The protonation constants (K
iH) of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (H4dota) were redetermined at 25 °C in 0.1 M NMe4Cl. The log Ki
H values (i = 1â5) are 12.6, 9.70, 4.50, 4.14 and 2.32, respectively. The stability constants of [Ce(dota)]â and [Yb(dota)]â were determined by pH-potentiometry as log KCeL = 24.6 and log KYbL = 26.4. The formation rates of [Ce(dota)]â and [Yb(dota)]â were studied by the stopped-flow method at much higher pH values than before. In the range pH 3.5â9.3 the spectra were interpreted in terms of the formation of diprotonated, [Ce(H2dota)]+, and (at around pH > 8) monoprotonated, [Ce(Hdota)], intermediates. These two species are characterized by the same spectra (and structure). At pH < 7.5 the formation rates of the complexes were directly proportional to the OHâ concentration. The rate constants, kOH, characterizing the formation of [Ce(dota)]â and [Yb(dota)]â are 2.7 Ã 105 and 9.3 Ã 107Mâ1 sâ1, respectively. However, at pH > 7.5 the order of reaction in [OHâ] is higher than one. The results were interpreted in terms of rate-controlling deprotonation of the monoprotonated intermediate [Ln(Hdota)] followed by rearrangement of the deprotonated intermediate into the product. The deprotonation is a general base-catalysed process, which occurs with the assistance of a H2O molecule at pH < 7.5. At higher pH values the OHâ-assisted deprotonation of the intermediate, as another pathway, increases the formation rate of the complexes. By assuming these reaction pathways a general rate expression was deduced and it was shown that kOH = kHLnHL/K
HLnHLKw, where K
HLnHL is the protonation constant of the monoprotonated intermediates [Ce(Hdota)] and [Yb(Hdota)], K
HCeHLÂ =Â 4.4Â ÃÂ 108, K
HYbHLÂ =Â 2.5Â ÃÂ 108 and kLnHL is the rate constant, characterizing the H2O-assisted deprotonation of the intermediates, kCeHLÂ =Â 18.5 and kYbHLÂ =Â 245 sâ1. At pHÂ >Â 8 the OHâ-assisted deprotonation of the monoprotonated intermediate needs to be considered in the formation rates of [Ce(dota)]â; the rate constant for this pathway is kOHCeHLÂ =Â 1.9Â ÃÂ 107Mâ1 sâ1.
在 25°C 的 0.1 M NMe4Cl 溶液中重新测定了
1,4,7,10-四氮杂环十二烷-1,4,7,10-四乙酸(H4dota)的质子化常数(K iH)。对数 Ki H 值(i = 1â5 )分别为 12.6、9.70、4.50、4.14 和 2.32。通过 pH 电位法测定,[Ce(dota)]â 和[Yb(dota)]â 的稳定常数分别为 log KCeL = 24.6 和 log KYbL = 26.4。通过停流法研究了[Ce(dota)]â和[Yb(dota)]â在比以前更高的 pH 值下的形成率。在 pH 值为 3.5â9.3 的范围内,光谱被解释为形成了二质子化的[Ce(H2dota)]+ 和(在 pH 值>8 左右)
单质子化的[Ce(Hdota)]中间体。这两种物质具有相同的光谱(和结构)。在 pH 值小于 7.5 时,络合物的形成速率与 OH 浓度成正比。形成[Ce(dota)]â和[Yb(dota)]â的速率常数kOH分别为2.7 Â Â 105和9.3 Â Â 107Mâ1 sâ1。然而,当 pH 值大于 7.5 时,[OHâ] 的反应顺序大于 1。这些结果被解释为
单质子化中间体[Ln(Hdota)]发生了速率控制型去质子化反应,然后去质子化中间体重新排列为产物。在 pH 值小于 7.5 的情况下,去质子化过程是在
H2O 分子的帮助下进行的。在较高的 pH 值下,中间体的 OH 辅助去质子化是另一种途径,会提高络合物的形成率。通过假定这些反应途径,推导出了一般速率表达式,并表明 kOHÂ =Â kHLnHL/K HLnHLKw,其中 K HLnHL 是
单质子化中间体 [Ce(Hdota)] 和 [Yb(Hdota)] 的质子常数,K HCeHLÂ =Â 4.4 Â Â 108,K HYbHLÂ =Â 2.5 Â Â 108,kLnHL 是速率常数,表征 辅助的中间产物的去质子化过程,kCeHLÂ =Â 18.5,kYbHLÂ =Â 245 sâ1 。在 pH 值大于 8 的条件下,在计算[Ce(dota)]的形成速率时需要考虑
单质子化中间体的羟基辅助去质子化;该途径的速率常数为 kOHCeHL=Â 1.9Â Â 107Mâ1 sâ1。