Electronic Control of Molecular Configuration Instability via Vibronic Coupling. Pseudo Jahn-Teller Stabilization of Vertically Excited States of F2CO, N2H2 and H2C2O Molecules

Abstract

Abstract. The pseudo Jahn-Teller effect is employed to explain the origin of distortions of carbonyl fl uoride, diazene and ketene molecules in the lowest singlet and triplet excited states. The ground state and the excited electronic states of considered molecules were calculated by ab initio SCF CI method with the use of the 631G-type basis set. The corresponding pseudo Jahn-Teller coupling constants are estimated by fi tting the ab initio data for the adiabatic potential curves of the molecules to the general formula, obtained from the vibronic theory. With these data an explanation of both the larger instability of the triplet excited states as compared with the singlet ones and the elongation of the corresponding bond distances in the excited states is obtained.Keywords: Pseudo Jahn-Teller effect, stereochemistry, excited states, carbonyl fl uoride, diazene, ketene. IntroductionThe stereochemistry of molecular systems is determined by their electronic structure and the permutation symmetry of identical atoms. The latter means that from the viewpoint of symmetry any molecule must possess the spatial structure with the highest possible symmetry. However, depending on the electronic state the high-symmetry nuclear confi guration can become unstable, and the molecule is distorted. Any perturbation of molecular system (excitation, ionization, coordination to transition metal complexes etc.) changes its electronic state and, as a consequence, may lead to distortion of its nuclear confi guration.The problem of electronic control on nuclear confi guration instability has already some history. In the case of degenerate electronic states the confi guration instability is a direct consequence of the Jahn-Teller effect (See, e.g. in [1][2][3]). It has also been shown that the only reason of instability of the high-symmetry nuclear confi gurations and structural distortions of any molecular system in the non-degenerate state is the pseudo Jahn-Teller effect (PJTE), that is the vibronic mixing of considered state with the appropriate by symmetry excited states [1,2,[4][5][6][7]. Hence the instabilities should be sought for in the electronic states which mix strongly under nuclear displacements, and vice versa. If there is instability, there should be corresponding excited states that cause the instability of the reference confi guration. The electronic control of the instabilities via vibronic coupling has been considered in detail in a series of papers [4][5][6][7][8][9][10][11][12][13].One of the examples of electronic control on nuclear confi guration instability is the stabilization of vertically excited states. The available experimental and theoretical data indicate that in the electronically excited states molecules may adopt equilibrium geometries very different from those in the ground states. Moreover, the distortions of some molecules in the lowest triplet state are stronger than in the excited singlet state of the same symmetry. In [14] the pseudo Jahn-Teller effect was used to explain the origin of the carbo...

Recent advances in ketene chemistry

“…16 Electron scattering by ketene has been studied by computational methods using the R-matrix method for energies ranging from 0 to 10 eV, 17 and the calculated vertical excitation energies of the first two excited states are in good agreement with experimental results. The electron scattering calculations predict two π* shape resonant states, one core-excited shape resonant state and one Feshbach resonant state.…”

Structural Distortions of Coordinated Ketene Molecule Induced by the Pseudo Jahn-Teller Effect

“…The second triplet excited 3 A 1 state is stabilized by the out-of-plane distortion to the pyramidal nuclear confi guration also of C s symmetry, but the refl ection plane in this case is perpendicular to the molecular plane of the C 2v structure (Figure 1e). Previously [19] we have shown that instability of the high-symmetry C 2v nuclear confi guration of ketene molecule in the 3 A 2 and 3 A 1 excited states and the corresponding low-symmetry in-plane and out-of-plane distortions are due to the pseudo Jahn-Teller mixing of these states with higher excited 3 B 1 state. From this statement, some conclusions can be drawn, which may be useful in the analysis of distortions of coordinated ketene molecule.…”

Inversion isomerism induced by the Pseudo Jahn-Teller Effect in X2CE molecules (X = H, F; E = O, S, Se) in their reduced and excited states

“…A more thorough analysis of the origin of stabilization of the vertically excited states of carbonyl fluoride molecule F2CO in its first excited singlet and triplet electronic states based on the numerical estimations of the PJTE parameters was performed in Ref. [27]. The analysis confirmed that this distortion is produced entirely by the vibronic mixing of these states with the higher excited states of appropriate symmetry.…”