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

Abstract

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 14 N 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 V 3 = 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.

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

“…We observed for the methyl group at the 2-position of 2,4-dimethylthiazole (6) an unexpectedly low barrier of 19.1 cm −1 [192], the same situation as that of 2-methylthiazole (molecule (3) in Figure 3) [98,99]. Though slightly higher, the values of 122.1 cm −1 and 63.3 cm −1 found for the 4-and the 5-methyl groups, respectively, of 4,5-dimethylthiazole (5) [85] are still too low to be explained by steric effects, especially if compared to the intermediate value of 396.7 observed for the 4-methyl group of the 2,4-isomer ( 6), as well as those of the dimethylfurane (1, 2) [88,190], dimethylthiophene (1, 2) [89], and dimethyl-pyrrole (3, 4) [87,191] families. This again hints at electronic effects as the reason.…”

“…Only four dimethyl-substituted planar five-member rings, 4,5-dimethylthiazole [85], 2,5-dimethylpyrrole [87], 2,5-dimethylfuran [88], and 2,5-dimethylthiophene [89], are published in the literature. Studies in progress include the 2,3-isomers of dimethylfuran [190] and dimethylthiophene as well as the 2,4-isomers of dimethylpyrrole [191] and dimethylthiazole [192].…”

“…This often enables interactions between the LAMs and other fragments of the molecules. Selected examples include studies on a number of benzene derivatives featuring two methyl internal rotations such as 4-methylacetophenone [77], three isomers of dimethylfluorobenzene [78][79][80], three isomers of dimethylbenzaldehyde [81], three isomers of dimethylanisole [82][83][84], or some fivemembered aromatic rings such as 4,5-dimethylthiazole [85], 2-acetyl-5-methylfuran [86], and the 2,5-isomers of dimethylpyrrole [87], dimethylfuran [88], and dimethylthiophene [89]. In these studies, it was proven that steric hindrance arising from a neighboring substituent increases the torsional barrier of a methyl group attached to the aromatic ring, but also that related effects can be electronically transferred across the π-conjugated system and influence the methyl torsion.…”

Microwave Spectrum of Two‐Top Molecule: 2‐Acetyl‐3‐Methylthiophene

“…[39] [a] C. Dindić Institute of Physical Chemistry RWTH Aachen University [39] LAM containing molecules with conjugated π-double bonds are of special interest as electronic effects can be transferred across a longer range, often enabling interactions between two internal rotors. Examples are investigations on a number of benzene derivatives featuring two methyl internal rotations such as three isomers of dimethylanisole, [40][41][42] three isomers of dimethylbenzaldehyde, [43] o-and m-xylene, [44,45] two isomers of dimethylfluorobenzene, [46,47] 4-methylacetophenone, [48] or some five-membered aromatic rings like the 2,5-isomers of dimethylthiophene, [49] dimethylfuran, [50] and dimethylpyrrole [51] or 2acetyl-5-methylfuran [52] and 4,5-dimethylthiazole. [53] In these studies, it was clearly proven that steric hindrance arising from a neighboring substituent increases the torsional barrier of a methyl group attached to the aromatic ring, but also that electrostatic effects can be transferred across the π-conjugated system and influence the methyl torsion.…”

Millimeter-Wave Spectroscopy of Methylfuran Isomers: Local vs. Global Treatments of the Internal Rotation

“…BELGI -C on the other hand allows, for a given vibrational state, the fit of several torsional states together within the same rotation-torsion Hamiltonian matrix, see Section 4.1.2 . A comparison of the results obtained with a “local” approach with those issued from a “global” approach has already been introduced in several papers [ 23 , 25 , 26 ]. Local approaches have shown their efficiency and high speed, especially in the first steps of assignments, but they sometimes fail when the potential barrier is low or when reaching higher excited torsional states.…”