2014ApJ...787...72B

Highly accurate quantum chemical calculations beyond CCSD(T) have been used to study the molecular cation l-C₃H⁺ which is the carrier of harmonically related radio lines observed in the Horsehead photodissociation region and toward Sgr B2(N). Excellent agreement with spectroscopic and radioastronomical measurements is obtained for the rotational constant, with the calculated value of B₀= 11246.4 MHz only 1.5 MHz or 0.01% above the experimental value. The unusually large ratio of centrifugal distortion constants D₀(exp.)/D_e(theor.) = 1.80 is attributed to the shallow CCC bending potential of l-C₃H⁺ and is quantitatively reproduced by variational calculations within a pseudo-triatomic model. A comparative study of centrifugal distortion constants in a series of four linear interstellar molecules (C₃N^–, C₃O, l-C₃H⁺, and C₃) is made and some general conclusions are drawn.