O-17 NMR study of chromium(VI) ions in water

A mathematical model for oxygen exchange out of labeled (e.g., O-17) Na2CrO4 into solvent H2O according to the dimerization reaction 2CrO(4)(2-) + 2H(+) reversible arrow (1/K-a(HCrO4-)) 2HCrO(4)(-) reversible arrow (alpha/beta) Cr2O72- + H2O is described. Two rates of isotopic change are identified: (1) a rapid change intimately associated with the attainment of chemical equilibrium, but not identical with it (rate constants nu(1) and nu(2), respectively), in which the fractional label in monomeric HCrO4- + CrO42- (p) decreases more rapidly than that in the dimer Cr2O72- (q), i.e. p > q, followed by (2) a slower decrease in both (q > p) until isotopic equilibrium with the solvent is established, p = q = r (rate constant nu(3)). Visible-UV and O-17 NMR spectra have been used to characterize the elusive HCrO4- ion. Vibrational fine structure is seen in the visible spectrum (C-3v symmetry), and integration of the monomeric (HCrO4- + CrO42-) O-17 absorption agrees with only four O atoms in HCrO4-. A pH-dependent shift to higher frequency (delta CrO42- = 812 ppm; delta H-CrO4- = 860 ppm) is used to give pK(a(HCrO4-)) = 5.80 (I = 1.0 M, 25 degrees C), and comparisons with the integrated Cr2O72- signal (delta(Cr2O72-) = 1115 ppm) give K-d's for dimerization (= alpha/beta) of 81 M(-1) (I = 1.0 M) and 132 M(-1) (I = 6.0 M), at 25 degrees C. Dimerization and hydrolytic rate constants (alpha and beta) have been obtained under the conditions of the O-17 exchange experiments; they are shown to contain spontaneous, buffer, and [H+] and [OH-] contributions in agreement with earlier investigations. O-17 exchange out of enriched Na2CrO4 has been followed in aqueous solutions over the concentration range 0.01-2.0 M and over the pH range 6.38-13.0 at 25 degrees C and constant ionic strength (I = 1.0, 6.0). The dimerization reaction (eq 1) contributes only at the highest Cr(VI) concentration (2.0 M), and then only slightly (i.e., similar to 10% at pH 7.3). The rate data have been interpreted in terms of the direct exchange paths (CrO42-)-O-17 + H2O --> (k(1)) (CrO3O2-)-O-17 + (H2O)-O-17; (HCrO4-)-O-17 + H2O --> (k(2)) (HCrO3O-)-O-17 + (H2O)-O-17; (H2CrO4)-O-17 + H2O --> (k(3)) (H2CrO3O)-O-17 + (H2O)-O-17; (Cr2O72-)-O-17 + H2O --> (k(4)) (Cr2O6O2-)-O-17 + (H2O)-O-17, with rate constants k(1) = 7.2 x 10(-8) M(-1) s(-1), k(2) = 7.6 x 10(-4) M(-1) s(-1), k(3) = 1.7 x 10(5) M(-1) s(-1) (I = 1.0 M, 25 degrees C), and k(4) = 4.1 x 10(-3) M(-1) s(-1) (I = 6.0 M, 25 degrees C). The monomeric paths (but not the rate constants) agree with those found in an earlier study, but that involving Cr2O72- is new. Rate constants for O-17 exchange are compared with those of other substitution reactions of HCrO4- and Cr2O72-.