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 spectrometer operating in the frequency range from 2 to 26.5 GHz. Only one stable conformer observed as expected and confirmed by quantum chemical calculations carried out complement experimental analysis. two equivalent methyl groups cause each rotational transition split into four torsional species, which is combined with quadrupole hyperfine splittings same order magnitude arising 14N nucleus. This results complicated feature. spectral assignment done separately for species. Two global fits were XIAM code BELGI-C2v-2Tops-hyperfine code, modified version BELGI-C2v-2Tops giving satisfactory root-mean-square deviations. potential barriers internal rotation determined be V3 = 317.208(16) cm?1. parameters obtained high accuracy, providing all necessary ground state information further investigations higher ranges on excited torsional-vibrational states.