SOLVENT ISOTOPE EFFECTS AS A PROBE OF GENERAL CATALYSIS AND SOLVATIONIN PHOSPHORYL TRANSFER

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.