Vibronic Coupling Approach Research Articles

The simulated photoelectron spectrum of 1-propynide

The negative ion photoelectron spectrum of 1-propynide is computed by employing the multimode vibronic coupling approach. A three-state quasidiabatic Hamiltonian, H(d), is reported, which accurately represents the ab initio determined equilibrium geometries and harmonic frequencies of the ground X (2)A(1) state as well as the low-lying Jahn-Teller distorted components of the A (2)E excited state. It also reproduces both the minimum energy crossing point (MECP) on the symmetry-required (2)E(x)-(2)E(y) conical intersection seam and the MECP on the same symmetry (2)A(1)-(2)E(x) conical intersection seam. H(d) includes all terms through second order in internal coordinates for both the diagonal and off-diagonal blocks. It is centered at the (2)E(x)-(2)E(y) MECP and is determined using ab initio gradients and derivative couplings near both the (2)E(x)-(2)E(y) MECP and the X (2)A(1) equilibrium geometry. This construction is enabled by a recently reported normal equation based algorithm. The C(3v) symmetry of the system is used to significantly reduce the computational cost of the ab initio treatment. This H(d) is then expressed in a vibronic basis that is chosen for its ability to reduce the dimension of the vibronic expansion. The vibronic Hamiltonian matrix is diagonalized to obtain a negative ion photoelectron spectrum for 1-propynide-h(3). The determined spectrum compares favorably with previous spectroscopic results. In particular, the lines attributable to the (2)E state are found to be much weaker than those corresponding to the (2)A(1) state of 1-propynyl. This diminution of the (2)E state is attributable principally to the (2)E(x)-(2)A(1) conical intersection rather than an intrinsically small electronic transition moment for the production of the (2)E state.

Effects of multimode Jahn-Teller coupling on the photodetachment spectrum of nitrate anion ( [formula omitted

The multimode static and dynamic Jahn-Teller effects in the 2E″ and 2E′ excited states of the nitrate radical (NO 3) and their impact on the vibronic structure of the photodetachment spectrum of the nitrate anion are investigated theoretically. The analysis rests on the quadratic vibronic coupling approach and large-scale multi-reference configuration interaction calculations for the determination of the respective coupling parameters. The possible pseudo-Jahn-Teller interactions between the ground 2 A 2 ′ and excited 2E′ electronic states of NO 3 are also included in the vibronic coupling model. The results of the 2E″ state are compared with the experimental recording of Neumark and co-workers, while those for the 2E′ state come as a prediction. The deviations between theoretical and experimental vibronic structures of the 2E″ photoelectron band are discussed.

Multi-state vibronic interactions in the fluorobenzene radical cation: The importance of quadratic coupling terms

The multi-mode multi-state vibronic interactions in the set of X ∼ 2 B 1 – D ∼ 2 A 1 electronic states of the monofluoro benzene radical cation are investigated theoretically, based on a quadratic vibronic coupling approach. The underlying ionization potentials and coupling constants are obtained from ab initio coupled-cluster calculations. Previous investigations (relying on the linear coupling approach) are extended by including all (diagonal) quadratic coupling constants for the totally symmetric modes in the five coupled electronic states. The inclusion of these quadratic couplings is found to be essential to correctly reproduce the low-energy conical intersections between different sets of electronic states. The photoelectron, mass-analyzed threshold ionization and photoinduced Rydberg ionization spectra are re-analyzed. The lowering of the minimum energies of intersection is shown to be crucial to lead to an efficient population transfer from higher excited states of the cation to the ground state. The implications of these findings for understanding the fluorescence dynamics of fluorinated benzene radical cations are discussed.

Magnetic moments of multielectron mixed-valency dimer clusters in a dynamic intermediate vibronic coupling approach

The vibronic aspect of the Jahn-Teller effect is considered for multielectron mixed-valency dimer clusters. A basic intermediate-coupling set is used in the numerical diagonalization of the vibronic Hamiltonian. The temperature dependence of the vibronic magnetic moment is derived for symmetrical and distorted mixed-valency dimer clusters. Strong vibronic interaction can suppress double exchange and lead to an antiferromagnetic ground state for the cluster. Distortions that eliminate the inversion center localize the surplus electron and suppress the ferromagnetic effect caused by double exchange.

Calculation of Duschinsky effect in phenol

The mode mixing in the first electronic state of phenol, known as Duschinsky effect, responsible for dissymmetry between absorption and emission spectra, reduction of vibrational wavenumbers in S 1 and cross sequences, has been calculated by means of a vibronic coupling approach. Although the results reproduce qualitatively the observed trend, they also show that a more quantitative analysis requires more accurate starting force fields.

A vibronic coupling approach for the interpretation of the g-value temperature dependence in type-I copper proteins

The e.s.r. spectra of copper proteins containing type-I (pseudo-tetrahedral) centres show a gradual decrease of the g‖ values with increasing temperature. This fact is interpreted on the basis of a vibronic coupling mechanism which is effective in pseudo-tetrahedral copper(II) complexes, but not in the square-planar ones. A theory is developed to account for the shift of the g‖ values as well as for the d-d transition energies. A functional role of the vibronic coupling is also suggested.