Low-resolution microwave studies of substituted ethyl- and isopropylbenzenes

True, Nancy S.; Farag, M. S.; Bohn, Robert K.; MacGregor, Malcolm A.; Radhakrishnan, J.

doi:10.1021/j100246a018

Cited by 41 publications

(20 citation statements)

References 2 publications

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“…For molecules of this size, the relative intensities of low resolution microwave spectra are not easily related to internal rotational barriers and may well reflect the threshold energy for intramolecular vibrational redistribution. Hence the data (7) are not contradictory to the results of the present work. The long-range proton-proton coupling constants for isopropylbenzene in CS2 and acetone solutions cannot discriminate between a purely twofold barrier and one containing an additional significant fourfold component.…”

Section: Resultssupporting

confidence: 70%

See 1 more Smart Citation

Theoretical and experimental data on the internal rotational potential in isopropylbenzene

Schaefer¹,

Sebastian²,

Penner³

1988

Can. J. Chem.

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. Can. J. Chcm. 66, 1495 (1 988). Geometry-optimized STO-3G MO computations of the internal rotational potential about the C,,I-C,,I bond in isopropylbenzene yield V(kJ/mol) as 14.1(3) sin" -2.8(3) sin' 2 4 + 1.4(3) sin"$; $ is the angle by which the a C-H bond deviates from the benzene plane. This result provides no support for the very low internal barrier once hypothesized from microwave data for isopropylbenzene in the vapor phase, nor for the local minimum at $ = 90' inferred from hyperfine coupling constants in .rr electron radicals of isopropylbenzene derivatives. An alternative explanation in the literature for such microwave spectra is therefore a more likely one. Similar MO computations with the 6-31G basis at three values of $ can be fit by 13.40 sin" -2.6 1 sin' $ for the energies in kJ/mol. In CS2 and acetone-d6 solutions, the long-range proton-proton coupling constants imply the independence of the rotational potential of solvent polarity and also that the barrier is rather lower than that computed for the free molecule. An apparent twofold barrier of 6.5 kJ/mol is consistent with the coupling constants but so is a combination of twofold and fourfold barrier components amounting to 9.7, -3.5 and 7.2, -1.5 kJ/mol. The coupling constants do not allow discrimination among these possibilities. The internal rotational potential is not altered significantly in the presence of a 4-hydroxyl substituent in isopropylbenzene. En solutions dans le CS2 ou dans I'acetone-d6, les constantes de couplage proton-proton longue distance impliquent que le potentiel la rotation ne depend pas de la polarit6 du solvant et que la barri6re est vraisemblablement plus faible que celle calcul6e pour la molCcule libre. Une barrikre apparente binaire de 6,5 kJ/mol est en accord avec les constantes de couplage; toutefois, une combinaison de composantes de barrikres binaire et quaternaire qui s'klkvent B 9,7, -3,5 et 7,2 et -1,5 kJ/mol fournit aussi un tel accord. Les constantes de couplage ne permettent pas de departagcr entre ces deux possibilites. La presence d'un substituant hydroxy-4 sur l'isopropylbenzkne ne modifie pas le potentiel a la rotation interne d'une faqon importante.[Traduit par la revue] Introduction The recent literature contains some interesting and somewhat contradictory information on the potential function governing the internal rotation of the isopropyl group about the Cs,,2-Csp3 bond linking it to the benzene ring. In a number of radicals containing the cumyl moiety ( I ) , the temperature dependence of the hyperfine coupling interaction with the methine proton was interpreted to mean that 2 lies 5-7 kJ/mol above 1 in energy and that 3 lies at a local minimum in the potential energy, being 2.5-3.3 kJ/mol less stable than 1. It was suggested that the rotational potential arises mainly from steric repulsion between the rotating methyl group and the neighbouring aromatic C-H bonds in the anion, cation, and neutral radicals. Apparently, the electronic structures of the radicals have only a small influence on t...

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Section: Resultssupporting

confidence: 70%

“…It does not appear that the barrier to internal rotation about the CS,2-C,3-bond is significantly altered by the hydroxyl substituent. It therefore seems unlikely that the apparently very low barrier reported for 4-isopropylbenzaldehyde in the vapor phase (7) has its origin in a substituent effect.…”

Section: The Other Long-range Coupling Constantsmentioning

confidence: 99%

Theoretical and experimental data on the internal rotational potential in isopropylbenzene

Schaefer¹,

Sebastian²,

Penner³

1988

Can. J. Chem.

