Isotope effects on isotope effects: equilibrium nitrogen isotope effects on the basicity of pyridine in protium and deuterium oxides

Abstract

The {sup 14}N/{sup 15}N isotope effect on the pK{sub a} of pyridinium ion is 18% larger in D{sub 2}O than in H{sub 2}O: pK{sub a}{sup 14} {minus} pK{sub a}{sup 15} = {minus}0.0090{sub 7} {plus minus} 0.0001{sub 6} in H{sub 2}O and {minus}0.0107{sub 2} {plus minus} 0.0001{sub 5} in D{sub 2}O. Valence mode cutoff models for pyridinium ion and pyridine account for both values and suggest the following: (1) The value of pK{sub a}{sup 14} {minus} pK{sub a}{sup 15} for C{sub 5}H{sub 5}NH{sup +} would be near {minus}0.0136 in the absence of hydrogen bonding. (2) The C{sub 5}H{sub 5}NH{sup +}{hor ellipsis}OH{sub 2} and C{sub 5}H{sub 5}N{hor ellipsis}HOH hydrogen bonds reduce the magnitude of pK{sub a}{sup 14} {minus} pK{sub a}{sup 15}by about 0.0031 and 0.0014, respectively. (3) The H{sub 2}O/D{sub 2}O isotope effect on pK{sub a}{sup 14} {minus} pK{sub a}{sup 15} is a consequence of the reduced masses for the C{sub 5}H{sub 5}ND{sup +} vibrational modes being more sensitive to the {sup 14}N/{sup 15}N mass difference than are the reduced masses for the C{sub 5}H{sub 5}NH{sup +} modes; contributions from differences between C{sub 5}H{sub 5}N{hor ellipsis}HOH and C{sub 5}H{sub 5}N{hor ellipsis}DOD hydrogen bonds are very small. In general, H/D isotope effects onmore » other equilibrium isotope effects are expected to be significant whenever two conditions are satisfied: (1) The two isotopic atoms are in separate reactant molecules but in the same product molecule. (2) Motion of both isotopic atoms occurs in at least one vibrational mode of the product for which hv/k{sub b}t has a value significantly greater than 2.« less