Jahn-Teller effect in the magnesium phthalocyanine anion

We present an ab initio study of the static Jahn–Teller (JT) effect in the magnesium phthalocyanine anion (MgPc−). The neutral MgPc has D4h symmetry with a doubly degenerate eg affinity level and 14 B1g plus 14 B2g JT active modes. MgPc− is thus a physical realization of a eg ⊗ (B1g ⊕ B2g) JT problem. We carried out a first principles DFT/LDA calculation of the electronic structure, full vibrational spectrum, and linear electron–vibration coupling constants for this system. From that we calculate a static JT energy gain of 50 meV – essentially due to a B1g distortion – characterizing this as a relatively weak-coupling system. It is anticipated that quantum mechanical dynamic JT tunneling between the two equivalent B1g distorted JT minima should be quite important. Two B2g distorted saddle points are identified as the relevant transition states, with a barrier of 27 meV above the minima. Previous work on this JT problem is also addressed.