| http://www.w3.org/ns/prov#value | - In this case, it is particularly preferred that at least 3 of R1, R2, R3 and R4 are an alkyl group, each alkyl group has from 1 to 5 carbon atoms, the aryl group is a phenyl group and the sum of the carbon atom numbers of R1, R2, R3 and R4 is not more than 20.
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