Barriers to internal rotation in neopentylbenzenes substituted on the benzyl group. A <sup>13</sup>C NMR band shape study

Andersson, Sven Ingmar; Drakenberg, Torbjörn

doi:10.1002/omr.1270211206

Cited by 10 publications

(8 citation statements)

References 56 publications

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“…When X = C(CH3)3, the relative stability of 1 rises to more than 20 kJ/mol (8,9), but drops (10) to about 10 kJ/mol when X is Si(CH3)3, Ge(CH3),, or Sn(CH3),, interpretable as a hyperconjugative stabilization (1 1, 12) of 1 because substitution by chlorine at the ortho positions changes the internal barrier by only a small amount. By way of contrast, such substitution in ethylbenzene leads to a very large increase in the stability of 1 (13).…”

Section: Introductionmentioning

confidence: 99%

Theoretical and experimental approaches to the barrier to rotation about the Csp²—Csp³ bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

Schaefer¹,

Sebastian²,

Penner³

1991

Can. J. Chem.

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. Can. J. Chem. 69,496 (1991). The 'H nuclear magnetic resonance spectra of benzyl silane and benzyl trichlorosilane, obtained in CS2 and benzene-d6 solutions, are analyzed. The long-range coupling constants between the methylene and para ring protons are used to derive apparent twofold barriers about the Csp2-Csp3 bonds of 7.4 2 1.6 and 8.1 + 1.1 kJ/mol for the silane and the trichlorosilane, respectively. These are higher than that for ethylbenzene and are attributed mainly to the stabilization of the perpendicular conformer, that with the C-Si bond in a plane perpendicular to the phenyl plane, by u-IT conjugation (hyperconjugation) of the C-Si bond and the IT electron system. Molecular orbital computations confirm the predominantly twofold nature of the internal barrier in benzyl silane and also for benzyl germane and stannane. The calculated barriers for the silane derivatives are rather higher than the experimental values. The computed barriers have magnitudes that appear to change with X in much the same order as do the hyperconjugative interactions deduced in other ways for CH2X(CH3)3 groups (X = Sn, Ge, Si, C). The angles CCX in benzyl-X (X = CH3, SiH3, SiC13, GeH3, SnH3) are all computed to decrease smoothly as sin2$, where $ is the angle by which the C-X bond twists out of the phenyl plane.Key words: conformations, benzyl silane and trichlorosilane; NMR, benzyl silane and trichlorosilane; MO calculations, benzyl silane and trichlorosilane. Des calculs d'orbitales molCculaires confirment la nature principalement binaire de la barrikre interne dans le benzyl silane ainsi que dans les benzyl germane et stannane. Les barrikres calculCs pour les dCrivCs silanes sont sensiblement plus hautes que les valeurs expCrimentales. La variation des amplitudes des barrikres calculCes pour les dCrivCs silanes avec X semble suivre l'ordre des interactions d'hyperconjugaison dCduites sur d'autres bases pour les groupements CH2X(CH3), (X = Sn, Ge, Si, C). Les calculs indiquent que les angles CCX, des benzyl-X (X = CH3, SiH3, SiC13, GeH3 et SnH,), doivent diminuer doucement comme le sin2$, oh IJJ est l'angle que fait la liaison C-X avec le plan du noyau phCnyle.

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Section: Introductionmentioning

confidence: 99%

Theoretical and experimental approaches to the barrier to rotation about the Csp²—Csp³ bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

Schaefer¹,

Sebastian²,

Penner³

1991

Can. J. Chem.

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“…The V 2 terms in compounds 4 and 5 do not differ greatly from those of the analogous carbon compounds. The same does not hold for compound 6 , the V 2 term being lower than in the analogous carbon derivatives, the barrier of which is most likely affected by the presence of strong through-space effects. ,, …”

Section: Resultsmentioning

confidence: 94%

“…The same does not hold for compound 6, the V 2 term being lower than in the analogous carbon derivatives, the barrier of which is most likely affected by the presence of strong through-space effects. 11,26,27 Hyperconjugative Character of the SisC and SisH Bonds. The electron donor and acceptor character of the SisC bond in compounds 1-6 was examined within the framework of the NBO approach 19 and the procedure described in previous papers 11,20,21 was adopted.…”

Section: Resultsmentioning

confidence: 99%

The Rotational Barriers of Groups Containing Silicon in Substituted Benzenes. A Theoretical Approach to the Silicon Substituent Effect

