ANALYSIS AND FIT OF THE HIGH-RESOLUTION VISIBLE SPECTRUM OF THE TWO-EQUIVALENT-TOP MOLECULE BIACETYL
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Date
2006
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Publisher
Ohio State University
Abstract
The $A {^1\!A_u}(S_1)-X {^1\!A_g}(S_0)$ LIF spectrum of biacetyl (CH$_3$C(=O)C(=O)CH$_3$) shows a long progression in the torsional vibrations of the two methyl tops. The $S_1$ torsional level pattern can be qualitatively understood}, 122-132 (2005).} using local mode ideas applied to the two equivalent methyl rotors and symmetry ideas from the PI group $G_{36}$. For the present rotational analysis, we assigned a $G_{36}$ symmetry species, two local-mode torsional quantum numbers, and rotational quantum numbers $J_{Ka,Kc}$ to each observed torsion-rotation level. Transitions were globally fit with a two-equivalent-top computer program, which uses the molecular principal axis system, a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. The program is similar to our two-inequivalent-top program for N-methylacetamide.}, 28-42 (2004).} Last year, we refit 179 previously published}, 5165-5173 (2002).} rotational lines involving 7 torsional tunneling sublevels with zero or one quantum of torsional excitation in the $S_1$ state of biacetyl, using 14 parameters (5 for $S_0$, 9 for $S_1$) to obtain a standard deviation of 0.0041 \wn. We have now extended that fit by adding 131 unpublished lines involving 5 tunneling sublevels with two quanta of torsional excitation, using 19 parameters to obtain a standard deviation of 0.0040 \wn. Including transitions with still higher torsional excitation leads to a degraded standard deviation. We thus suspect that a number of higher-order terms describing top-top-overall-rotation interactions are missing from the present model, but the possibility of misassignments in the higher-energy regions must also be considered.
Description
Author Institution: Department of Physics, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan; Optical Technology Division, NIST, Gaithersburg, MD 20899-8441, USA; Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan; Institute of Atomic and Molecular Sciences, Academia Sinica,; Taipei 106, P.O. Box 23-166, Taiwan