Main features in the room-temperature rotational spectrum of acrylamide. (a) Strong lines of the syn conformer in the millimeter wave region consisting of a pair of b-type transitions involving K_a = 0, 1 and K_c = J energy levels. (b) Groups of b-type R-branch transitions near the sub-millimeter wave region. A diffuse group on the left embraces doubly degenerate transitions with the same K_a. Here, the leading transition is J = K_a = 17 ← 16 and the value of J increases by 2 while the value of K_a decreases by 1 with each successive line. The compact group on the right encompasses degenerate transitions with two different K_a. In the present group, degenerate K_a = 0, 1, K_a = 1, 2, ... K_a = 6, 7 transitions are observed. Only J quantum numbers are labeled with the red combs for simplicity. (c) A portion of the spectrum showing the rotational transitions of syn and skew acrylamide that involve 22 _2,20 and 22 _3,20 energy levels. While two a-type and two b-type transitions are observed for the skew conformer, only two b-type transitions are found for the syn form in agreement with their dipole moment components.