Hydrogens in Reactions:
Hydrogens normally may be omitted from SMILES. Only for certain
"special" cases must hydrogens be shown. These cases include:
Charged hydrogens ([H+]),
hydrogens connected to other hydrogens, ([H][H]),
hydrogens connected to more than one atom,
isotopic hydrogens ([2H]).
For reactions, if a hydrogen is mapped, it is also "special"
and must be shown.