Ab initio calculations on the excited states of π-systems. I. Valence excitations in acetylene

doi:10.1016/0301-0104(86)85119-9

Chemical Physics

Volume 102, Issues 1–2, 15 February 1986, Pages 77-89

https://doi.org/10.1016/0301-0104(86)85119-9 Get rights and content

Abstract

The electronic ground state and the first valence excited states of acetylene (³Σ⁺_u, ^1,3Σ⁻_u and ^1,3Δ_u) are investigated with the aid of ab initio methods applying large and flexible basis sets. Electron correlation has been taken into account on various levels of sophistication, including SCF CI, MC SCF and MC SCF CI methods. Equilibrium geometries are determined for all states mentioned, excitation energies, vertical and 0–0 transitions are reported and the dependence of the computed results on the type of wavefunction is discussed. Harmonic force constants and vibrational frequencies are calculated for all states treated in this paper and compared to experimental values.

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Ground and excited state geometries via Mukherjee's multireference coupled-cluster method
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Citation Excerpt :
Our results for acetylene in Section 3.2 explicitly show an important advantage of EOMEE-CCSD over Mk-MRCCSD. While we can confirm the earlier reported [70,72,78] energetic order for the low-lying singlet and triplet states, we find qualitative mismatches between the Mk-MRCCSD and EOMEE-CCSD results for the 1B2 and 1Bu excited states because more than one configuration contributes significantly to these states. EOMEE-CCSD is more convenient for such electronic states since it is not subject to any restriction with respect to the electronic structure of the target state as long as there is a well-defined reference state.
A comprehensive study of molecular equilibrium structures is conducted to benchmark the multireference coupled-cluster (CC) method suggested by Mukherjee and coworkers (Mk-MRCC). We determine equilibrium structures and adiabatic excitation energies by applying the Mk-MRCC method within the singles and doubles (SD) approximation to ground and excited states of various small and medium-sized molecules. The results are compared to those obtained using other multireference or single-reference CC methods. For most molecules with a multireference ground state, it is found that equilibrium structures and excitation energies computed at the Mk-MRCCSD, equation-of-motion CCSD, multireference averaged-quadratic CC, and CCSD(T) levels of theory qualitatively agree, while the single-reference CCSD method often leads to distinctly different results.
Conical intersections and strong nonadiabatic coupling effects in singlet-excited acetylene: An ab initio quantum dynamical study
2008, Chemical Physics
The conically intersecting potential energy surfaces of the S₁ and S₂ excited states of acetylene and the resulting strong nonadiabatic couplings are investigated theoretically. The adiabatic potential energy surfaces are obtained from high-level MRCI calculations. They are diabatized using the concept of regularized diabatic states and then used as a basis for the subsequent wave-packet dynamical treatment of the nuclear motion. All three angular degrees of freedom are included in the present study, while the bond lengths are kept frozen. The importance of the nonadiabatic interactions for the fine structure of the VUV spectrum of acetylene in the 6.5–8 eV excitation energy range is established. The electronic populations display an S₂ → S₁ internal conversion process on the order of 50 fs, which is, however, incomplete owing to the relatively small S₂–S₁ energy gap and the present reduced-dimensionality treatment. Snapshots of the wave-packet as well as angular probability densities are analyzed and reveal, for the first time, an incipient excited-state cis–trans isomerization in this system.
Ab initio non-adiabatic coupling elements: Conical intersections of the C<inf>2</inf>H<inf>2</inf> molecule
2005, Chemical Physics Letters
Citation Excerpt :
In this Letter, we report on a preliminary study of the NACTs and the ci-points related to the four lower states of this molecular system with symmetry of 1A′. Other than being the natural extension of the C2H radical, acetylene is considered one of the more fundamental molecules in chemistry and as such was exposed to numerous spectroscopic [4–8] and theoretical studies [9–15]. As far as the cis are concerned we mention the recent experiment by Wittig and co-workers [8] who studied the photo-dissociation of acetylene and verified the tendency of this process to form C2H(A2Π).
In this Letter, we report on ab initio non-adiabatic coupling terms, the relevant conical intersection (ci)-points and the corresponding adiabatic potentials as obtained for the C₂H₂ (acetylene) molecule. The emphasis in this study is on the region, where the four lower A′-states of this molecule form three (in fact four) cis all located close to each other so that if the molecule is excited to the fourth state it may decay to its ground state without the intervention of spin–orbit couplings.
Zeeman quantum beat observed by IR-UV double resonance LIF spectroscopy of acetylene in the Ã1A<inf>u</inf> 3ν<inf>3</inf>′+ν<inf>6</inf>′ and 3ν<inf>3</inf>′+ν<inf>4</inf>′ ungerade vibrational states
2001, Chemical Physics Letters
Citation Excerpt :
