Institut für Chemie, Sekr. C2, Technische Universität Berlin, D-10623 Berlin, Germany.
J Phys Chem A. 2010 Apr 1;114(12):4437-45. doi: 10.1021/jp912012g.
The radical HSO is an oxidation product of pollutants such as H(2)S and CH(3)SH in Earth's atmosphere. For the first time, the interaction of HSO and its tautomer HOS with single water molecules to yield the hydrates HSO.nH(2)O and HOS.nH(2)O was studied for n = 1-3, applying the high-level G3X(MP2) theory. A large number of structures corresponding to local minima on the potential energy surfaces has been identified. While gaseous HSO is more stable than HOS, the enthalpy diffference between HSO.nH(2)O and HOS.nH(2)O decreases with increasing degree of hydration and becomes practically zero for n = 3. Thus, in aqueous solution as well as in fog and rain droplets, HOS is expected to compete with HSO. The barrier for the tautomerization of HSO to HOS is dramatically lowered by the presence of water molecules since a cyclic transition state allows a concerted proton shift within the system of neighboring hydrogen bonds. The corresponding activation enthalpy of only 73.5 kJ mol(-1) predicted for the transformation of HSO.2H(2)O into HOS.2H(2)O may be compared to the 202 kJ mol(-1) reported for the tautomerization of the unhydrated gaseous HSO/HOS molecules. The impact of water of hydration on the fundamental vibrational modes of HSO and HOS has also been studied. Furthermore, HOS is predicted to dimerize at low temperatures to give two van der Waals molecules with singlet (symmetry C(2)) or triplet configuration (symmetry C(2h)), the latter being more stable than the singlet isomer. The disproportionation of 2HSO to H(2)S and SO(2) is predicted to be exothermic by -263.5 kJ mol(-1). The reaction of HSO with ozone to HSO(2) and O(2) is also strongly exothermic by -274.0 kJ mol(-1) and seems to proceed without any barrier. HOS forms a 1:1 van der Waals complex with O(3); the redox reaction of its two components is calculated as exothermic by -410.9 kJ mol(-1) and results in a rather stable adduct between HOSO and O(2) with the structure of a peroxo isomer of HOSO(3). This unprecedented hydrogen peroxosulfite radical might open a novel route to atmospheric sulfate without the intermediate formation of SO(2) and SO(3).
氢过硫酸根自由基（HSO）是大气中 H(2)S 和 CH(3)SH 等污染物的氧化产物。本文首次应用 G3X(MP2)理论研究了 HSO 和其互变异构体 HOS 与单个水分子相互作用生成水合物 HSO.nH(2)O 和 HOS.nH(2)O（n = 1-3）的反应。确定了大量对应于势能表面局部最小值的结构。尽管气态 HSO 比 HOS 更稳定，但随着水合程度的增加，HSO.nH(2)O 和 HOS.nH(2)O 之间的焓差减小，当 n = 3 时实际上为零。因此，在水溶液以及雾和雨滴中，预计 HOS 将与 HSO 竞争。水分子的存在极大地降低了 HSO 异构化为 HOS 的势垒，因为环状过渡态允许在相邻氢键系统内协同质子转移。对于 HSO.2H(2)O 转化为 HOS.2H(2)O 的转化，预测仅需 73.5 kJ mol(-1)的活化焓，可与未水合气态 HSO/HOS 分子的异构化报告的 202 kJ mol(-1)相比较。还研究了水合作用对 HSO 和 HOS 基本振动模式的影响。此外，预测 HOS 在低温下二聚形成两个具有单重态（对称 C(2))或三重态构型（对称 C(2h))的范德华分子，后者比单重异构体更稳定。2HSO 歧化为 H(2)S 和 SO(2)的反应预测为放热反应，焓变为-263.5 kJ mol(-1)。HSO 与臭氧反应生成 HSO(2)和 O(2)也是强烈的放热反应，焓变为-274.0 kJ mol(-1)，并且似乎没有任何障碍。HOS 与 O(3)形成 1:1 范德华配合物；其两个组分的氧化还原反应计算为放热反应，焓变为-410.9 kJ mol(-1)，并导致 HOSO 和 O(2)之间形成一个相对稳定的加合物，其结构为 HOSO(3)的过氧异构体。这种前所未有的过硫酸氢根自由基可能为大气硫酸盐的形成开辟了一条新途径，而无需中间形成 SO(2)和 SO(3)。
