| http://www.w3.org/ns/prov#value | - It is also possible that the structure of formula I be such that R1 and R4, which may be the same or different, are an alkyl group, an alkoxy group, an aryl group, or a heterocyclic group, and at least one of R1 and R4 may be substituted by a halogen atom, and R2 and R3, which may be the same or different, are a hydrogen atom, an alkyl group, an alkoxy group, an aryl group or a heterocyclic group
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