Formation of Distinct Isomers via Chemical Reaction in a Supersonic Jet: The Reaction of Sulfur Trioxide and Thioacetic Acid Studied by Rotational Spectroscopy

The reaction between sulfur trioxide (SO3) and thioacetic acid (thiol form, CH3COSH) is studied by microwave spectroscopy in a supersonic jet. With multiple isotopic substitutions, and supported by MP2 and density functional theory calculations, the spectra unambiguously establish the formation of two distinct isomers. The first, denoted the O–S isomer, is CH3C(=S)OSO2OH and involves a new bond between the oxygen of the thioacetic acid and the sulfur of the SO3. The second, denoted the S–S isomer, CH3C(=O)SSO2OH, involves the new bond between the sulfur of the acid and the sulfur of the SO3. The O–S isomer is entirely analogous to the product identified in previous studies of the RCOOH + SO3 and C6H5COSH + SO3 reactions, but the formation of the S–S isomer is new. Calculations place the S–S isomer ∼8 kcal/mol lower in energy than the O–S form, suggesting that the latter is a kinetically controlled product. The mechanism by which the S–S isomer forms is unclear but may involve either a set of independent pathways or initial formation of the O–S isomer, followed by rapid rearrangement. Participation by a third body is likely. Without input from an external energy source, the formation of two isomers in a supersonic expansion is unusual.

本研究采用超声射流（supersonic jet）条件下的微波光谱法（microwave spectroscopy），对三氧化硫（SO3）与硫代乙酸（巯基形式，CH3COSH）之间的反应展开探究。通过多重同位素取代实验，并结合二级莫勒-普莱塞特微扰理论（MP2）与密度泛函理论（density functional theory）计算的辅助验证，光谱数据确凿证实了两种不同异构体的生成。第一种被命名为O-S型异构体，其结构式为CH3C(=S)OSO2OH，新形成的化学键连接于硫代乙酸的氧原子与三氧化硫的硫原子之间。第二种为S-S型异构体，结构式为CH3C(=O)SSO2OH，其新化学键形成于硫代乙酸的硫原子与三氧化硫的硫原子之间。O-S型异构体与此前针对RCOOH + SO3及C6H5COSH + SO3反应所鉴定出的产物结构完全一致，但S-S型异构体的生成则为首次发现。计算结果显示，S-S型异构体的能量比O-S型异构体低约8千卡每摩尔（kcal/mol），这表明O-S型异构体属于动力学控制产物。目前S-S型异构体的生成机制尚不明确，其可能经由多条独立反应路径生成，或是先形成O-S型异构体再经快速重排得到。反应过程中很可能有第三体参与。在无外部能量输入的情况下，超声膨胀（supersonic expansion）过程中同时生成两种异构体的现象较为罕见。