On Tue, Oct 23, 2018 at 1:08 PM Chris Earnshaw <cgearns...@gmail.com> wrote:
> Interesting - I do hope your idea works out! > > This prompted me to see what happens with azulene, which is another case > where the envelope is aromatic but neither of the individual rings are > based on a simple neutral representation. This ends up being related to > Peter's example; the input SMILES c1ccccc2c1ccc2 gets converted to > c1cc2cccccc-2c1, where the fusion bond is perceived as 'pure single' rather > than aromatic as it should be, so we do indeed end up with two non-aromatic > rings embedded in an aromatic envelope. > Yes; and it's cool that crystallography shows that the fusion bond is considerably longer than the ring bonds. However, a common representation of azulene is as a cyclopentadienyl anion fused to a tropylium cation. But this picture consider the fusion bond to be aromatic. > Is there any way to define the aromaticity of an individual ring based on > its mapping to an aromatic envelope? Are there any cases where that > wouldn't be true? > In naphthalene, the fusion bond and both rings would be considered aromatic, if that's what you're getting at. [image: image.png] >>> >> This is a very cute example, because every bond and every atom is aromatic, yet one ring composed of fully aromatic bonds and atoms is not (considered) aromatic. (But if you "pyridonize" the left ring, it becomes aromatic, too.) -P.
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