Cd. Bryan et al., SOLVENT ISOTOPE EFFECTS AS A PROBE OF GENERAL CATALYSIS AND SOLVATIONIN PHOSPHORYL TRANSFER, Canadian journal of chemistry, 74(6), 1996, pp. 931-938
Phosphoryl transfer to methanol from tris(p-nitrophenyl) phosphate (PN
NN), methyl bis(p-nitrophenyl) phosphate (PMNN), and dimethyl p-nitrop
henyl phosphate (PMMN) exhibits general base catalysis by acetate ion
but no detectable catalysis by acetic acid. For PNNN, acetate catalysi
s produces normal solvent isotope effects k(ROH)/k(ROD) of 1.68 +/- 0.
01 at high ionic strength (0.475) and 1.77 +/- 0.04 at low ionic stren
gth (0.048). A linear proton inventory indicates most simply that the
isotope effect arises from a one-proton catalytic bridge in the transi
tion state, although this model cannot strongly be distinguished from
a generalized solvation effect. Reactions of methoxide ions produce sl
ight inverse isotope effects k(ROD)/k(ROH) of 1.1-1.2, far smaller tha
n the inverse effect of about 2.5 expected for complete and uncompensa
ted desolvation of the reactant-state methoxide ion. The transition st
ate is thus stabilized by substantial interaction with the solvent. Th
e proton inventory for the least reactive substrate PMMN (relative rat
e constant 1) is suggestive of transition-state stabilization by a com
bination of one-proton catalytic bridge(s) and distributed sires, whil
e the proton inventory for the most reactive substrate PNNN (relative
rate constant 1388) suggests only generalized transition-state solvati
on (many distributed sites); the proton inventory for PMNN, a substrat
e of intermediate reactivity (relative rate constant 60), suggests an
intermediate situation. The data are consistent with a model in which
transition states with exterior concentrations of charge favor stabili
zation of the charge by isotope-fractionating one-proton bridges, whil
e transition states with distributed charge favor stabilization of the
charge by many distributed sites.