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“…As such, the interactions that determine the rotational barrier are dominated by intermolecular interactions. Ethyl and isopropyl group rotational barriers in isolated molecules similar to that studied here [6][7][8][9] are in the 1 -8 kJ mol −1 range, but in the solid state they are effectively infinite. [1][2][3][4][5][10][11][12] In the solid state, rotational barriers in the 5-50 kJ mol −1 range can be determined by observing the temperature and the nuclear magnetic resonance ͑NMR͒ frequency dependence of 1 H spin-lattice relaxation rates.…”

Section: Introductionmentioning

confidence: 89%

“…In a large class of planar aromatic compounds, the rotational barrier for methyl group rotation in ethyl [1][2][3][4][5][6][7][8][9] and isopropyl 1,7,[10][11][12] groups tends to be dominated by intramolecular interactions in both the isolated molecule [6][7][8][9] and in the crystal. [1][2][3][4][5][10][11][12] The barriers for ethyl and isopropyl group rotations in these compounds, however, are much larger in the crystal than in the isolated molecule because these groups lack rotational symmetry.…”

Section: Introductionmentioning

confidence: 99%

The quenching of isopropyl group rotation in van der Waals molecular solids

Wang¹,

Rheingold

DiPasquale

et al. 2008

The Journal of Chemical Physics

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X-ray diffraction experiments are employed to determine the molecular and crystal structure of 3-isopropylchrysene. Based on this structure, electronic structure calculations are employed to calculate methyl group and isopropyl group rotational barriers in a central molecule of a ten-molecule cluster. The two slightly inequivalent methyl group barriers are found to be 12 and 15 kJ mol −1 and the isopropyl group barrier is found to be about 240 kJ mol −1 , meaning that isopropyl group rotation is completely quenched in the solid state. For comparison, electronic structure calculations are also performed in the isolated molecule, determining both the structure and the rotational barriers, which are determined to be 15 kJ mol −1 for both the isopropyl group and the two equivalent methyl groups. These calculations are compared with, and are consistent with, previously published NMR 1 H spin-lattice relaxation experiments where it was found that the barrier for methyl group rotation was 11Ϯ 1 kJ mol −1 and that the barrier for isopropyl group rotation was infinite on the solid state NMR time scale.

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“…The spectrum of the gas was scanned under low-resolution conditions at about 50 mtorr pressure at room temperature or in dry ice-cooled cells 10 in a Stark-modulated Hewlett-Packard 8460A spectrometer from 18-40 GHz. High-resolution microwave spectra were also obtained on Fourier transform pulsed-jet microwave spectrometers.…”

Section: Experimental and Computational Methodsmentioning

confidence: 99%

Four conformers characterized in allyl nitrite

Lee¹,

Breeze²,

Bohn³

et al. 2002

Chirality

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Allyl nitrite, O=NOCH(2)CH=CH(2), can exist in conformations which include syn (S) and anti (A) configurations of O=N-O-C, anti or gauche (G) configurations of N-O-C-C, and syn or skew (Sk) configurations of O-C-C=C. Four of them have been observed and characterized by their microwave spectra and quantum chemical calculations to have AGSk' (0.0 kcal/mol), AGSk (0.2(2) kcal/mol), SASk (0.2(2) kcal/mol), and SAS (0.9(2) kcal/mol) configurations about the three successive dihedral angles. All four are observed in microwave studies of the vapor at 300 and 200 K and the first three are seen in pulsed-jet beams. The SAS form apparently relaxes to the SASk conformer during expansion in the pulsed-jet. Quantum chemical calculations reveal that the two possible conformers with anti-anti configurations of the O=N-O-C-C fragment, AAS and AASk, are transition states, unstable configurations, or of marginal stability, consistent with structures observed in this and other previously studied alkyl nitrites. The remaining four possible conformers are predicted to have relative energies within 1 kcal/mol of the observed conformers but were not observed experimentally.

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Low-resolution microwave studies of substituted ethyl- and isopropylbenzenes

Cited by 41 publications

References 2 publications

Theoretical and experimental data on the internal rotational potential in isopropylbenzene

Theoretical and experimental data on the internal rotational potential in isopropylbenzene

The quenching of isopropyl group rotation in van der Waals molecular solids

Four conformers characterized in allyl nitrite

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