  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) C-C 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.
