TY - JOUR
T1 - Cis-trans isomerization in the S1 state of acetylene
T2 - Identification of cis-well vibrational levels
AU - Merer, Anthony J.
AU - Steeves, Adam H.
AU - Baraban, Joshua H.
AU - Bechtel, Hans A.
AU - Field, Robert W.
N1 - Funding Information:
We thank Dr. Jon Hougen (NIST, Gaithersburg) for valuable discussions on the group theory aspects, and Dr. Michelle (Silva) Clark for recording the CH data. At MIT, this work was supported by the U.S. Department of Energy (DOE) Grant No. DE-FG0287ER13671. A.J.M. thanks the Academia Sinica, Taiwan, for the award of a Distinguished Visiting Professorship, and the Natural Sciences and Engineering Research Council of Canada for partial support of this work. J. H. Baraban acknowledges the support of a NSF Graduate Research Fellowship. 13 2 2
PY - 2011/6/28
Y1 - 2011/6/28
N2 - A systematic analysis of the S1-trans (A∼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 (A∼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)10.1063/1.3570823 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 6 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. 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.
AB - A systematic analysis of the S1-trans (A∼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 (A∼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)10.1063/1.3570823 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 6 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. 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.
UR - http://www.scopus.com/inward/record.url?scp=79960160124&partnerID=8YFLogxK
U2 - 10.1063/1.3599091
DO - 10.1063/1.3599091
M3 - Article
AN - SCOPUS:79960160124
SN - 0021-9606
VL - 134
JO - Journal of Chemical Physics
JF - Journal of Chemical Physics
IS - 24
M1 - 244310
ER -