Cis-trans isomerization in the S1 state of acetylene: Identification of cis-well vibrational levels

Abstract

A systematic analysis of the S(1)-trans (Ã(1)A(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 (13)C 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 S(1)-cis isomer (Ã(1)A(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 (13)C isotope shift of the 46 175 cm(-1) level (assigned here as cis-3(1)6(1)). The S(1)-cis zero-point level is deduced to lie near 44 900 cm(-1), and the ν(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.

Reduced dimension rovibrational variational calculations of the S1 state of C2H2. II. The S1 rovibrational manifold and the effects of isomerization

“…Several decades of spectroscopy performed by many workers, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] as well as numerous theoretical studies, [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] have shown that the molecule is transbent, and have given a complete vibrational assignment up to 4500 cm −1 of vibrational energy. A second conformer, where the molecule is cis-bent, was discovered recently.…”

“…A second conformer, where the molecule is cis-bent, was discovered recently. 13 It lies 2670 ± 10 cm −1 above the trans conformer, 34 and the barrier to isomerization is estimated to lie 4979 cm −1 (plus or minus a few hundred cm −1 ) above the trans minimum. 33 Since the S 1 -cis state transforms as A 2 in the C 2v point group, transitions to it from the + g electronic ground state are dipole-forbidden.…”

A study of the bending motion in tetratomic molecules by the algebraic operator expansion method

“…66,68 There are three fundamental bending vibrations of cis-S 1 , with symmetry a 2 , b 2 , a 1 , as shown in Fig. 12.…”

Probing cis-trans isomerization in the S1 state of C2H2 via H-atom action and hot band-pumped IR-UV double resonance spectroscopies

“…Further detailed analyses by Watson et al, [5][6][7] the Crim group, 8,9 Yamakita and Tsuchiya, [10][11][12] and Merer et al [13][14][15][16][17] have led to an essentially complete assignment of the vibrational and rotational structure up to 4500 cm −1 above the zero-point level of theà state. One of the most interesting results of these analyses has been the discovery 17,18 of bands that belong to the cis-bent isomer of theà state, the zero-point level of which lies about 2670 cm −1 higher in energy than that of the trans-bent isomer. The existence of the cis isomer had been predicted by several theoretical studies, [19][20][21][22][23][24] but it was thought that theà state of the cis isomer would be unobservable because it transforms as 1 A 2 in the C 2v point group; thus, transitions to it from the 1 Σ + g ground state are electric dipole forbidden.…”

“…In addition, interactions occur between cis and trans levels at similar energies. 18,28,30 These are the mechanisms by which the cis levels obtain their intensity by mixing with nearby trans levels, though the selection rules and level shifts are different for even and odd K-values. Combining these two effects, the resulting K-staggering varies in a seemingly random fashion from level to level.…”