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

Abstract

Using 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.

14N nuclear quadrupole coupling and methyl internal rotation in 3-methylpyrrole investigated by microwave spectroscopy

“…The planar moment of 1.5639 uÅ 2 of 3MP ( 3) is very close to that of N-methylpyrrole (1) (1.652 uÅ 2 ) [35] and 2MP (2) (1.6115 uÅ 2 ) [10], confirming that there are two out-of-plane hydrogen atoms. Regarding the methyl torsion, the V3 potential value changes significantly if the ring atom or the substitution at the heteroatom is varied.…”

“…The V3 value predicted at the MP2/cc-pVTZ level of theory is -22.8% lower than the experimental value. At the same level, the difference of 8.8% observed for 2MP is much smaller [10]. Obviously, even within the same class of molecules, in this case methylsubstituted pyrroles, a level of theory which gives calculated results close to the experimental values for one isomer may not give such a satisfactory agreement for another isomer.…”

“…The BELGI-Cs-hyperfine code succeeded to reproduce the experimental data of N-tert-butylacetamide [24], 3-nitrotoluene [30], and recently 2MP [10] with a satisfactory rms deviation. For 3MP, with 14 floated parameters (and fixing the two parameters F and Dab), the rms deviation of 2.9 kHz obtained with BELGI-Cshyperfine is almost the measurement accuracy.…”

“…It is known from a number of previous studies that the method introduced by Bailey [17] to compute the 14 N NQCCs is rather reliable [10,[18][19][20]. Therefore, we also apply this method for 3MP.…”

“…Recently, the microwave spectrum of a mono-methyl substituted pyrrole derivative, 2-methylpyrrole (2MP), was studied, revealing torsional splittings from the three-fold hindered internal rotation of the methyl group and hyperfine structures from the quadrupole coupling of the 14 N nucleus [10]. The molecular structure as well as the effective V3 potential and the 14 N nuclear quadrupole coupling constants (NQCCs) were determined with high accuracy.…”