Taddei

1998

J. Chem. Inf. Comput. Sci.

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The potential energy profile for a number of derivatives of benzene substituted with the SiH 3 , SiH 2 CH 3 , and CH 2 SiX 3 (X ) H, F, Cl, CH 3 ) groups was calculated at ab initio (HF/6-31G*//HF/6-31G*) level, and the energy barriers were estimated. The rotational barrier is low for the molecules with silicon directly bonded to the phenyl ring, but higher values are found for the CH 2 SiX 3 groups, which depend on X as well. The hyperconjugative effect of the substituents was estimated from the natural bond orbital (NBO) theory of donor acceptor properties: the SiH 2 CH 3 group was found to be electron-acceptor in character, and the CH 2 SiX 3 were found to be electron-donor groups. The rotational barrier of these molecules is mainly determined by hyperconjugation, steric effects being smaller than in the analogous carbon derivatives. The SisH and SisC bonds display an electron-acceptor character of similar magnitude when silicon is attached to the phenyl ring, but the SisC bond has a prevailing donor character when carbon is bonded to the phenyl ring.

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“…Substitution by another methyl group at the P carbon atom of isobutylbenzene, to produce neopentylbenzene, raises this barrier to greater than 20 kJ/mol (29,30); the methyl groups are now unable easily to avoid the ortho C-H bonds of the benzene moiety. All these statements refer to experimental results in solution.…”

Section: Resultsmentioning

confidence: 99%

“…The substitution by one more methyl group, to give neopentylbenzene, 3, causes at least a second doubling of the experimental internal barrier (29,30), presumably because of the greater difficulty experienced by the methyl groups in avoiding close H,H contacts with the ortho C-H bonds at the height of the barrier. The equilibrium betweetz 5 arzd 6 in solution he symmetrical conformer, 6, is not detectable at low temperatures in a jet-cooled stream (10).…”

Section: The Appuretzt Twofold Burriermentioning

confidence: 99%

The conformational behavior of isobutylbenzene in CS₂/C₆D₁₂ solution at 300 K

Schaefer¹,

Lee²

1992

Can. J. Chem.

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The 1H nuclear magnetic resonance spectrum of isobutylbenzene as a dilute solution in CS2/C6D12 at 300 K is analyzed. The gauche conformer (statistical weight 2) predominates, as was previously shown for a jet-cooled stream of this compound. The apparent twofold barrier to rotation about the Csp2—Csp3 bond is 10.5 ± 0.5 kJ/mol, as must be the case for the analogous gauche conformer of n-propylbenzene. The internal barriers are compared for ethylbenzene, gauche isobutylbenzene, and neopentylbenzene. AM1 and STO-3G MO computations of the conformer stabilities and barriers are reported, the latter being in better agreement with experiment.

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Barriers to internal rotation in neopentylbenzenes substituted on the benzyl group. A ¹³C NMR band shape study

Cited by 10 publications

References 56 publications

Theoretical and experimental approaches to the barrier to rotation about the Csp²—Csp³ bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

Theoretical and experimental approaches to the barrier to rotation about the Csp²—Csp³ bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

The Rotational Barriers of Groups Containing Silicon in Substituted Benzenes. A Theoretical Approach to the Silicon Substituent Effect

The conformational behavior of isobutylbenzene in CS₂/C₆D₁₂ solution at 300 K

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Barriers to internal rotation in neopentylbenzenes substituted on the benzyl group. A 13C NMR band shape study

Cited by 10 publications

References 56 publications

Theoretical and experimental approaches to the barrier to rotation about the Csp2—Csp3 bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

Theoretical and experimental approaches to the barrier to rotation about the Csp2—Csp3 bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

The Rotational Barriers of Groups Containing Silicon in Substituted Benzenes. A Theoretical Approach to the Silicon Substituent Effect

The conformational behavior of isobutylbenzene in CS2/C6D12 solution at 300 K

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Barriers to internal rotation in neopentylbenzenes substituted on the benzyl group. A ¹³C NMR band shape study

Theoretical and experimental approaches to the barrier to rotation about the Csp²—Csp³ bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

Theoretical and experimental approaches to the barrier to rotation about the Csp²—Csp³ bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

The conformational behavior of isobutylbenzene in CS₂/C₆D₁₂ solution at 300 K