Pure Appl. Chem., Vol. 71, No. 2, pp. 253–264, 1999.
https://doi.org/10.1351/pac199971020253?doi.org
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According to Huckel MO theory, acepentalene (5) would have a triplet ground state, its central carbon atom would be highly pyramidalized ( ΣCCC=332.48o) [2]. More recent density functional theory (B3LYP/6-31G*) calculations placed the singlet state of acepentalene (5) with Cs symmetry 2.5 kcal/mol lower than the triplet state (pyramidalization at the central carbon ΣCCC=340.58o). According to calculated nucleus independent chemical shifts (NICS) and magnetic susceptibility exaltations, acepentalene is antiaromatic, whereas the closed shell acepentalenediide (7) with a C3v-symmetric structure (pyramidalization at the central carbon ΣCCC=350o) is aromatic by these criteria [3].