Convergent energies and anharmonic vibrational spectra of Ca2H2 and Ca2H4 constitutional isomers

Abstract

Three constitutional isomers of both Ca₂H₂ and Ca₂H₄ have been characterized with molecular electronic structure theory. Correlation methods as complete as CCSDT(Q) and basis sets as large as cc-pwCV5Z have been used to converge the relative energies within chemical accuracy (≤1 kcal mol⁻¹). Anharmonic vibrational frequencies were computed using second-order vibrational perturbation theory employing CCSD(T)/cc-pwCVTZ cubic and quartic force-fields and a CCSD(T)/cc-pwCVQZ quadratic force field. The monobridged [Ca(μ₂-H)CaH] and dibridged [Ca(μ₂-H)₂Ca] isomers of Ca₂H₂ were predicted to lie 6.5 and 12.9 kcal mol⁻¹ below the energy of the classical HCaCaH linear isomer, respectively. Despite the energetic favorability of the bridged Ca₂H₂ isomers, we conclude (surprisingly) that only the higher energy linear structure has been observed in the laboratory. At 0 K, the tribridged [Ca(μ₂-H)₃CaH] isomer of Ca₂H₄ is predicted to be enthalpically favored by 0.9 kcal mol⁻¹ in comparison to the enthalpy of the dibridged [HCa(μ₂-H)₂CaH] structure. Comparison of experiment with our computed frequencies suggests that the observed vibrational features arise from both the dibridged and the tribridged Ca₂H₄ structures.