Stretch-bend combination polyads in the ùA_u state of acetylene, C₂H₂

Journal Article · Mon May 18 00:00:00 EDT 2009 · Journal of Molecular Spectroscopy

DOI:https://doi.org/10.1016/j.jms.2009.05.005· OSTI ID:1557819

Steeves, Adam H. ^[1]; Bechtel, Hans A. ^[1]; Merer, Anthony J. ^[2]; Yamakita, Nami ^[3]; Tsuchiya, Soji ^[4]; Field, Robert W. ^[1]

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1. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
2. Academia Sinica, Taipei (Taiwan); Univ. of British Columbia, Vancouver, BC (Canada)
3. Japan Women’s Univ., Tokyo (Japan)
4. National Chiao Tung Univ., Hsinchu (Taiwan)

Rotational analyses are researched for a number of newly-discovered vibrational levels of the S₁-trans (ùA_u) state of C₂H₂. These levels are combinations where the Franck–Condon active $$\nu'_{2}$$ and $$\nu'_{3}$$ vibrational modes are excited together with the low-lying bending vibrations, $$\nu'_{4}$$ and $$\nu'_{6}$$. The structures of the bands are complicated by strong a- and b-axis Coriolis coupling, as well as Darling–Dennison resonance for those bands that involve overtones of the bending vibrations. The most interesting result is the strong anharmonicity in the combinations of $$\nu'_{3}$$ (trans bend, a_g) and $$\nu'_{6}$$ (in-plane cis bend, b_u). This anharmonicity presumably represents the approach of the molecule to the trans–cis isomerization barrier, where ab initio results have predicted the transition state to be half-linear, corresponding to simultaneous excitation of $$\nu'_{3}$$ and $$\nu'_{6}$$. The anharmonicity also causes difficulty in the least squares fitting of some of the polyads, because the simple model of Coriolis coupling and Darling–Dennison resonance starts to break down. The effective Darling–Dennison parameter, K₄₄₆₆, is discovered to increase rapidly with excitation of $$\nu'_{3}$$, while many small centrifugal distortion terms have had to be included in the least squares fits in order to reproduce the rotational structure correctly. Fermi resonances become important where the K-structures of different polyads overlap, as happens with the 2¹3¹B¹ and 3¹B³ polyads (B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S₁-trans state in order to search for possible S₁-cis (¹A₂) levels. Here, along with results from other workers, identifies at least one K sub-level of every single vibrational level expected up to a vibrational energy of 3500 cm^–1.

• Research Organization: Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES); Engineering Research Council of Canada; National Science Council of Taiwan
• Grant/Contract Number: FG02-87ER13671
• OSTI ID: 1557819
• Journal Information: Journal of Molecular Spectroscopy, Vol. 256, Issue 2; ISSN 0022-2852
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Acetylene
Cis–trans isomerization
Vibration–rotation analysis