Novel Effect Induced by Pseudo-Jahn-Teller Interactions: Broken Cylindrical Symmetry in Linear Molecules

摘要

It is shown that in linear molecules the pseudo-Jahn-Teller (PJT) interaction of a E or II term with a A term induces a bending instability that is angular dependent, reducing the symmetry of the adiabatic potential energy surface from expected D_(∞h) to D_(4h) and C_(∞v) to C2_(v )or C_(4v). This spontaneously broken cylindrical symmetry (BCS) emerges from the solution of the vibronic coupling equations of the PJT effect (PJTE) problems (E+A)?w, (II+A)?w, (II+I+A)?w, and (A+A)?w, where w includes linear, quadratic, and fourth order vibronic coupling terms, and it is confirmed by ab initio calculations for a series of triatomic molecules with ground or excited A terms. The BCS is due to the angular symmetry of the electronic wave functions of the Δ term, ~cos 2φ, and ~sin 2φ, split by the fourth order vibronic coupling, which in overlap with the other symmetry wave functions of the ￡ or II term provides for the periodical symmetry of the added covalency that facilitates the bending. The mechanism of this PJT-induced BCS effect is discussed in detail; the numerical values of the vibronic coupling parameters for the molecules under consideration were estimated by means of combining separate ab initio calculations of some of them with a procedure fitting the analytical expressions to ab initio calculated energy profiles. It is also shown that the bending of linear molecules in A states, similar to II states, is exclusively a PJT (not Renner-Teller) effect. The BCS revealed in this paper illustrates again the predicting power of the PJTE.