Atom map is part of Isomeric SMILES. Recent change.
The map is always the last part of the atom expression, delimited by
the colon character.
Why the change?
+ Convenient. A single string defines all relevant connectivity data.
+ Processing maps is automatic.
+ Hydrogens mapped easily / correctly.
- Harder to read reaction SMILES with maps.
- Isomeric SMILES are larger.
-/+ Maps are considered and reordered during canonicalization.