Pseudo Jahn–Teller origin of bending instability of triatomic molecules

For several decades the bending of linear molecules in degenerate states has been assumed to be due to the Renner–Teller effect (RTE), whereas the pseudo Jahn–Teller effect (PJTE) has been completely ignored. But in a recent publication of ab initio calculations performed on a series of triatomics it was shown that the contribution of the PJTE to their bending instability is much larger than that of the RTE. This raised the question of whether the finding of the essential role of the PJTE in the instability is a particular case for the molecules studied or a general trend. The problem is important not only for rationalizing the experimental observations and ab initio calculations of such systems, but also for a correct interpretation of their spectra, as the PJTE, by admixing excited states of different symmetry (opposite parity) to the ground one, influences significantly their spectroscopic properties. To contribute to the solution of this problem we performed ab initio   calculations of electronic structure and energy profiles of a group of triatomic systems including CH2+,H2O+,SiH2+,PH2, and HNF which have a Π twofold degenerate state in the linear configuration, but are bent in the equilibrium geometry. For them we formulated the combined RTE and PJTE problem and, by calculating ab initio the Renner–Teller vibronic coupling constant g and estimating the other parameters by fitting the PJTE equations to the ab initio calculated energy profiles, we show that in all the cases under consideration the PJTE contribution is significantly larger than that of the RTE. The parameters of the equilibrium geometry and electronic structure of these systems obtained in our calculations are in good agreement with those reported in other publications.