Highly Accurate Quantum-chemical Calculations For The Interstellar Molecules C3 And L-c3h+

Composite potential energy surfaces with coupled-cluster contributions up to CCSDTQP were constructed for C$_3$ and l-C$_3$H$^+$ and used in the calculation of spectroscopic properties. The use of very large AO basis sets and the consideration of higher-order correlation beyond CCSD(T) is of utmost importance for C$_3$ in order to arrive at quantitative spectroscopic data. The first detection of l-C$_3$H$^+$ in the interstellar medium was reported by Pety et al.,\footnote{J. Pety, P. Gratier, V. Guzm\'{a}n, E. Roueff, M. Gerin et al., \textit{Astron. Astrophys.} \textbf{2012}, \textit{A68}, 1-8.} who attributed 9 radio lines observed in the horsehead photodissociation region to that species. That assignment was questioned by the recent theoretical work of Huang et al.\footnote{X. Huang, R. C. Fortenberry, T. J. Lee, \textit{Astrophys. J. Lett.} \textbf{2013}, \textit{768:L25}, 1-5.} However, our more accurate calculations are well in support of the original assignment. The calculated ground-state rotational constant is $B_0 = 11248$~MHz, only $0.03\%$ off from the radio astronomical value of $11244.9512\pm0.0015$~MHz. The ratio of centrifugal distortion constants D$_0$(exp.)/D$_e$(theor.) of 1.8 is quite large, but reasonable in comparison with C$_3$O\footnote{P. Botschwina, R. Oswald, \textit{J. Chem. Phys.} \textbf{2008}, \textit{129}, 044305.} and C$_3$