Illustrations of the main characteristics of the room-temperature rotational spectrum of propiolamide. (a) An example of a quartet of a-type and b-type R-branch rotational transitions between K_a = 0, 1, K_c = J pairs of energy levels. (b) Quadruply degenerate lines consisting of a pair of a-type and a pair of b-type transitions involving the same K_a = 0, 1 pairs of levels. These strong lines appear in the spectrum approximately every 6.06 GHz, which corresponds to the value of 2C. (c) An example of a group of high-J quadruply degenerate transitions. The leading transition is J = 71 ← 70 and the value of J decreases by 1 with each successive line running to higher frequencies. The K_a quantum number is increasing away from the leading line in such a way that the first line corresponds to four degenerate K_a = 0, 1 transitions, the second to K_a = 1, 2, the third to K_a = 2, 3, and so on. (d) An example of ¹⁴N nuclear quadrupole hyperfine structure observed in the millimeter wave spectrum. The F = J ± 1 hyperfine components typically appear overlapped, while the F = J component is, in many cases, relatively well separated.