热水溶液中硫代硫酸盐分子内硫同位素的交换

分子内不同结构位置上两个同种元素的原子间的直接同位素交换,服从假一级化学反应速度定律。这两位置原子间的同位素交换分数F仅为反应时间t的函数,且有ln(1-F)=-(kf+kr)t。由此分析Uyama等(1985)的实验结果,表明热液中硫代硫酸盐(HSSO3H)分子内-SO3H和-SH之间的直接硫同位素交换很慢,它们之间实际存在的较快速同位素交换是通过与硫化物反应形成多硫化物这个可逆反应来实现的。

INTRAMOLECULAR EXCHANGE OF SULFUR ISOTOPES WITHIN TH1OSULFATE IN HYDROTHERMAL SOLUTION

For some compounds, such as thiosulfate, there are two isotopically exchangeable atoms that occur in structurally non-equivalent positions within a molecule. The overall exchange reaction of intramolecular isotopes for a compound (AB) with two exchangeable atoms (A and B),in general, may be expressed by a reversible reaction:A^*B↔^*ABwhere the components with asterisks represent those with heavy isotopes (e.g.34S) and those without asterisks lighter isotopes (e.g. 32S). Similar to an tsotopic exchange reaction of two-compound, the intramolecular isotope exchange will obey a pse-udo first-order rate law, i.e.,In(1-F)=-(k_f+k_r)_t tin which F, kf and kr refer to the traction of isotopic exchange, the overall rate constants for the forward and reverse reactions, respectively. For a direct intramolecular exchange, thus, its values of ln(1-F) at various durations will lie on a straight line in plot of in(1-F) versus t.The experimental data (i.e., Run 150-A) on sullur isotopic exchange between inner (-SO₃H) and outer (-SH) thiosulfate (HSSO₃H) in hydrothermal solution reported by Uyama et al. (1985), however, did not obey the pseudo first-order rate law. This suggests that the sullur isotopic exchange between -SO₃H and -SH of tni. osulfate occurring in Run 150-A is not a direct exchange of intramolecular isotopes within thiosulfate. In solution, the faster exchange of sulfur isotopes between -SO₃H and -SH actually proceed through the reaction with sulfides and, then, the formation and disproportionation of polysulfides. The overall rate constant for intramolecular exchange of sulfur isotopes within thiosulfate approximates to 5×10^-5s^-1 at 150℃.