Photodissociation Dynamics of the Zn⁺(Acetylene) and Zn⁺(Ethylene) Cation-π Complexes

Zn⁺(acetylene) and Zn⁺(ethylene) ion-molecule complexes are investigated in the gas phase with selected-ion photofragment imaging. UV photodissociation produces respectively Zn⁺ and C₂H₂⁺ or Zn⁺ and C₂H₄⁺ fragment channels, revealing both simple bond cleavage and charge-transfer dissociation in these complexes. Imaging of each fragment channel reveals considerable kinetic energy release (KER), which provides upper limits on the bond dissociation energies (BDEs): D₀ ≤ 1.04 ± 0.20 eV (24.0 ± 4.6 kcal/mol) for Zn⁺-(C₂H₂) and D₀ ≤ 0.82 ± 0.18 eV (18.9 ± 4.2 kcal/mol) for Zn⁺-(C₂H₄). Agreement with previous data from spectroscopic measurements on Zn⁺(C₂H₄) suggest that these upper limits are near the true BDE for each of these complexes. Density functional theory (DFT) calculations explore the bonding and structures of the Zn⁺(C₂H₂) and Zn⁺(C₂H₄) ions, employing the B3LYP, M06, M06-L, and MN15-L functionals. Time-dependent DFT (TD-DFT) computations at the B3LYP/def2-QZVP level characterize the excited states of these complexes. Zn⁺(C₂H₂) dissociates via absorption to the bound ²B₂ excited state followed by curve crossing to the ²A₁ charge-transfer excited state, whereas Zn⁺(C₂H₄) dissociates via direct excitation of the charge-transfer state.