Excited states of acetylene: a CASPT2 study

Abstract

Valence and low-lying Rydberg states of acetylene (C 2 H 2 ) are reexamined in the singlet as well as in the triplet manifold. The major goal of this work is a better understanding of the valence states that contribute to the low-energy electron-energy-loss spectrum recorded under conditions where transitions to triplet states are enhanced. An appropriate theoretical treatment of these states has to include the low-lying Rydberg states because of their energetic proximity to some of the valence states. The CASSCF/CASPT2 method provides a suitable framework for such a task. For some important states the geometry was optimized at the CASPT2 level to allow a comparison with the results of other highly accurate methods that have been applied to acetylene in the past.

Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2

“…These parameters, which are listed below Table 6. Included in the data set are two levels, with K"=1 and 3, which are not seen in the jet spectra, but which cause perturbations in the 3 4 level at higher J values; they are described in Ref. [48].…”

“…Much of the emphasis is on the lowest singlet excited state, S 1 ( Ã 1 A u ), where ab initio calculations [1][2][3][4][5][6] indicate that there is a comparatively low barrier to trans-cis isomerization and, at higher energy, a conical intersection with the second singlet excited state, S 2 . Perturbations by triplet states are widespread in the S 1 state, [7][8][9][10][11][12][13][14] where the density of such perturbations depends on the specific vibrations that are excited, and the onset of predissociation [15,16] leads to interest in the mechanisms of the near-threshold photodissociation of acetylene.…”

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

“…1, and even more complicated configurations may occur, in which the potential function has coexisting minima corresponding to linear, cis-bent and trans-bent configurations. [1][2][3][4][5][6][7][8] Also a molecule may change its point group symmetry upon electronic excitation as shown many years ago [9][10][11] and as demonstrated recently. 12 A commonly used method to describe tetratomic molecules is the force-field method.…”

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

“…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. Recent calculations 25 place the barrier to cis-trans isomerization about 5000 cm −1 above the minimum energy of the trans isomer.…”

“…Simultaneous excitation of ν 3 (trans bend, a g ) progressively destroys the bending polyad structure 16 because the saddle point on the potential surface at the isomerization barrier results in very large anharmonicity in the 3 m 6 n combination levels. The reason for this is that the pathway to the predicted [19][20][21][22][23][24] half-linear geometry at the saddle point is a CCH local bending motion formed from the combination of the normal coordinates Q 3 and Q 6 . At the saddle point, the local bending vibrational intervals must go through a minimum, in similar fashion to the "Dixon dip" 27 of the large amplitude bending intervals in triatomic molecules.…”