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M. L. SENENT, Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia, C.S.I.C., Madrid 28006, Spain.; M. HOCHLAF, Université Paris-Est, Laboratoire de Modélisation et Simulation Multi Echelle (FRE 3160 CNRS), Champs sur Marne, F-77454 Marne-la-Vallée,Cedex 2, France.; H. MASSÓ, Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia, C.S.I.C., Madrid 28006, Spain.; N. INOSTROZA, Universidad Andrés Bello, Departamento de Química, Facultad de Ecología y Recursos Naturales, Santiago, Chile..
Structures, vertical excitation energies to the first electronic states and spectroscopic parameters for the ground electronic state are determined for several small Cn, CnH and CnSi astrophysically important molecular species \footnoteH.~Massó, M.L.~Senent, P.~Rosmus and M.~Hochlaf, J.Chem.Phys. \underline\textbf124, 234304 (2006); M.L.~Senent H.~Massó and M.~Hochlaf, ApJ , \underline\textbf670, 1510 (2007); H.~Massó, V.~Veryazov, P.A.~Malmqvist, B.O.~Roos and M.L.~Senent, J.Chem.Phys. \underline\textbf127, 154318 (2007); M.~Hochlaf, C.~Nicolas and L.~Poisson, J.Chem.Phys. \underline\textbf127, 014310 (2007) N.~Inostroza, M.~Hochlaf, M.L.~Senent and J.R.~Letelier (submitted 2008). The main aim of these investigations is to help the interpretation of several previous astrophysical observations \footnoteJ.~Goicoechea, J.~Cernicharo, H.~Massó and M.L.~Senent, ApJ. \underline\textbf609, 225 (2004); J.~Cernicharo, J.R.~Goicochea and Y.~Benilan, ApJ \underline\textbf580, L157, 2002. State-of-the art ab initio calculations are performed for this purpose. All species display isomerism although the inter-transformation processes, that involve various electronic states, are not well known. Anharmonic spectroscopic parameters are computed from full-dimensional Potential Energy Surfaces which are mapped close to their respective local minima. In light of these calculated properties (band positions, spectroscopic parameters, predictions of ro-vibronic effects, spin-spin constants,..) assignments for FIR astrophysical observed bands, are discussed. vspace1em \noindent