The structure and low-barrier methyl torsion of 3-fluorotoluene

Nair, K. P. R.; Herbers, Sven; Nguyen, Ha Vinh Lam; Grabow, Jens‐Uwe

doi:10.1016/j.saa.2020.118709

Cited by 17 publications

(18 citation statements)

References 56 publications

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“…In molecules belonging to the low-barrier-class, the internal rotation of methyl groups is hindered with a barrier height lower than 200 cm -1 . Vinyl acetate (151.5 cm -1 ) [2], N-ethylacetamide (73.5 cm -1 ) [3] 3fluorotoluene (17 cm -1 ) [4] m-and p-methylanisole (36.6 and 49.6 cm -1 , respectively) [5,6], ptolualdehyde (28.1 cm -1 ) [7], and acetamide (25 cm -1 ) [8], for example, fall in this class. The intermediate-barrier-class comprises of molecules with barrier heights from about 200 to 600 cm -1 , i.e.…”

Section: Introductionmentioning

confidence: 99%

Steric effects on two inequivalent methyl internal rotations of 3,4-dimethylfluorobenzene

Melan

Khemissi

Nguyen

2021

Spectrochimica Acta Part A: Molecular and Biomolecular Spectros

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The microwave spectrum of 3,4-dimethylfluorobenzene was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz with the goal of quantifying the steric effects on the barriers to internal rotation of the two inequivalent methyl groups. Due to these torsional motions, splittings of all rotational transitions into quintets were observed and fitted with residuals close to measurement accuracy. The experimental work was supported by quantum chemical calculations, and the B3LYP-D3BJ/6-311++G(d,p) level of theory yielded accurate optimized geometry parameters to guide the assignment. The three-fold potential values of 456.19(13) cm −1 and 489.77(15) cm −1 for the methyl groups at the meta and para position, respectively, deduced from the experiments are compared with the predicted values and those of other toluene derivatives.

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Section: Introductionmentioning

confidence: 99%

Steric effects on two inequivalent methyl internal rotations of 3,4-dimethylfluorobenzene

Melan

Khemissi

Nguyen

2021

Spectrochimica Acta Part A: Molecular and Biomolecular Spectros

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“…Only for a small number of molecules, e.g. acetone, 12 o-xylene, 13 cis-methyl formate, 14 3-fluorotoluene, 15 and methyl methacrylate, 16 experimental structures in the presence of methyl internal rotation(s) have been determined. Different methods to treat these LAMs such as the combined axis method, 17 the rho axis method, 18 and the effective Hamiltonian method 19 were applied but the reliability of each method for structure determination purposes still remains unclear.…”

Section: Introductionmentioning

confidence: 99%

Equilibrium Structure in the Presence of Methyl Internal Rotation: Microwave Spectroscopy and Quantum Chemistry Study of the Two Conformers of 2-Acetylfuran

et al. 2021

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For 2-acetylfuran, quantum chemistry predicted and proton magnetic resonance study reported two conformers, anti and syn, differing in the position of the carbonyl group with respect to the O1C2 bond of the furan ring. The microwave spectrum of the title molecule was recorded in the frequency range from 2 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer, confirming the presence of both conformers. Spectroscopic parameters such as the rotational and centrifugal distortion constants could be determined with high precision. The spectra of all 13 C-and 18 O-isotopologues of the energetically more favorable anti-conformer could be assigned, allowing the experimental determination of bond lengths and bond angles from the heavy atom substitution rs and the semi-experimental equilibrium 𝑟 e SE structures. Splittings arising from the internal rotation of the acetyl methyl group could be resolved for both conformers as well as for all assigned isotopologues, from which the barrier to methyl internal rotation was determined. The torsional barrier is largely invariant at around 319 cm 1 in the parent species of anti-2-acetylfuran and its isotopologues, showing that though isotopic substitution greatly influences the rotational properties of the molecule and causes a different microwave spectrum, its effect on the methyl torsion is negligible. On the other hand, conformational effects play a decisive role, as the torsional barrier of 239.780(13) cm 1 found for syn-2-acetylfuran differs significantly from the value for anti-2-acetylfuran. The results are compared and discussed with other methyl substituted furan derivatives and acetyl group containing ketones for a better understanding of different effects influencing molecular geometry parameters and methyl internal rotations.

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“…The determination of the internal rotation barriers offers insight into the changes in the electronic structure of toluene caused by the substituent effects. A vast number of fluoro substituted toluenes have been investigated, such as the three isomers of fluorotoluene [1][2][3][4], the six isomers of difluorotoluene [5][6][7][8][9], and 2,6-dimethylfluorobenzene [10], but studies on chloro-substituted toluenes are rather scarce. The reason is probably the quadrupole moment of the chlorine nucleus coupling the nuclear spin I = 3/2 to the end-over-end rotation of the molecule, thereby causing a hyperfine structure (hfs) in addition to the methyl torsional splittings.…”

Section: Introductionmentioning

confidence: 99%

Internal rotation and chlorine nuclear quadrupole coupling in 2-chloro-4-fluorotoluene explored by microwave spectroscopy and quantum chemistry

Nair

Herbers

Bailey

et al. 2021

Spectrochimica Acta Part A: Molecular and Biomolecular Spectros

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2-Chloro-4-fluorotoluene was investigated using a combination of molecular jet Fourier transform microwave spectroscopy in the frequency range from 5 to 21 GHz and quantum chemistry. The molecule experiences an internal rotation of the methyl group, which causes a fine splitting of all rotational transitions into doublets with separation on the order of a few tens of kHz. In addition, hyperfine effects originating from the chlorine nuclear quadrupole moment coupling its spin to the end-over-end rotation of the molecule are observed. The torsional barrier was derived using both the rho and the combined-axis-method, giving a value of 462.5(41) cm -1 . Accurate rotational constants and quadrupole coupling constants were determined for two 35 Cl and 37 Cl isotopologues and compared with Bailey's semi-experimental quantum chemical predictions. The gas phase molecular structure was deduced from the experimental rotational constants supplemented with those calculated by quantum chemistry at various levels of theory. The values of the methyl torsional barrier and chlorine nuclear quadrupole coupling constants were compared with the theoretical predictions and with those of other chlorotoluene derivatives.

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The structure and low-barrier methyl torsion of 3-fluorotoluene

Cited by 17 publications

References 56 publications

Steric effects on two inequivalent methyl internal rotations of 3,4-dimethylfluorobenzene

Steric effects on two inequivalent methyl internal rotations of 3,4-dimethylfluorobenzene

Equilibrium Structure in the Presence of Methyl Internal Rotation: Microwave Spectroscopy and Quantum Chemistry Study of the Two Conformers of 2-Acetylfuran

Internal rotation and chlorine nuclear quadrupole coupling in 2-chloro-4-fluorotoluene explored by microwave spectroscopy and quantum chemistry

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