Spectroscopic Studies of Large-Amplitude Motions in Methyl-Rotor-Containing Molecules Important to Fundamental Physics, Astronomy and Chemical Dynamics

提案摘要******这项NSERC发现拨款申请的主要重点是对基础和实际重要性的复杂有机气相分子的精密光谱学和表征。被研究的分子是(i)含有内部转子的物质，已知其对探测基本常数的变化具有增强的敏感性，并且已知其促进扭转介导的振动能量再分配；（ii）天文学界迫切需要含硫分子的数据，以便了解硫的化学性质和稠密分子云中“缺硫”的问题。****未来的工作将以我们过去的成功为基础，并将其扩展到以下方向：****(1)甲基转子物种的扭转增强超细分裂。我们成功地模拟了甲醇中意想不到的、以前未见过的超细双峰，它们是由一种全新的扭转介导的甲基质子自旋-旋转效应产生的，这是一种质子自旋和整体旋转的振幅扭转运动的独特组合。这提出了一些有趣的问题，我们将解决：(i)这种扭转介导的效应是否存在于其他甲基转子分子中；（ii）我们能否开发分析工具来预测和模拟大范围物种的这些分裂？（iii）这种增强效应是否与扭转参数ρ（分子顶部转动惯量与整个分子转动惯量之比）有关；（iv）这些分裂对天文发现有什么影响？****(2)星际介质中的硫化学尚不清楚。这个问题很重要，因为含硫分子被用来探测恒星形成区域的物理结构，并被提议作为化学时钟，通过了解它们的物理条件来获得关于热核年龄的信息。然而，ISM密集区域的硫含量明显低于扩散区域，这就造成了“缺硫问题”。我们将以含硫化合物所需的基准实验室数据库为目标，以帮助理解密集分子云中缺失的硫问题。****(3)高分辨率光谱揭示了甲醇所有振动模式的复杂相互作用网络，一直到3 μm区域，甲基硫醇和甲醇的同位素变体既有相似之处，也有显著的差异。在激发振动状态下观察到的扭转模式与基态的扭转模式有明显的偏差，从减少的扭转分裂到完全倒置的扭转模式。我们的发现需要一种新的方法来处理振动被激发时的大振幅运动。实验观察和从头算分子模型的结合将导致对扭转-振动耦合和动力学的复杂本质的新见解

Summary of Proposal******The primary focus of this NSERC Discovery Grant application is on precision spectroscopy and characterization of complex organic gas-phase molecules of fundamental and practical importance. Molecules being investigated are (i) internal-rotor-containing species known to have enhanced sensitivity for probing variation of fundamental constants and known to promote torsion-mediated vibrational energy redistribution; and (ii) sulphur-containing molecules for which data are urgently needed by the astronomical community in order to understand sulphur chemistry and the “missing Sulphur” question in dense molecular clouds.****Future work will build on our past success and extend it in the following directions:****(1) Torsion-enhanced hyperfine splittings in methyl-rotor species. We have successfully modeled unexpected and previously unseen hyperfine doublets in methanol that arise from a totally new torsionally-mediated methyl proton spin-rotation effect, a unique combination of large-amplitude torsional motion with proton spin and overall rotation. This raises interesting questions that we will address: (i) does this torsionally-mediated effect exist in other methyl-rotor molecules; (ii) can we develop analytical tools to predict and model these splittings for a wide range of species; (iii) is this enhanced effect related to the torsional parameter ρ (ratio of moment of inertia of the top to that of the whole molecule); and (iv) what are the implications of these splittings for astronomical findings?****(2) Sulphur chemistry in the interstellar medium is not well understood. This problem is important as sulphur-bearing molecules are used to probe the physical structure of star-forming regions and have been proposed as chemical clocks to obtain information about the age of hot cores from a knowledge of their physical conditions. However, significantly less sulphur is found in dense regions of the ISM than in diffuse regions, creating the “missing Sulphur problem”. We will target benchmark laboratory databases needed for sulphur-containing compounds to assist understanding of the missing sulphur question in dense molecular clouds.****(3) High-resolution spectra have revealed complicated networks of interactions in all vibrational modes of methanol all the way up to the 3 μm region, Methyl mercaptan and isotopic variants of methanol show close similarities but also dramatic contrasts. The observed torsional patterns in the excited vibrational states deviate significantly from that in the ground state, ranging from reduced torsional splittings to completely inverted torsional patterns. Our findings call for a new way of treating large-amplitude motion when vibrations are excited. The combination of experimental observations and ab initio molecular modeling will lead to new insights into the complex nature of the torsion-vibration coupling and dynamics.**