The valence-excited states T1–T4 and S1–S2 of acetylene: A high-level MR-CISD and MR-AQCC investigation of stationary points, potential energy surfaces, and surface crossings

Abstract

Valence-excited singlet (S1,S2) and triplet (T1–T4) states of acetylene have been studied by means of extended multireference electron correlation techniques (MR-CISD, MR-CISD+Q, and MR-AQCC). Extrapolations to the basis set limit have been performed. Minima and saddle points have been calculated using a recently developed analytic gradient method for excited states. Planar as well as nonplanar structures have been considered. In particular, the existence of an asymmetric, planar cis-type minimum on the S2 surface has been confirmed conclusively. Moreover, an intersection S1/S2 has been located close to this minimum. This situation will most probably affect the interpretation of the absorption bands attributed to the trans 1 1Bu state. In-plane and out-of-plane saddle points for cis–trans isomerization have been determined and characterized by harmonic vibrational analysis. Several interesting surface crossings for different electronic states (S1/S2, T2/T3, and S1/T3) have been characterized. Implications of the flatness of the T3 surface around linear structures and the location of the S1/T3 crossing seam on the anomalities observed in the ZAC spectrum of the à 1Au state are discussed.

Reduced dimension discrete variable representation study of cis–trans isomerization in the S1 state of C2H2

“…Ab initio calculations have long predicted a cis minimum [12][13][14] , but experimental confirmation has been hard to come by. This is because the S 1 -S 0 electronic transition is forbidden in C 2v geometries, and any spectroscopic observation of the cis conformer is therefore expected to manifest itself in the form of weak transitions or indirect effects, necessarily resulting from vibronic mechanisms.…”

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

“…Up to quadratic terms in the elements of the Lie algebra there are nine terms in (8). Up Table I with contour plot setting ϕ = 0 in (a) and the cuts r 1 = r 2 and r 1 = −r 2 in panel (b).…”

“…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.…”