The photoelectron spectrum of the isopropoxide anion: Nonadiabatic effects due to conical intersections and the spin-orbit interaction

Abstract

The negative ion photoelectron spectrum of the isopropoxide anion is determined using the multimode vibronic coupling approach. The simulated spectrum is based on a two state quasidiabatic Hamiltonian for the isopropoxy radical, H(d), which includes all terms through second-order order in all internal coordinates and accurately represents the vicinity of the ab initio determined equilibrium geometry of the ground electronic state as well as the minimum energy crossing point (MECP) on the symmetry-allowed (2)A(")-(2)A(') accidental seam of conical intersection. H(d) is centered at the (2)A(")-(2)A(') MECP and is determined from ab initio gradients and derivative couplings using a normal equations based algorithm. Spin-orbit effects are included using a generalization of a procedure due to Child and Longuet-Higgins. The nonadiabatic interactions coupling the (2)A(") and (2)A(') states are very similar to those found in an Exe Jahn-Teller system although the requisite symmetry is absent. The simulated photoelectron spectrum for isopropoxide-h(7) is compared to a measured photoelectron spectrum and the results of a dispersed fluorescence experiment on the isopropoxy radical. The nominal A (2)A-X (2)A splitting of 68 cm(-1) from the dispersed fluorescence experiment is confirmed. This splitting is shown to be a consequence of the accidental Jahn-Teller symmetry and the spin-orbit interaction so that the standard designation of this spectral feature as the A-X splitting is not appropriate. This spectral feature is better thought of as the spin-orbit splitting of a nearly degenerate ground state. It is further shown that the intensities and line positions are such that the origin band of the nominal A (2)A state and that of the X (2)A state could not be distinguished with the resolution available in the photoelectron experiment. The photoelectron spectrum of the completely deuterated analog, isopropoxide-d(7), is also reported and discussed.