SIMBAD references

Highly correlated ab initio calculations are employed for the structural and spectroscopic characterization of small odd chains of type C_2n+1H, considering neutral forms, cations, and giving special attention to the anions. This work confirms the stability of the linear carbon chains and carbon clusters containing three-body rings. The smallest species, C₃H, displays three stable structures, whereas C₅H possesses at least 8 neutral isomers and 11 and 10 isomers with a negative or a positive charge. The equilibrium geometries, which can be candidates for laboratory and astrophysical detection, are studied using the RCCSD(T)-F12 and MRCI/CASSCF levels of theory, specifying properties for various electronic states. Four different stable isomers are confirmed for the C₅H^– anion. They are two rings and two chains, all showing singlet ground electronic states. The viability of the triplet linear form of C₅H^– (C_∞v_(X³Σ^–)) postulated in previous works, is not confirmed because it appears to be really dependent on the electron correlation energy denoting instability. A quasi-linear singlet (C_s (X¹A')) represents a secondary minimum. Electronic state crossing occurs close to the linear structure where spin-orbit effects are negligible. The most stable structure of C₅H^– is a three-carbon cycle in which rotational constants have been determined to be A₀ = 35479.86 MHz, B₀ = 3618.29 MHz, and C₀ = 3280.10 MHz. Its dipole moment is relatively large (6.4086 D).