Theoretische Untersuchung des Renner–Teller Effekts in asymmetrischen vieratomigen Molekülen

Abstract

In this work the vibronic energy levels are calculated for the two lowest lying Renner-Teller (RT) coupled electronic states of the ketenyl radical HCCO and the photodetachment spectrum of the corresponding anion is simulated. he splitting of potential energy surfaces (PESs) of degenerated electronic states for the transition from linear to bent molecular geometry and the resulting interaction of the electronic states is called Renner-Teller effect. The effect exhibits an irregular order of the vibronic energy levels. From the mathematical point of view, the condition for Born-Oppenheimer approximation is not fulfilled and nonadiabatic coupling have to be taken into account. For this reason a generalisation of the ansatz of Pople and Longuet-Higgins (from triatomic molecules to asymmetric tetra-atomic molecules) for the Renner-Teller effect has been worked out. The coupling of the electronic states is considered by means of the difference of adiabatic PESs. The developed model is applied to the HCCO radical including the calculation of multidimensional PESs. The comparison of calculated vibronic energy levels with didactic models shows, how the RT effect influences the energy levels and which energy levels undergo level shifts or level splittings. Also the magnitude of the observed level splittings (up to 80 wave numbers) can be determined. Furthermore, the simulation of the photodetachment spectrum of the HCCO anion using the Franck-Condon approximation is compared with experimental investigations of Neumark and coworkers. Most experimental peaks can be assigned to transitions to vibronic states of HCCO and the photodetachment spectrum of HCCO anion can be understood. The barrier height to linearity, which is highly sensitive to basis set and method, can be restricted to values between 700 wave numbers and 900 wave numbers. The neglected couplings between bending mode and stretching mode are linked with the differences in the intensity profiles between theoretical and experimental spectrum and show the direction for further developments.