Rate of Exchange of Halide Nuclei with Mercury (II) in Aqueous Solution

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Abstract
Linewidths ΔHms of the I127nuclear magnetic resonance of water solutions of KI in the presence of varying amounts of HgII have been measured at temperatures in the range 273° to 373°K. At constant [I—], ΔHms depends linearly on the concentration of HgII: ΔHms =kms [HgII], where kms ranges from 1.9×103 G M —1 at [I—]=4.0M to 3.7×105 G M —1 at [I—]=0.005M (300°K). By means of the modified Bloch equations the results may be interpreted as due to a rapid exchange of iodine nuclei among the species I—, HgI4 2—, and HgI3(OH2)—. Broadening of the I127nuclear magnetic resonance is produced by relatively short relaxation times of I127 in the mercury complexes; T 2(I—)=1.7×10—4 sec, T 2(HgI4 2—)=1.1×10—7 sec (300°K). Biomolecular rate constants for the following exchange reactions have been obtained: HgI 4 2− + I − → lim k 1 I − + HgI 4 2− , HgI 3 ( OH 2 ) − + I − → lim k 2 I − + HgI 3 ( OH 2 ) − , HgI 3 ( OH 2 ) − + I − → lim k 3 H 2 O + HgI 4 2− . At 300°K, k 1=8.8×107 M —1 sec—1, k 2=5.7×109 M —1 sec—1, and k 3=7.1×108 M —1 sec—1, ΔE 1 ‡=1.8±0.6 kcal mole—1. The rates appear to be diffusion limited and may be interpreted by an appropriate solution of the Smoluchowski diffusion equation. Similar but less extensive measurements have been made on KBr solutions containing added HgBr2. Again exchange appears to be diffusion controlled and T 2(HgBr4 2—) is much shorter than T 2(Br—). The relatively small value of k 1 is attributed to a negative entropy of activation associated with dielectric polarization of the solvent by the ions involved. The observed relaxation rate of I127 in HgI4 2— combined with an estimate of the quadrupolar coupling constant of I127 in solid K2HgI4 allows an estimation of the rotational correlation time τ c of HgI4 2— in solution, which at 25°C is equal to 1.4×10—11 sec. This time is a factor of six shorter than estimated from the Debye expression for τ c .