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Title: Cis-trans isomerization in the S 1 state of acetylene: Identification of cis-well vibrational levels

Abstract

A systematic analysis of the S1-trans (Ã 1A u) state of acetylene, using IR-UV double resonance along with one-photon fluorescence excitation spectra, has allowed assignment of at least part of every single vibrational state or polyad up to a vibrational energy of 4200 cm–1. Four observed vibrational levels remain unassigned, for which no place can be found in the level structure of the trans-well. The most prominent of these lies at 46 175 cm -1. Its 13C isotope shift, exceptionally long radiative lifetime, unexpected rotational selection rules, and lack of significant Zeeman effect, combined with the fact that no other singlet electronic states are expected at this energy, indicate that it is a vibrational level of the S1-cis isomer (ÃA 1A 2). Guided by ab initio calculations [J. H. Baraban, A. R. Beck, A. H. Steeves, J. F. Stanton, and R. W. Field, J. Chem. Phys. 134, 244311 (2011)] of the cis-well vibrational frequencies, the vibrational assignments of these four levels can be established from their vibrational symmetries together with the 13C isotope shift of the 46 175 cm -1 level (assigned here as cis-3 16 1). The S 1-cis zero-point level is deduced to lie near 44 900 cmmore » -1, and the ν6 vibrational frequency of the S 1-cis well is found to be roughly 565 cm -1; these values are in remarkably good agreement with the results of recent ab initio calculations. The 46 175 cm -1 vibrational level is found to have a 3.9 cm -1 staggering of its K-rotational structure as a result of quantum mechanical tunneling through the isomerization barrier. Such tunneling does not give rise to ammonia-type inversion doubling, because the cis and trans isomers are not equivalent; instead the odd-K rotational levels of a given vibrational level are systematically shifted relative to the even-K rotational levels, leading to a staggering of the K-structure. Furthermore, these various observations represent the first definite assignment of an isomer of acetylene that was previously thought to be unobservable, as well as the first high resolution spectroscopic results describing cis-trans isomerization.« less

Authors:
 [1];  [2];  [3];  [4];  [3]
  1. Inst. of Atomic and Molecular Sciences, Taipei (Taiwan); Univ. of British Columbia, Vancouver, BC (Canada)
  2. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States); Univ. of California at San Francisco, San Francisco, CA (United States)
  3. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
  4. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
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)
OSTI Identifier:
1557653
Grant/Contract Number:  
FG02-87ER13671
Resource Type:
Accepted Manuscript
Journal Name:
Journal of Chemical Physics
Additional Journal Information:
Journal Volume: 134; Journal Issue: 24; Journal ID: ISSN 0021-9606
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Merer, Anthony J., Steeves, Adam H., Baraban, Joshua H., Bechtel, Hans A., and Field, Robert W. Cis-trans isomerization in the S1 state of acetylene: Identification of cis-well vibrational levels. United States: N. p., 2011. Web. doi:10.1063/1.3599091.
Merer, Anthony J., Steeves, Adam H., Baraban, Joshua H., Bechtel, Hans A., & Field, Robert W. Cis-trans isomerization in the S1 state of acetylene: Identification of cis-well vibrational levels. United States. doi:10.1063/1.3599091.
Merer, Anthony J., Steeves, Adam H., Baraban, Joshua H., Bechtel, Hans A., and Field, Robert W. Tue . "Cis-trans isomerization in the S1 state of acetylene: Identification of cis-well vibrational levels". United States. doi:10.1063/1.3599091. https://www.osti.gov/servlets/purl/1557653.
@article{osti_1557653,
title = {Cis-trans isomerization in the S1 state of acetylene: Identification of cis-well vibrational levels},
author = {Merer, Anthony J. and Steeves, Adam H. and Baraban, Joshua H. and Bechtel, Hans A. and Field, Robert W.},
abstractNote = {A systematic analysis of the S1-trans (Ã1Au) state of acetylene, using IR-UV double resonance along with one-photon fluorescence excitation spectra, has allowed assignment of at least part of every single vibrational state or polyad up to a vibrational energy of 4200 cm–1. Four observed vibrational levels remain unassigned, for which no place can be found in the level structure of the trans-well. The most prominent of these lies at 46 175 cm-1. Its 13C isotope shift, exceptionally long radiative lifetime, unexpected rotational selection rules, and lack of significant Zeeman effect, combined with the fact that no other singlet electronic states are expected at this energy, indicate that it is a vibrational level of the S1-cis isomer (ÃA1A2). Guided by ab initio calculations [J. H. Baraban, A. R. Beck, A. H. Steeves, J. F. Stanton, and R. W. Field, J. Chem. Phys. 134, 244311 (2011)] of the cis-well vibrational frequencies, the vibrational assignments of these four levels can be established from their vibrational symmetries together with the 13C isotope shift of the 46 175 cm-1 level (assigned here as cis-3161). The S1-cis zero-point level is deduced to lie near 44 900 cm-1, and the ν6 vibrational frequency of the S1-cis well is found to be roughly 565 cm-1; these values are in remarkably good agreement with the results of recent ab initio calculations. The 46 175 cm-1 vibrational level is found to have a 3.9 cm-1 staggering of its K-rotational structure as a result of quantum mechanical tunneling through the isomerization barrier. Such tunneling does not give rise to ammonia-type inversion doubling, because the cis and trans isomers are not equivalent; instead the odd-K rotational levels of a given vibrational level are systematically shifted relative to the even-K rotational levels, leading to a staggering of the K-structure. Furthermore, these various observations represent the first definite assignment of an isomer of acetylene that was previously thought to be unobservable, as well as the first high resolution spectroscopic results describing cis-trans isomerization.},
doi = {10.1063/1.3599091},
journal = {Journal of Chemical Physics},
number = 24,
volume = 134,
place = {United States},
year = {2011},
month = {6}
}

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