Rotation methyl group

Fast Tyr ring flips methyl group rotations 2. Slow hinge bending between domains ... 

Some molecules are not rigid as they contain functional groups that have a certain orientational freedom. For example, dimethyl acetylene, CH3-CSC-CH3, in which the two methyl groups rotate with little hindrance, cannot be described by a specific geometry,... 

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

As expected for C2 symmetry, the 2-propyl cation 6 is chiral and racemizes by methyl group rotation, passing through a Cs transition state with a calculated barrier of ca. 0.7 kcal/mol. This process is rapid on the NMR time scale, even at temperatures well below 77 K. 

The tert-butyl cation structure (7) with Cs symmetry is better suited for hyperconjugation than the C3h form and is thus energetically slightly favored.29 The energy surface for methyl-group rotation is however very flat. 

Fig. 1.3 Relaxation map of polyisoprene results from dielectric spectroscopy (inverse of maximum loss frequency/w// symbols), rheological shift factors (solid line) [7], and neutron scattering pair correlation ((r(Q=1.44 A )) empty square) [8] and self correlation ((t(Q=0.88 A" )) empty circle) [9],methyl group rotation (empty triangle) [10]. The shadowed area indicates the time scales corresponding to the so-called fast dynamics [11]...

At low temperature the material is in the glassy state and only small ampU-tude motions hke vibrations, short range rotations or secondary relaxations are possible. Below the glass transition temperature Tg the secondary /J-re-laxation as observed by dielectric spectroscopy and the methyl group rotations maybe observed. In addition, at high frequencies the vibrational dynamics, in particular the so called Boson peak, characterizes the dynamic behaviour of amorphous polyisoprene. The secondary relaxations cause the first small step in the dynamic modulus of such a polymer system. 

Fig. 4.34 Comparison between the descriptions of the elastic intensity at 260 K (filled triangle) and 280 K filled circle) in terms of the EISF corresponding to a methyl-group rotation (solid lines) and to a 2-site jump (dashed lines), (Reprinted with permission from [195]. Copyright 1998 American Chemical Society)...

A. Jones, Clark University, Massachusetts When the methyl group is attached to the backbone or even to a side chain it has a whole distribution of correlation times associated either with the backbone or the side chain in addition to its methyl group rotation. Our assumption in these models is that they are independent so that the methyl group Ti would be calculated on the... 

Sepiolite clay (<100 mesh) was heated in air at 120°C in order to remove the zeolitic and surface bound water molecules. The partially dehydrated clay mineral was subsequently exposed to acetone vapor at room temperature for a period of four days. H and 29Si CP MAS-NMR experiments revealed that the acetone molecules penetrated into the microporous channels of the sepiolite structure. Broad line 2H NMR studies using acetone-d6 revealed that, in addition to fast methyl group rotations, the guest acetone-d6 molecules were also undergoing 2-fold re-orientations about the carbonyl bond. The presence of acetone-d6 molecules adsorbed on the exterior surfaces of the sepiolite crystals was also detected at room temperature. 

In Fig. 5b, which was obtained at 30°C, the powder pattern displays a severely distorted, intermediate rate line shape. This line shape is characteristic of both fast methyl group rotation and 2 fold molecular re-orientation about the carbonyl bond at a rate comparable to the reciprocal of the quadrupolar coupling constant ( 105 Hz). At room temperature, therefore, the acetone-d6 molecules in the microporous channels of sepiolite are able to undergo restricted re-orientations. 

Taken from Ref.19 b taken from Ref.69 c taken from Ref. 8 d taken from Ref.67 taken from Ref, 731 r taken from Ref.,0) phr of sulfur h % of dicumyl peroxide number of crosslinking units per weight average PMA chain j average number of repeat units between crosslinks k concentration of chains between network joints, (mole/cm3)x 104 1 methyl group rotation m segmental motion motions within the crystalline part of the polymer... 

Methyl-substituted malonaldehyde (a-methyl-/3-hydroxyacrolein) provides an opportunity to study the role of asymmetry of the potential profile in the proton exchange. In the initial and final states, one of the C-H bonds of the methyl group is in the molecular plane and directed toward the proton position. The double well potential becomes symmetric only due to methyl group rotation over tt/6, when the C-H bond lies in the plane perpendicular to the molecular one. As a result, proton tunneling occurs in combination with CH3 hindered rotation and the... 

Figure 8.5. The methylcyclopropyl radical in various conformations, (a) The non-planar-bisected conformation is stable, (b) The non-planar-straddled conformation corresponds to methyl group rotation by 60°. (c) The planar eclipsed configuration is the transition state, (d) The planar-bisected structure corresponds to conversion without rotation. (From Zebretto et al. [1989].)...

Steele, W.V., Chirico, R.D., Nguyen, A., Knipmeyer, S.E. (1995) Vapor pressure, high-temperature heat capacities, critical properties, derived thermodynamic functions, and barriers to methyl-group rotation, for the six dimethylpyridines. J. Chem. Thermodyn. 27, 311-334. 

This result suggests, if it is assumed that a C-H heteronuclear dipolar relaxation mechanism is operative, that methyl protons dominate the relaxation behavior of these carbons over much of the temperature range studied despite the 1/r dependence of the mechanism. The shorter T] for the CH as compared to the CH2 then arises from the shorter C-H distances. Apparently, the contributions to spectral density in the MHz region of the frequency spectrum due to backbone motions is minor relative to the sidegroup motion. The T p data for the CH and CH2 carbons also give an indication of methyl group rotational frequencies. 

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