Accurate ab initio predictions of ionization energies and heats of formation for cyclopropenylidene, propargylene and propadienylidene

The ionization energies (IEs) of cyclopropenylidene (c-C₃H ₂), propargylene (HCCCH) and propadienylidene (H₂CCC) have been computed using the CCSD(T)/CBS method, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasi-perturbative triple excitation effect (CCSD(T)). The zero-point vibrational energy correction, the core-valence electronic correction, the scalar relativistic effect and the high level correction beyond the CCSD(T) excitations have also been made in these calculations. The CCSD(T)/CBS values for the IE(C-C₃H₂) and IE(HCCCH) of 9.164, 8.987 eV are in good agreement with the experimental values of (9.15±0.03) and (8.96±0.04) eV. The CCSD(T)/CBS calculations yield the IE values of 10.477 and 10.388 eV for the ionization transitions H ₂CCC→H₂CCC⁺(²A₁, C_2v) and H₂CCC→H₂CCC⁺( ²A′, C_s). respectively. On the basis of the Franck-Condon factor consideration, the IE of (10.43±0.02) eV determined in the previous single-photon ionization experiment most likely corresponds to the ionization threshold for the H₂CCC→H₂CCC ⁺(²A₁, C_2v) transition. Although the precision of the experimental IE measurements for c-C₃H₂, HCCCH, and H₂CCC is insufficient to pin down the accuracy of the theoretical calculations to better than ±30 meV. the excellent agreement between the experimental and theoretical IE values observed in the present study indicates that the CCSD(T)/CBS calculations together with high-order correlation corrections are capable of yielding reliable IE predictions for simple hydrocarbon carbenes and bi-radicals. We have also reported the heats of formation at 0 K (ΔH°_f0) and 298 K (ΔH° ₂₉₈) for c-C₃H₂/c-C₃H ₂⁺, HCCCH/HCCCH⁺, and H₂CCC/H ₂CCC⁺. The available experimental ΔH° _f0 and ΔH°_f298 values for c-C₃H ₂/c-C₃H₂⁺, HCCCH/HCCCH⁺ are found to be in good accord with the CCSD(T)/CBS predictions after taking into account the experimental uncertainties. ©2006 Chinese Physical Society.