Illustration of the loop observed in the ^rR branches of m-DCB, with δ = Δ(K_a + K_c − J) = 0 ← 1; case of the ^rR(17) branch. For the two series of transitions, the two band heads are highlighted in orange. (top trace) Intensity of the transitions as a function of the frequency; (middle trace) transition frequency as a function of J″; and (bottom trace) reduced energy of the involved energy levels as a function of J(J + 1). Reduced energies and intensities are from an SPCAT prediction file obtained using the final set of constants (Table 1 in the main text); reduced energies were calculated using the following formula: E_reduced = E − hc × 1∕2(B + C)J(J + 1), where E is in wavenumber and B and C are in megahertz.