What is this SMILES notation: C[Si](C)C |^1:1|

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?




The SMILES syntax in question is neither mentioned in the [Daylight Theory Manual](http://www.daylight.com/dayhtml/doc/theory/index.html), nor in the [OpenSMILES specification](http://www.opensmiles.org/opensmiles.html).


It is not part of the canonical SMILES syntax, but an extension, developed by the [ChemAxon](https://www.chemaxon.com/) company.


According to their [documentation](https://www.chemaxon.com/marvin-archive/latest/help/formats/cxsmiles-doc.html#cxsmiles), extensions are tagged by leading and trailing `|` (pipe) symbols. `^1:`, followed by an atom index, indicates a 



> 
> monovalent radical center: 
> 
> 
> 


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