The effects of proton tunneling, 14N quadrupole coupling, and methyl internal rotations in the microwave spectrum of ethyl methyl amine

Koziol, Kenneth J.; Stahl, Wolfgang; Nguyen, Ha Vinh Lam

doi:10.1063/5.0025650

Cited by 6 publications

(6 citation statements)

References 27 publications

(37 reference statements)

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“…35 This level was applied because it has yielded reliable rotational constants to guide the assignment in many of our previous investigations. [36][37][38] As expected from the proton magnetic resonance study on 2AF 2 and studies on similar molecules 9,34,39 two conformers were found at α = 0° (called syn) and 180° (called anti) on the potential energy curve illustrated in Figure 2. This is essentially the same as the energy difference found for the two conformers of 2-acetyl-5-methylfuran.…”

Section: Quantum Chemical Calculationssupporting

confidence: 63%

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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Section: Quantum Chemical Calculationssupporting

confidence: 63%

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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“…Calculations at this level have given predicted equilibrium rotational constants Be which matched almost exactly the experimentally deduced rotational constants for 2,6-dimethylfluorobenzene [32]. For comparison, optimizations were also carried out at the ab initio MP2/6-311++G(d,p) level of theory, a level which yielded satisfactory results for many molecules of different classes of compounds in the literature [38][39][40][41][42]. Finally, we performed calculations at the MP2/6-31G(d,p) level, because it has provided rotational constants with values close to the experimental ones for many molecules containing a phenyl ring [43][44][45].…”

Section: Geometry Optimizationssupporting

confidence: 53%

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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“…In general, methyl groups attached to double bonds show barrier heights ranging from approximately 500 cm −1 to 750 cm −1 . This is much smaller than the values of about 1000 cm −1 found for methyl groups at the end of an alkyl chain [10,41,42]. We suspect the orbital interactions to be the reason.…”

Section: Propenyl Methyl Torsioncontrasting

confidence: 57%

The two-top molecule 3-penten-2-one: Acetyl methyl torsion in α,β-unsaturated ketones

Andresen

Schwell

Nguyen

2022

Journal of Molecular Structure

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The effects of proton tunneling, 14N quadrupole coupling, and methyl internal rotations in the microwave spectrum of ethyl methyl amine

Cited by 6 publications

References 27 publications

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

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

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

The two-top molecule 3-penten-2-one: Acetyl methyl torsion in α,β-unsaturated ketones

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