prov:value
| - wherein X1, X2, X3, and X4 each, independently of the others, is a hydrogen atom, a halogen atom, a hydroxy group, an amine group, an ammonium group, a cyano group, a pyridine group, a pyridinium group, an ether group, a carbonyl group, a thiocarbonyl group, an aldehyde group, a ketone group, an ester group, an amide group, a sulfate group, a sulfonate group, a sulfonic acid group, a sulfide group
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