The Jahn-Teller and pseudo-Jahn-Teller effects in the propyne radical cation

The Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) effects in the X̃²E, òE and B̃²A₁ electronic states of the propyne radical cation are investigated with the aid of ab initio quantum chemistry calculations and first principles quantum dynamics simulations. For the latter, both time-independent and time-dependent quantum mechanical methods are employed. Standard vibronic coupling theory is used to construct a symmetry consistent vibronic Hamiltonian in a diabatic electronic basis. Taylor series expansion of the elements of the diabatic electronic Hamiltonian is carried out and the parameters that appear in the expansion are derived from the ab initio calculated adiabatic electronic energies. It is found that the JT effect is weak in the X̃²E state as compared to that in the òE state. Because of large energy separation, the PJT coupling among the JT-split components of the X̃²E state with the neighboring states is also very weak. However, the PJT coupling of the B̃²A₁ state with the JT split components of the òE state has some impact on the dynamics in the coupled òE-B̃²A₁ electronic states. The vibronic spectrum of each of these states is calculated and compared with the experimental results. The nonradiative internal conversion dynamics of excited cationic states is examined. Interesting comparison is made with the JT and PJT coupling effects in the nuclear dynamics of the X̃²E-òE-B̃²B₂ electronic states of the isomeric allene radical cation.