Alternation of Singlet and Triplet States in Carbon-Based Chain Molecules and Its Astrochemical Implications: Results of an Extensive Theoretical Study

A variety of homologous carbon chains (HCnH, HCnN, CnS, CnO, and OCnO) are found to exhibit an appealing even-odd effect. Chains containing a number of carbon atoms of a certain parity possess singlet ground states, while members of opposite parity have triplet ground states. From a general perspective, it is important that this even-odd effect confounds straightforward chemical intuition. Whether the most stable form is a triplet or a singlet is neither simply related to the fact that the species in question is a "normal" (closed-shell, nonradical) molecule nor a (di)radical or to the (e.g., cumulene-type) CC bond succession across the chain. From a computational perspective, the present results are important also because they demonstrate that electron correlations in carbon-based chains are extremely strong. Whether the "gold-standard" CCSD(T) (coupled-cluster expansions with single and double excitations and triple excitations corrections) framework suffices to describe such strongly correlated systems remains an open question that calls for further clarification. Most importantly for astrochemistry, the present results may explain why certain members are not astronomically observed although larger members of the same homologous series are detected; the missing species are exactly those for which the present calculations predict triplet ground states.