VIBRONIC COUPLING IN LINEAR-MOLECULES AND LINEAR-TO-BENT TRANSITIONS - HCN

The hamiltonian describing vibronic coupling between Π and Σ electronic states of linear molecules is derived. In contrast to the well-known Renner-Teller coupling of the two components of a Π electronic state, the Σ-Π vibronic coupling term is linear in the bending coordinate. The symmetry properties of the hamiltonian are analyzed. A numerical procedure which allows the calculation of the spectrum (e.g. the absorption or the photoelectron spectrum) for arbitrary values of the coupling constant is presented. The adiabatic potential energy curves are investigated. A nonlinear equilibrium geometry results for the lowest of the interacting states if the vibronic coupling is strong enough. Intensity borrowing effects and non-adiabatic effects due to the vibronic coupling are qualitatively discussed and the importance of the simultaneous coupling to totally symmetric vibrational modes is pointed out. The inclusion of the totally symmetric modes influences the spectrum in a nontrivial way and is essential for descr in the photoelectron spectrum of HCN and DCN. The energies and coupling constants entering the hamiltonian are calculated by ab initio methods including the effects of electron correlation. The ab initio calculation predicts strong vibronic interaction between the closely spaced 2Π and 2Σ states responsible for the first band in the photoelectron spectrum. By slightly readjusting some of the parameters excellent agreement between the computed and the experimental spectrum is obtained, confirming that the very complex structure of the spectrum can be fully understood in terms of Σ-Π vibronic coupling and excitation of the CN stretching mode. The second example considered is the 9 eV band in the absorption spectrum of HCN and DCN. The vibronic coupling mechanism explains the experimentally observed strong excitation of the bending mode and the complexity of the vibrational structure of this band. © 1979.