Browsing by Author "Miller, Terry A."

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Manifestations of Torsion-CH Stretch Coupling in the Infrared Spectrum of CH3OO
(2016-07-14)
Hsu, Kuo-Hsiang
;
Huang, Yu-Hsuan
;
Lee, Yuan-Pern
;
Huang, Meng
;
Miller, Terry A.
With a step-scan Fourier-transform spectrometer we recorded temporally resolved infrared absorption spectra of CH3OO radicals that were produced upon irradiation of CH3COCH3 and O-2 at 193 nm in a flowing mixture. At a resolution of 0.15 cm(-1), the rotational structure of the nu(2) band of CH3OO near 2954.4 cm(-1) is partially resolved and shows an unexpectedly broadened, and somewhat distorted, Q-branch. A 4D model Hamiltonian, consisting of three CH stretches and the methyl torsion, was developed to explore the origins of this broadening. The vibrational progressions predicted by the model Hamiltonian and the rotational contours of the nu(2) band, based on experimental ground-state rotational parameters and their values scaled by their calculated ratios for the upper state, produced simulations in satisfactory agreement with the observed spectrum. These results provide new inSight into the vibrational couplings in CH3OO.
Modeling the CH Stretch/Torsion/Rotation Couplings in Methyl Peroxy (CH3OO)
(2017-12-21)
Huang, Meng
;
Miller, Terry A.
;
McCoy, Anne B.
;
Hsu, Kuo-Hsiang
;
Huang, Yu-Hsuan
The manifestations of CH stretch/torsion/ rotation coupling in the region of the CH stretch fundamentals are explored in the CH3OO radical. Following our earlier study of the fundamental in the totally symmetric CH stretch (the fundamental), this work focuses on the other two CH stretch fundamentals, nu(1) and nu(9), which would be degenerate in the absence of a barrier in the potential along the methyl torsion coordinate. The simplest model, which assumes a decoupling of the CH stretch vibrations from the torsion, fails to reproduce several important features of the spectrum. Specifically, the absence of a strong peak around the origin of the nu(1), fundamental and broadening of the strong peak near the origin in the observed spectrum of the nu(9) fundamental are not captured by this model. The origins of these features are explored through two more sophisticated treatments of the torsion/CH stretch couplings. In the first, a four-dimensional potential based on the three CH stretches and the torsion is developed and shown to reproduce both of these features. On the basis of the results of these calculations, the calculated parameters are adjusted to simulate the recorded spectrum. To further explore the torsion/CH stretch couplings in CH3OO, a 9-state model Hamiltonian is developed and discussed. The implications of various types of couplings on the observed energy level patterns are also discussed.
Rovibronic bands of the (A)over-tilde <-(X)over-tilde transition of CH3OO and CD3OO detected with cavity ringdown absorption near 1.2-1.4 mu m
(2007-07-28)
Chung, Chao-Yu
;
Cheng, Chi-Wen
;
Lee, Yuan-Pern
;
Liao, Hsin-Yi
;
Sharp, Erin N.
We have recorded several rovibronic bands of CH3OO and CD3OO in their (A) over tilde <-(K) over tilde transitions in the range of 1.18-1.40 mu m with the cavity ringdown technique. While the electronic origins for these species have been reported previously, many newly observed rovibronic bands are described here. The experimental vibrational frequencies (given as nu in the unit cm(-1) in this paper) for the COO bending (nu(8)) and COO symmetric stretching (nu(7)) modes in the (A) over tilde state are 378 and 887 cm(-1) for CH3OO, and 348 and 824 cm(-1) for CD3OO, respectively. In addition, two other vibrational frequencies were observed for the (A) over tilde state of CD3OO, namely, nu(5) (954 cm(-1)) and nu(6) (971 cm(-1)). These experimental vibrational frequencies for the A state of both CH3OO and CD3OO are in good agreement with predictions from quantum-chemical calculations at the UB3LYP/aug-cc-pVTZ level. The enhanced activity of the nu(5) vibrational mode in CD3OO is rationalized by mode mixing with the nu(7) mode, as supported by calculations of multidimensional Franck-Condon factors. In addition, many hot bands involving the methyl torsional mode (nu(12)) are observed for both normal and deuterated methyl peroxy. These bands include the "typical" sequence transitions and some "atypical" ones due to the nature of the eigenvalues and eigenfunctions which are a consequence of the low, but very different, torsional barriers in the (X) over tilde and (A) over tilde states. In addition, the 12(2)(2) band in CH3OO and the 12(3)(3) band in CD3OO show quite different structures than the origin bands, an effect which results from tunneling splittings comparable to the rotational contour. (C) 2007 American Institute of Physics.