Section of the rotational spectrum of acetone in natural isotopic composition (in black) in the region of the oblate paired transition of ¹³CH₃C(O)CH₃ with and . The red trace shows the simulation from our final parameters in Table 4. The internal rotation pattern (1, 1)+(1, 2):(0, 1)+(1, 0):(1, 1) is particularly easy to identify because of its simple (1 + 1):(2 + 2):2 = 1 : 2 : 1 intensity ratio and the torsional substates are given in the ERHAM notation (in blue), see Table 2. No other line in the shown section can be assigned based on the data from our present study. Thus, the dense spectrum also demonstrates the challenge of assigning ¹³CH₃C(O)CH₃ lines in natural abundance.