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

International audienceUsing two molecular jet Fourier transform microwave spectrometers, the rotational spectrum of 2methylpyrrole 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 14 N nucleus, all lines show a quadrupole hyperfine structure. The microwave spectra were analyzed 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-Cshyperfine 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 14 N nuclear quadrupole coupling constants are discussed and compared with other methyl substituted pyrroles as well as other aromatic five-membered rings