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

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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.**