A b i n i t i o calculations on HC+5, a cation of interest to astrochemistry

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Abstract
HC+5, a cation of astrochemical interest, has been studied by the coupled electron pair approximation (CEPA) using a basis set of 111 contracted Gaussian‐type orbitals (cGTOs). It has a large equilibrium dipole moment of −2.88 D with the positive end at the proton site and is thus a suitable candidate for future investigation by radioastronomy or microwave spectroscopy. Most promising for a study in the infrared region of the spectrum is the Fermi resonance doublet ν2/ν4+ν5 with band origins calculated at 2199 and 2174 cm−1. Using larger basis sets of 146 and 160 cGTOs, the equilibrium proton affinity of C5 is calculated to be 887.7 and 886.6 kJ mol−1, respectively. For the proton affinity at 298 K, a value of 860±5 kJ mol−1 is recommended.