Portion of the room-temperature millimeter-wave spectrum of methoxyamine showing a group of a-type R-branch transitions for J′ ← J′′ = 10 ← 9. The asymmetry splitting for the K_a = 1 transitions in this case reaches almost 11 GHz and decreases rapidly with increasing K_a until it is completely unresolved for K_a ≥ 5. Small internal rotation A−E splitting can be seen for two K_a = 4 rotational transitions. Forbidden x-type transitions (ΔK_a = 0, ± 2,..., ΔK_c = 0, ± 2,...) that gain intensity through the so-called intensity borrowing from allowed a-type transitions between the same pairs of levels are situated outside the A lines and are labeled with an E^∗.