Jahn-Teller Distortions in Molecules Containing Group 15 Elements

Abstract

Small molecules containing group 15 elements exhibit specific structural characteristics due to the group 15 atom acting as a center for symmetry reduction. It is well known that high symmetry structures with degenerate electronic states can undergo symmetry breaking via a first order Jahn-Teller effect. However, not so well understood is symmetry breaking when the electronic state is non-degenerate. Second order Jahn-Teller effects are a specific subset of such general vibronic coupling interactions and occur frequently in group l5 substituted systems. The trigonal pyramidal ground state of the EX3 molecules E=N, P, As, Sb and Bi and X=F, C1, Br and I is of C3v symmetry, reduced from trigonal planar D3h symmetry by a second order Jahn-Teller effect which couples the low energy A1' symmetry state with an excited A2" symmetry state via the a2" mode of vibration. Conversely, the EX3 molecules are capable of undergoing inversion from the C3v symmetry ground state through a D3h symmetry transition state. This D3h symmetry state exhibits significant configuration interaction. Furthermore, the symmetry of the inversion transition state can be reduced to C2V by a second order Jahn-Teller effect which couples the low energy D3h symmetry A1'electronic state with a higher energy E' state via two available e' modes of vibration. The transition state structure for inversion can therefore be of D3h or C2v symmetry depending on the magnitude of Jahn-Teller coupling. In this work the energy barrier to inversion is determined for the EX3 molecules and trends elucidated with respect to variation in the central group l5 element and group l7 ligands. Substitution of a group l5 element into an aromatic ring system can result in a planar or distorted ring structure depending on the relative magnitude of the aromatic or Jahn-Teller stabilisation effects. In this work the higher group 15 analogs of pyridone and hydroxy-pyridine are examined with respect to aromaticity and structure. Inversion potential energy surfaces are characterised for those keto-tautomers which become distorted. The effect of group l5 atom substitution on the ketoenol tautomeric equilibrium is investigated.