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14N Nuclear quadrupole coupling and methyl internal rotation in the microwave spectrum of 2-methylpyrrole

Using two molecular jet Fourier transform microwave spectrometers, the rotational spectrum of 2-methylpyrrole was recorded in the frequency range from 2 to 40 GHz. From the torsional splittings due to the internal rotation of the methyl group a barrier height of 279.7183(26) cm−1 was deduced. Because of the 14N nucleus, all lines show a quadrupole hyperfine structure. The microwave spectra were analysed using the XIAM and BELGI-Cs-hyperfine codes. The XIAM code enabled us to reproduce the whole data set with a root-mean-square deviation of 5.6 kHz while the BELGI-Cs-hyperfine code could provide a better root-mean-square almost by a factor of 2 compared to that of XIAM. The experimental results were complemented by quantum chemical calculations. The values of the methyl torsional barrier and the 14N nuclear quadrupole coupling constants are discussed and compared with other methyl substituted pyrroles as well as other aromatic five-membered rings. GRAPHICAL ABSTRACT