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Top, esters used for LFER experiments (X substituents are identified in Table 4). Bottom, the substrates p-nitrophenyl phosphate (4NPP) and p-nitrophenyl methylphosphonate (4NPMP) with the positions indicated at which isotope effects were measured. 4NPMP has two, and 4NPP has three nonbridging oxygen atoms (marked a). The scissile (leaving group) oxygen atoms are indicated by b, and the nitrogen atoms for 15N KIEs are marked c.
