Ab initio calculations of vibronic coupling. Applications to symmetry-forbidden vibronic spectra and internal conversion in ethylene

A formalism for the calculation of vibronic coupling and transition dipole moments for symmetry-forbidden transitions has been presented. The approach involves ab initio calculations of vibrational frequencies, normal coordinates, transition moments for allowed transitions and transition matrix elements over the one-electron electric field operator at the atomic centers. The approach is applied to the calculations of vibronic spectra for the π-3p transitions in ethylene. The vibronic coupling is used to calculate the rates of internal conversion between the π-3p and π-3s states of C2H4 which are found to be in the 109–1011 s−1 range.