Section of the reduced energy plot of 3-hydroxypropenal. Data points of υ = 0⁺ are shown in blue, those of υ = 0⁻ in red; selected K_a series are indicated to the left. Resonant interactions with ΔK_c = 2 are indicated by yellow circles. They occur at K_c = 43 for υ = 0⁺ and K_c = 45 for υ = 0⁻ for 45 ≤ J ≤ 48. The resonant K_c pair increases gradually with J, K_c = 44 and 46 are resonant for 49 ≤ J ≤ 58. Two ΔK_a = 3 interactions are resonant at K_a = 21/18 and J = 35 as well as K_a = 22/19 and J = 49 for ortho and J = 50 for para, which is signaled by green circles. Further ΔK_a = 3 or 5 interactions are usually more local and are not highlighted.