Clues to NaCN formation

Collision excitation of c-C3H-(X1A1) by He

Accurate modeling of anionic abundances in the interstellar and circumstellar media requires calculations of collisional data with the most abundant species that are usually He atoms and H₂ molecules. In this paper, we focus on smaller cyclic molecular anion, c-C₃H^-, an astrophysical candidate, following the detection of larger C_nH^- carbon chains. From a new three-dimensional potential energy surface, the rotational (de-)excitation of the c-C₃H^-(X¹A₁) anion by collision with He is investigated. The surface is obtained in the supermolecular approach at the CCSD(T)-F12/aug-cc-pVTZ level of theory. Fully quantum close-coupling calculations of inelastic integral cross sections are performed on a grid of collisional energies large enough to ensure the convergence of the state-to-state rate coefficients for the 34 first rotational levels up to j_Ka,Kc = 7_7,0 of c-C₃H^- and temperatures ranging from 5 to 100 K. For this collisional system, rate coefficients exhibit a strong dominance in favor of 2_1,2 → l_1,1 downward transition. This transition was previously used for the detection of the cyclic parent c-C₃H. The c-C₃H^--He rate coefficients (∼10^-11 cm³ s^-1) are of the same order of magnitude as those of the detected anions C_nH^- (as C₂H^-, C₄H^-, and C₆H^-) in collision with He and one order of magnitude smaller than those with H₂. The critical densities of H₂ were also estimated, and a discussion on the validity of the local thermodynamic equilibrium conditions is carried out. This work represents the contribution to understanding and modeling abundances and chemistry of hydrocarbon radicals, C_nH, in astrophysical media.

Alkaline and Alkaline-earth Cyanoacetylides: A Combined Theoretical and Rotational Spectroscopic Investigation

The metallic cyanoacetylides LiC3N, NaC3N, MgC3N and CaC3N have been investigated by combined spectroscopy measurements and theoretical calculations. The theoretical calculations predict for the four species that the linear isomer with formula MCCCN (M= Li, Na, Mg and Ca) is the most stable one. We used the laser ablation molecular beam Fourier transform microwave spectroscopy to synthesize these species by the reaction of metal vapors, produced by laser ablation, and the 3-bromo-2-propynenitrile (BrCCCN). The pure rotational spectra were observed by Fourier transform microwave spectroscopy in the 2-18 GHz frequency region only for LiCCCN and NaCCCN, while no spectral signatures for MgCCCN and CaCCCN could be detected. Finally, we have searched for LiCCCN and NaCCCN species towards the carbon-rich evolved star IRC + 10216 but only upper limits to their abundances have been obtained.