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When converting InCHI to canonical SMILES using Indigo, I will occasionally get an output SMILES string like this:

C[Si](C)C |^1:1|

What does the "|^1:1|" part mean? I can't find this explained anywhere in either SMILES grammar documentation, nor Indigo documentation, although other chemical software seem to accept it as a valid SMILES string. Any ideas on the meaning of this annotation?

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    $\begingroup$ This is part of an extended SMILES syntax used by ChemAxon. Please have a look at the docs $\endgroup$ – Klaus-Dieter Warzecha Mar 1 '16 at 21:38
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The SMILES syntax in question is neither mentioned in the Daylight Theory Manual, nor in the OpenSMILES specification.

It is not part of the canonical SMILES syntax, but an extension, developed by the ChemAxon company.

According to their documentation, extensions are tagged by leading and trailing | (pipe) symbols. ^1:, followed by an atom index, indicates a

monovalent radical center:

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