The observed transition frequencies were taken from
(1) M. C. McCarthy, M. J. Travers, A. Kovacs, C. A. Gottlieb, and P. Thaddeus, ApJS 113, 105 (1997).
Transitions with K_a = 2 or higher have not been observed and are likely not reliably predictable at present. Hence, these lines have been omitted from the prediction.
The calculated value for the dipole moment was taken from
(2) S. A. Maluendes and A. D. McLean, Chem. Phys. Lett. 200, 511 (1992).
At low temperatures, it may be necessary to discern between ortho-l-C₆H₂ and para-l-C₆H₂. The ortho states are described by K_a odd, the para states by K_a even. The nuclear spin-weights are 3 and 1 for ortho-l-C₆H₂ and para-l-C₆H₂, respectively. The J_KaKc = 1₁₁ is the lowest ortho state. It is approximately 9.60 cm^–1 above ground.