On Jan 22, 2020, at 14:12, Greg Landrum <greg.land...@gmail.com> wrote: > As an aside: it's not particularly relevant to this discussion, but I don't > understand why the wikipedia page says that the compound is anti-aromatic. I > think the standard definition of anti-aromaticity (agrees with the one linked > to from the acepentalene page) requires the ring system to have 4n electrons. > That definitely doesn't apply here to either the individual rings or the > system as a whole. The system as a whole has 10 electrons (4n+2), the > individual rings each have 5 (neither aromatic nor anti-aromatic), and the > outer envelope has 9 (again, neither aromatic nor anti-aromatic).
Because I didn't know either, I looked into it. I think that's because (to quote "Towards experimental determination of conical intersection properties:a twin state based comparison with bound excited states", Phys. Chem. Chem. Phys., 2011,13, 11872–11877 [*] ) > A Hückel MO analysis[21] leads to the conclusion that the ground state of the > conjugated tricyclic acepentalene I is a triplet state. DFT calculations > corrected this picture and showed a singlet global minimum distorted to C_s > symmetry with alternated single and double bonds,[22] which are well > described by the Lewis structures A(B,C). According to a B3LYP/6-31G* > calculation the lowest triplet state has also a high symmetric C_3v > configuration and lies 3.9 kcal/mol above the singlet ground state minimum. > Acepentalene I was characterized as an antiaromatic system [23] despite being > formally an aromatic 10 electron system: the resonance between each pair of > Kekule structures in this case involves only 4 electron pairs of the > pentalene fragments and it averts the resonance with the additional fifth > electron pair common for both the structures. Such a resonance is described > as an anti-combination of two Kekule structures: (A–B), (C–B) and (C–A). Just need to add B3LYP/6-31G* calculations to RDKit's aromaticity perception algorithm and everything will be fine. :) The "characterized as an antiaromatic system[23]" is "T. K. Zywietz, H. Jiao, P. v. R. Schleyer and A. de Meijere, J. Org.Chem., 1998, 63, 3417" at https://pubs.acs.org/doi/abs/10.1021/jo980089f . Cheers, Andrew da...@dalkescientific.com [*] https://www.researchgate.net/profile/Shmuel_Zilberg/publication/51175586_Towards_experimental_determination_of_conical_intersection_properties_A_twin_state_based_comparison_with_bound_excited_states/links/561bb5bc08ae6d17308b037f/Towards-experimental-determination-of-conical-intersection-properties-A-twin-state-based-comparison-with-bound-excited-states.pdf _______________________________________________ Rdkit-discuss mailing list Rdkit-discuss@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/rdkit-discuss
