Sulfurous acid (H2SO3) is an infamously elusive molecule. Although some theoretical papers have supposed possible roles for it in more complicated systems, it has yet to be experimentally observed. To aid experiment in detecting this molecule, we have examined the H2O + SO2 potential energy surface at the CCSDT(Q)/CBS//CCSD(T)-F12b/cc-pVTZ-F12b level of theory to resolve standing discrepancies in previous reports and predict the gas-phase vibrational spectrum for H2SO3. We find that sulfurous acid has two potentially detectable rotamers, separated by 1.1 kcal mol-1 ΔH0K with a torsional barrier of 1.6 kcal mol-1. The sulfonic acid isomer is only 6.9 kcal mol-1 above the lowest enthalpy sulfurous acid rotamer, but the barrier to form it is 57.2 kcal mol-1. Error in previous reports can be attributed to misidentified stationary points, the use of density functionals that perform poorly for this system, and, most importantly, the basis set sensitivity of sulfur. Using VPT2+K, we determine that the intense S=O stretch fundamental of each species is separated from other intense peaks by at least 25 cm-1, providing a target for identification by infrared spectroscopy.

中文翻译：

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亚硫酸和磺酸：预测红外光谱并将表面平整。

亚硫酸（H2SO3）是一个难以捉摸的分子。尽管一些理论论文认为它在更复杂的系统中可能发挥作用，但尚待实验观察。为帮助实验检测该分子，我们在以前的报告中检查了CCSDT（Q）/ CBS // CCSD（T）-F12b / cc-pVTZ-F12b的H2O + SO2势能面，以解决长期存在的差异并预测H2SO3的气相振动光谱。我们发现亚硫酸具有两个潜在可检测到的旋转异构体，它们之间的间隔为1.1 kcal mol-1ΔH0K，扭转势垒为1.6 kcal mol-1。磺酸异构体仅比最低焓的亚硫酸旋转异构体高6.9 kcal mol-1，但形成它的障碍是57.2 kcal mol-1。先前报告中的错误可归因于错误识别的固定点，使用对于该系统性能较差的密度泛函，最重要的是，硫的基本灵敏度。使用VPT2 + K，我们确定每种物种的强S = O拉伸基本特征与其他强峰之间的间隔至少为25 cm-1，从而为红外光谱识别提供了目标。