The Jahn–Teller effect in the lower electronic states of benzene cation. I. Calculation of linear parameters for the e[sub 2g] modes.

Jahn-Teller (JT) coupling parameters can be simply derived from features of the potential energy surfaces of the JT active vibrational modes of a molecule subject to that effect. Potential energy curves representing cuts of D[SUB2h] symmetry have been calculated using density functional theory for each of the four active e[SUB2g] modes of benzene cation in each of the lowest three degenerate electronic states. From these curves and the vibrational frequencies, the JT coupling parameters can be found (vibrational numbering follows Wilson's notation, coupling parameters are in Moffit's notation). For the C[SUB6]H[SUP+,SUB6]+&Xtilde;[SUP2]E[SUB1g] state the largest JT coupling parameter is provided by mode 6 (D = 0.48), followed closely by mode 8 (0.30) and then mode 9 (0.10). Mode 7 provides almost no stabilization. For the [SUP2]E[SUB2g] state, calculated coupling parameters are very large, particularly for modes 8 (1.36) and 6 (0.93). Modes 7 (0.10) and 9 (0.07) are smaller but finite. For the &Dtilde;[SUP2]E[SUBlu] state, mode 6 has an impressive linear coupling parameter of 4.12, able to support several vibrations below the JT cusp. Indications of a substantial quadratic stabilization for that mode result in the possibility of a completely static distortion in that state. Similar, but not identical, results are obtained for C[SUB6]D[SUP+,SUB6] . [ABSTRACT FROM AUTHOR]