The S1–Tn coupling in the energy region around the 3ν3′ state was discussed also by ab initio MO calculations. Lischka and Karpfen [12] showed that the curve crossing between the singlet S1 and the triplet T3 occurs in a planar geometry. On the other hand, based on the calculation of the transition state energies for the trans-to-cis isomerization reaction on the T1, T2, and T3 surfaces, Schaefer and co-workers [13] concluded that the triplet T3 electronic state couples at first with the singlet S1 electronic state in a geometry near the half-linear configuration.
It has been found that rotational levels of the ungerade 3ν₃^′+ν₆^′ vibrational state of acetylene in the $A ̃^{1} A_{u}$ state split into two or more levels, while those of the 3ν₃^′+ν₄^′ state indicate no splitting. The level splitting is attributed to the S₁–T_n coupling which has been proved by observation of the Zeeman quantum beat in the fluorescence decay from the split level under a weak magnetic field. From the present finding that the S₁–T_n coupling occurs only in the combination state of the in-plane ν₆^′ mode with the 3ν₃^′ state, it is concluded that the S₁ state couples with the T_n state in a C_2h planar geometry.
Complete basis set, G1, G2, G2MP2, and density functional theory computational studies of the lowest energy triplet potential energy surface for acetylene-vinylidene rearrangement
2000, Journal of Molecular Structure: THEOCHEM
Citation Excerpt :
From the spectroscopic data it was estimated that the reaction is 46.5±5.5 kcal/mol endothermic [7]. Computationally, the vinylidene–acetylene isomerization has been extensively studied for the last two decades [9–21], and there are several interesting points that can be drawn from the literature. One of the first computed activation barriers is with the HF ab initio method, and this activation barrier is relatively high.
The triplet acetylene–vinylidene potential energy surface was explored with the complete basis set, G1, G2, and G2MP2 ab initio methods, as well as with the hybrid, gradient-corrected, and local spin approximation density functional methods to explore the possibility of triplet acetylene transformation in metastable vinylidene. For stationary points (triplet trans-acetylene, triplet cis-acetylene, H and CCH radicals, and triplet vinylidene) in their minima with several transition state structures that combine this minima were located on the potential energy surface. It was estimated that a global minima on the potential energy surface is the triplet cis-acetylene. Even if the triplet trans-acetylene is formed by excitation of the singlet acetylene, it should be transferred into the more stable triplet cis-acetylene without barrier. Triplet cis-acetylene and triplet vinylidene have almost identical stabilities, but the activation barrier for the formation of triplet vinylidene from triplet cis-acetylene through the 1,2-hydrogen shift is estimated to be 50.4 kcal/mol. The C–H dissociation–recombination mechanism for the triplet cis-acetylene transformation into triplet vinylidene is even higher, suggesting that direct transformation of acetylene into vinylidene through acetylene excitation and then relaxing the formed products should not be an energetically preferable path. Detailed analysis of the triplet acetylene–vinylidene potential energy surface with both the ab initio and density functional theory methods were discussed.
The electronic states of propyne studied by optical (VUV) absorption, near-threshold electron energy-loss (EEL) spectroscopy and ab initio multi-reference configuration interaction calculations
1999, Chemical Physics
The VUV absorption and EELS spectra of propyne (CH₃CCH) have been reinvestigated, and reassigned in the light of multi-reference multi-root CI calculations of quadruple zeta quality containing both valence and Rydberg type functions. While the Rydberg states were considered as linear, and hence studied as vertical excitations, the known bending of valence states in alkynes led to a parallel investigation of the equilibrium structures of the lowest singlet and triplet valence states of cis and trans type in C_s symmetry, by GVB calculations. Some of the rotamers from these bent states are clearly saddle-points. The calculated vibration frequencies of the ground state have been compared with experiment.

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Chemical Physics

Abstract

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Cited by (71)

Ground and excited state geometries via Mukherjee's multireference coupled-cluster method

Conical intersections and strong nonadiabatic coupling effects in singlet-excited acetylene: An ab initio quantum dynamical study

Ab initio non-adiabatic coupling elements: Conical intersections of the C<inf>2</inf>H<inf>2</inf> molecule

Zeeman quantum beat observed by IR-UV double resonance LIF spectroscopy of acetylene in the Ã<sup>1</sup>A<inf>u</inf> 3ν<inf>3</inf><sup>′</sup>+ν<inf>6</inf><sup>′</sup> and 3ν<inf>3</inf><sup>′</sup>+ν<inf>4</inf><sup>′</sup> ungerade vibrational states

Complete basis set, G1, G2, G2MP2, and density functional theory computational studies of the lowest energy triplet potential energy surface for acetylene-vinylidene rearrangement

The electronic states of propyne studied by optical (VUV) absorption, near-threshold electron energy-loss (EEL) spectroscopy and ab initio multi-reference configuration interaction calculations