Strong theoretical support for the assignment of B11244 to l-C ₃H⁺

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 B 0 = 11246.4 MHz only 1.5 MHz or 0.01% above the experimental value. The unusually large ratio of centrifugal distortion constants D 0(exp.)De(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.