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Author(s):Huang, Meng
Contributor(s):Miller, Terry A.; McCoy, Anne B.
Subject(s):Large amplitude motions, internal rotation
Abstract:Large amplitude motions in methyl rotor systems have been well studied, especially the coupling between the CH${_3}$ torsion and the CH stretches. The CH${_3}$OO radical is a example of a system where this coupling is relatively small, but its effects still can be observed in the the infrared spectrum taken by the Lee group.footnote{K.-H.~Hsu, Y.-P.~Lee, M.~Huang, T.~A.~Miller, TD08, textit{68th International Symposium of Molecular Spectroscopy} (2013)} Rotational contour simulations based on an asymmetric rotor model show good agreement with the experimental spectrum except for an unexplained broadening of the Q-branch of one of the CH stretch features. The broadening is likely caused by low frequency torsional modes populated at room temperature resulting in sequence band transitions that are slightly shifted from the origin. A reduced dimension model involving the three CH stretches and the CH${_3}$ torsion is applied to CH${_3}$OO to simulate the observed spectrum. The CH stretches are described by a harmonically coupled anharmonic oscillator model in which the parameters depend on the CH${_3}$ torsion angle. Based on these calculations, the observed broadening of the Q-branch can be qualitatively explained by coupling between two CH stretch/CH${_3}$ torsion combination bands which differ by one quantum in torsional excitation. The textit{~{A}-~{X}} electronic transitions of halogenated methyl peroxy radicals, CH${_2}$XOO (X-Cl, Br, I), show a complementary structure. At room temperature multiple peaks have been observed in the region of the origin and OO stretch vibronic bands in all three radicals with the spectra for CH$_2$IO$_2$ being by far the most complex. This structure may again be the result of hot bands originating from excited torsional levels. Several theoretical models have been investigated to calculate the Franck-Condon factors that govern the structure. A calculation that models the I-C-O-O torsion using curvilinear internal coordinates and molecular geometry and harmonic torsion frequencies predicted by electronic structure calculations shows the best agreement between the CH${_2}$IOO experimental and simulated spectra. The multiple peak structure results from the change in X-C-O-O torsion dihedral between the textit{~{X}} state and textit{~{A}} states. Interestingly, a similar calculation with Cartesian displacement coordinates fails to explain the torsional structure. This study shows the importance of coordinate system choice if a significant displacement in the torsional coordinate occurs upon electronic excitation.
Issue Date:23-Jun-15
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:ACS
Genre:Conference Paper / Presentation
Date Available in IDEALS:2016-01-05

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