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Title: Stretch-bend combination polyads in the à 1A u state of acetylene, C 2H 2

Abstract

Rotational analyses are researched for a number of newly-discovered vibrational levels of the S 1- trans1A u) state of C 2H 2. 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, ag) and $$\nu'_{6}$$ (in-plane cis bend, bu). 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, K 4466, 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 2 13 1B 1 and 3 1B 3 polyads (B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S 1- trans state in order to search for possible S 1- cis ( 1A 2) 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.

Authors:
 [1];  [1];  [2];  [3];  [4];  [1]
  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)
Publication Date:
Research Org.:
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); Engineering Research Council of Canada; National Science Council of Taiwan
OSTI Identifier:
1557819
Grant/Contract Number:  
FG02-87ER13671
Resource Type:
Accepted Manuscript
Journal Name:
Journal of Molecular Spectroscopy
Additional Journal Information:
Journal Volume: 256; Journal Issue: 2; Journal ID: ISSN 0022-2852
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Acetylene; Cis–trans isomerization; Vibration–rotation analysis

Citation Formats

Steeves, Adam H., Bechtel, Hans A., Merer, Anthony J., Yamakita, Nami, Tsuchiya, Soji, and Field, Robert W. Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2. United States: N. p., 2009. Web. doi:10.1016/j.jms.2009.05.005.
Steeves, Adam H., Bechtel, Hans A., Merer, Anthony J., Yamakita, Nami, Tsuchiya, Soji, & Field, Robert W. Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2. United States. doi:10.1016/j.jms.2009.05.005.
Steeves, Adam H., Bechtel, Hans A., Merer, Anthony J., Yamakita, Nami, Tsuchiya, Soji, and Field, Robert W. Mon . "Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2". United States. doi:10.1016/j.jms.2009.05.005. https://www.osti.gov/servlets/purl/1557819.
@article{osti_1557819,
title = {Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2},
author = {Steeves, Adam H. and Bechtel, Hans A. and Merer, Anthony J. and Yamakita, Nami and Tsuchiya, Soji and Field, Robert W.},
abstractNote = {Rotational analyses are researched for a number of newly-discovered vibrational levels of the S1-trans (Ã1Au) 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, ag) and $\nu'_{6}$ (in-plane cis bend, bu). 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.},
doi = {10.1016/j.jms.2009.05.005},
journal = {Journal of Molecular Spectroscopy},
number = 2,
volume = 256,
place = {United States},
year = {2009},
month = {5}
}

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