| http://www.w3.org/ns/prov#value | - In some embodiments, R1, R2, R3 and R4 can independently be non aromatic or aromatic, including hetero aromatic, groups and not all the groups R1, R2, R3 and R4, if aromatic, have a substituent on the atom adjacent to the atom bound to A or C. In other embodiments, each non electron donating substituent on one or more of R1, R2, R3 and R4 may be non-polar.
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