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Acepentalene is not aromatic. While it has a planar cyclic structure with a conjugated π system, it does not meet Hückel's rule. Hückel's rule states that a compound is aromatic if it has a cyclic, planar, conjugated system with 4n+2 π electrons (where n is an integer). Acepentalene has 10 π electrons, which does not fit this criteria. Therefore, it is considered antiaromatic.