Accurateab initioPredictions of Ionization Energies and Heats of Formation for Cyclopropenylidene, Propargylene and Propadienylidene

Abstract

The ionization energies (IEs) of cyclopropenylidene (c-C3H2), 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 H2CCC H2CCC+(2A1, C2v) and H2CCC H2CCC+(2A0, Cs), respectively. On the basis of the FranckCondon 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, C2v) 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 (Hof0) and 298 K (Hof298) for c-C3H2/c-C3H2+, HCCCH/HCCCH+, and H2CCC/H2CCC+. The available experimental Hf0o and Hof298 values for c-C3H2/c-C3H2+, HCCCH/HCCCH+ are found to be in good accord with the CCSD(T)/CBS predictions after taking into account the experimental uncertainties.