The excited and ion states of Allene

Large scale ab initio calculations for the excited states of C3H4 and C3H4+ are reported here for the vertical ionization potentials (VIPs), excitation energies, and oscillator strengths using the IVO-CASCI and third order effective valence shell Hamiltonian (Hν) methods. The results are in excellent agreement with experiment and with those predicted by the MRSDCI and ADC(4) methods. The potential energy surfaces (PES) of C3H4+ show the presence of a conical intersection between the X2E and A2E states near 12.3 eV and a 145° dihedral angle, which strongly suggests that the experimental peak at 12.7 eV is most likely non-vertical in origin and is a shake-up type state. A similar conical intersection is also observed in the PES of C3H4 but at a relatively small dihedral angle (105°). We provide computations for quartet ion states, triplet neutral states, and potential curves for the twisting of neutral allene, quantities that have not previously been reported.