Skeletal flexing during methyl rotation in small dimethyl molecules

Ozkabak, Ali G.; Goodman, Lionel

doi:10.1016/0009-2614(91)90004-s

Cited by 24 publications

(23 citation statements)

References 21 publications

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“…As discussed in our previous studies, 10-13 this flexing is caused by the increased repulsion between methyl groups during rotation. The importance of this relaxation was first noted in early studies by Ha et al 17 Ozkabak and Goodman 14 showed that the fully relaxed potential surface (including the COC angle opening) gives the gearing frequency in good agreement with IR measurements. Moule and coworkers 16 subsequently showed that a three-dimensional hypersurface, with respect to the COC angle and the torsional angles of the methyl groups, gives a still better account of the spectroscopic details than the fully relaxed surface.…”

Section: Torsional Potential Surfacesmentioning

confidence: 81%

Influence of Protonation on Internal Rotation of Dimethyl Ether

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Goodman

2000

J. Phys. Chem. A

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A model is proposed for the influence of protonation on ether potential surfaces. The model invokes the quantum-mechanical effect of protonation on the oxygen lone pair and its consequence on the internal rotation barriers and associated torsional rotation energy levels. The barrier reduction and lowered torsional energy gaps lead to possible conformational changes at ambient temperatures. The model is applied to dimethyl ether. In this case, the torsional fundamental energies become comparable to, or below, the room-temperature Boltzmann energy (depending on the proton position relative to the dimethyl ether lone pair), so that the geometry distribution in the protonated species is driven toward the asymmetric staggered-eclipsed conformer, rather than the highly preferred symmetric (C 2V ) eclipsed-eclipsed conformer in dimethyl ether.

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Section: Torsional Potential Surfacesmentioning

confidence: 81%

Influence of Protonation on Internal Rotation of Dimethyl Ether

and

Goodman

2000

J. Phys. Chem. A

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“…The effect of C 3 symmetry deviations to the potential function, V( 'T), can be assessed by generating more than the required number of conformer energies and fitting them to the general form of the potential given in Eq. (3). For this reason, we carried out thirteen rotational conformer calculations according to the two models, rigid frame and fully relaxed.…”

Section: B Methyl Conformer Energiesmentioning

confidence: 99%

Effect of skeletal relaxation on the methyl torsion potential in acetaldehyde

Ozkabak

Goodman

1992

The Journal of Chemical Physics

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Fully-relaxed model ab initio calculations at Hartree–Fock/6–31G(d,p) and Mo/ller–Plesset (MP2)/6–31G(d,p) levels for acetaldehyde methyl conformers indicate significant skeletal flexing (e.g., the CH3 –C bond length changes by 0.006 Å) and methyl hydrogen folding. Thirteen methyl conformer energies at 15° intervals are used to assess the magnitudes of the torsional potential function expansion terms. Only two terms V3=373.8 and V6=3.4 cm−1 (both significantly different from those obtained from microwave and infrared analyses) are found to be important. These calculations clearly show that relaxation during methyl rotation (i.e., skeletal flexing and methyl hydrogen folding) is an important determinant of the torsional potential. Energy levels obtained from internal rotation potentials which include flexing simulate infrared torsional fundamental frequencies in CH3CHO and CD3CHO to within 1–2 cm−1 of the experimental values. In the absence of relaxation infrared torsional fundamental frequencies are poorly simulated by ab initio calculations.

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“…Rigid rotation freezes the geometry at the equilibrium geometry except for the dihedral angle that describes methyl torsion. The rigid rotation barrier energy of dimethyl ether is calculated to be nearly 900 cm -1 higher than the fully relaxed (and accepted) 1600 cm -1 simultaneous methyl rotation barrier, [1][2][3][4] illustrating the high strain in the rigid rotation SS conformer. The effect of relaxation is that the steric repulsion energy strongly decreases, but concomitantly the oxygen σ lone-pair reorganization energy strongly increases (π interaction energies (which include hyperconjugation effects), found to give important but not dominant barrier energy contributions, are relatively relaxation unaffected).…”

Section: Introductionmentioning

confidence: 99%

Role of Lone-Pairs in Internal Rotation Barriers

1997

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An in-depth understanding of internal rotation barrier formation and energetics for dimethyl ether, protonated dimethyl ether, methanol, and their sulfur analogs is provided by dissecting the barrier into Pauli exchange steric repulsion, σ-lone-pair reorganization, and π hyperconjugation. The combined natural bond orbital, symmetry, and relaxation analysis demonstrates that oxygen σ lone-pairs play an important (sometimes dominant) role in controlling barrier heights. In dimethyl ether the increased steric contact brought about by simultaneous rotation of the methyl groups causes the COC angle to widen, in turn increasing the σ lone-pair p character, which leads to large lone-pair reorganization energy. Steric repulsion contributes to the dimethyl ether >4 kcal/mol barrier energy in only a minor way even though the steric contact can be looked at as the origin of the lone-pair increased p character. Absence of a σ lone-pair in acid media predicts a drastically lowered barrier (i.e. ∼1 kcal/mol). In methanol the increase in O atom lone-pair p character and associated lone-pair reorganization energy is strongly reduced, leading to an also greatly lowered barrier. Lone-pair σ reorganization effects are smaller in the sulfur-containing compounds, and C-S (σ) bond weakening is predicted to become the dominant barrier energy controlling term.

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Skeletal flexing during methyl rotation in small dimethyl molecules

Cited by 24 publications

References 21 publications

Influence of Protonation on Internal Rotation of Dimethyl Ether

Influence of Protonation on Internal Rotation of Dimethyl Ether

Effect of skeletal relaxation on the methyl torsion potential in acetaldehyde

Role of Lone-Pairs in Internal Rotation Barriers

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