Abstract

Reaction channels and spectroscopic properties of a series of silicon- carbon-bearing isomers of SiC$_{3}$H$^{+}$ and SiC$_{3}$H, which are suitable species for astrophysical detection in carbon-rich sources, are calculated with correlated ab initio CCSD(T) and density functional theory methods. We present four isomers of SiC$_{3}$H$^{+}$ for which the electronic ground states have closed-shell configurations. For SiC$_{3}$H, we considered the same structures in order to present a complete study. The global minimum among the SiC$_{3}$H$^{+}$ isomers corresponds to the rhomboidal structure with a transannular bond in a $^{1}$A$_{1}$ electronic state (rb3-SiC$_{3}$H$^{+}$ C$_{2v}$ X$^{1}$A$_{1}$). The next minima correspond to a second rhomboid $^{1}$A$_{1}$ isomer and a linear isomer (X$^{1}${\ensuremath{\Sigma}}$^{+}$) with relative energies 0.86 and 0.93 eV, respectively at the CCSD(T)/cc-pvTZ level of theory. The most stable mono- hydrogenated silicon carbon isomer is linear, followed by two rhomboidal isomers, rb2-SiC$_{3}$H and rb3-SiC$_{3}$H (0.23 and 0.31 eV). For each structure, a set of spectroscopic parameters including their equilibrium structures, rotational constants, harmonic frequencies and dipole moment is presented. Furthermore, we discuss plausible formation pathways of SiC$_{3}$H$^{+}$ isomers which are classified as charge- exchange, ion-neutral and dissociative recombination reactions. These results show one favourable pathway to produce rb3-SiC$_{3}$H$^{+}$ from rb-SiC$_{3}$-3s. The formation energy of the cation's isomers coming from neutral isomers as linear l1-SiC$_{3}$H, rb3-SiC$_{3}$H and rb2-SiC$_{3}$H plus H$^{+}$ as reactants (charge-exchange reaction) are 203.8 kcal mol$^{-1}$ (8.84eV), 175.4 kcal mol$^{-1}$ (7.60 eV) and 195.2 kcal mol$^{-1}$ (8.46 eV), which provides us with evidence of the endergonic character of these reactions. As a consequence, it does not seem to be feasible to produce a cation from neutral reactant plus H$^{+}$ by a charge-exchange reaction that was proposed by UMIST.