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

摘要

Linewidths Dgr;Hmsof the I127nuclear magnetic resonance of water solutions of KI in the presence of varying amounts of HgIIhave been measured at temperatures in the range 273deg; to 373deg;K. At constant Imdash;, Dgr;Hmsdepends linearly on the concentration of HgII: Dgr;Hms=kmsHgII, wherekmsranges from 1.9times;103GMmdash;1at Imdash;=4.0Mto 3.7times;105GMmdash;1at Imdash;=0.005M(300deg;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 Imdash;, HgI42mdash;, and HgI3(OH2)mdash;. Broadening of the I127nuclear magnetic resonance is produced by relatively short relaxation times of I127in the mercury complexes;T2(Imdash;)=1.7times;10mdash;4sec,T2(HgI42mdash;)=1.1times;10mdash;7sec (300deg;K). Biomolecular rate constants for the following exchange reactions have been obtained:HgI42minus;plus;Iminus;rarr;k1Iminus;plus;HgI42minus;,HgI3lpar;OH2rpar;minus;plus;Iminus;rarr;k2Iminus;plus;HgI3lpar;OH2rpar;minus;,HgI3lpar;OH2rpar;minus;plus;Iminus;rarr;k3H2Oplus;HgI42minus;.At 300deg;K,k1=8.8times;107Mmdash;1secmdash;1,k2=5.7times;109Mmdash;1secmdash;1, andk3=7.1times;108Mmdash;1secmdash;1, Dgr;E1Dagger;=1.8plusmn;0.6 kcal molemdash;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 andT2(HgBr42mdash;) is much shorter thanT2(Brmdash;).The relatively small value ofk1is attributed to a negative entropy of activation associated with dielectric polarization of the solvent by the ions involved.The observed relaxation rate of I127in HgI42mdash;combined with an estimate of the quadrupolar coupling constant of I127in solid K2HgI4allows an estimation of the rotational correlation time tgr;cof HgI42mdash;in solution, which at 25deg;C is equal to 1.4times;10mdash;11sec. This time is a factor of six shorter than estimated from the Debye expression for tgr;c.