Part of the millimetre-wave spectrum of isobutene. All transitions are split due to the internal rotation (see text). However, most of the lines are observed as unresolved single lines (as shown for 4_0,4 ← 3_1,3 and 4_1,4 ← 3_0,3 transitions) or triplets (as shown for 26_19,7 ← 26_18,8 transition) because of the high barrier to the methyl internal rotation. The three components are assigned from left to right as A₁A₁, EE, and unresolved A₁E plus EA₁ with measured splitting A₁A₁ – EE of 0.3 MHz.