The experimental data was reported by
(1) M. C. Mc. Carthy and P. Thaddeus, 2002, J. Mol. Spectrosc. 211, 235.
Because of the limited input data, predictions were limited to 200 GHz and K_a = 2. It is advisable to view predictions even in the 3 mm region with some caution.
¹³C nuclear spin-rotation splitting may be resolvable at low values of J.
The dipole moment was assumed to agree with that of the main isotopic species, see e050503.cat.
At low temperatures, it may be necessary to discern between ortho-l-C₄H₂ and para-l-C₄H₂. 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-C₄H₂ and para-l-C₄H₂, respectively. The J_KaKc = 1₁₁ is the lowest ortho state. It is approximately 9.686 cm^–1 above ground.