Local vs global approaches to treat two equivalent methyl internal rotations and ¹⁴N nuclear quadrupole coupling of 2,5-dimethylpyrrole

The microwave spectrum of 2,5-dimethylpyrrole was recorded using a molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2 to 26.5 GHz. Only one stable conformer was observed as expected and confirmed by quantum chemical calculations carried out to complement the experimental analysis. The two equivalent methyl groups cause each rotational transition to split into four torsional species, which is combined with the quadrupole hyperfine splittings in the same order of magnitude arising from the ¹⁴N nucleus. This results in a complicated spectrum feature. The spectral assignment was done separately for each torsional species. Two global fits were carried out using the XIAM code and the BELGI-C_2v-2Tops-hyperfine code, a modified version of the BELGI-C_2v-2Tops code, giving satisfactory root-mean-square deviations. The potential barriers to internal rotation of the two methyl groups were determined to be V₃ = 317.208(16) cm⁻¹. The molecular parameters were obtained with high accuracy, providing all necessary ground state information for further investigations in higher frequency ranges and on excited torsional-vibrational states.