Methyl groups hindered rotation potential

Figure 7.10. Hindered rotation potentials for two inequivalent methyl groups I and II in a-toluene calculated by the atom-atom potential method. The values of V, V6, and 80 are equal to 29.8meV, -14.9meV, and 17.2° (I) and 23.0meV, -1.8meV, and 35° (II). Solid and dashed lines indicate the levels of torsional vibrations for CH3 and CD3 groups. The same potentials under pressure 4.6 kbar are shown in the right. (From Cavagnat et al. [1986].)...

Inspection of the optimized geometry indicates that dimerization of the car-bene center of 31b is hindered but still attained, as the facing methyl groups can rotate about one another in order to minimize the interaction potential during dimerization. The relatively easy isolation of the dimer of 31b supports this view. On the other hand, in 39, the flexible motion of bulky and rigid three-dimensional bicyclic substituents during the dimerization is not attained. 

Another sort of evidence supporting the bent-bond structure of the, double bond is provided by the information about restricted rotation, The bent-bond structure for propylene leads to the expectation that the potential function hindering the rotation of the methyl group would be nearly the same as in ethane, but with the barrier a little smaller than in ethane because two of the bonds on the adjacent carbon Atom (the bent bonds) are distorted the stable orientation would be the stag-... 

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... 

The wave functions describing rotation of a hindered methyl group have A and E symmetry in the threefold symmetric potential and are associated with nuclear spin functions having the same symmetry representations and corresponding to a total spin of the protons of three-halves and one-half,... 

In asymmetric tops like methyl alcohol, CHjOH, and symmetric rotors like CH SiH, the methyl group can undergo internal rotation relative to the rest of the molecule, traditionally called the frame (LS59, OM07). Although various different tops are considered here, all have three-fold symmetry. In such cases, the potential l hindering the internal rotation can be written ... 

The hindered rotor is a hybrid between a free rotor and a vibration. An example of this important mode is the methyl group rotation in such molecules as ethane, acetaldehyde, and toluene. The potential for such a mode in lowest order is given by (Pitzer, 1953) ... 

The internal dynamics of the methyl group immensely complicates the spectroscopy of these molecules. Of course, this aspect of the problem also provides much of the spectroscopic interest. When the methyl hydrogens of acetaldehyde oscillate around the CC axis, they experience forces arising from the CHO frame of the molecule which vary sinusoidally. As a result, the potential function for internal rotation can be represented by a cosine function in which the crest to trough distance measures the height of the potential barrier. Since the energy barrier to methyl rotation is low in acetaldehyde, the internal motion is one of hindered internal rotation, rather than torsional oscillation. 

Unusual types of temperature dependence of heat capacities can occur in molecular systems. A methyl group in an organic molecule often has a potential for rotating about its symmetry axis that has three peaks and three valleys over its 360° range of positions. It exhibits a vibrational motion called hindered rotation where it twists back and forth within one of the valleys. However, if the molecule is promoted to a higher energy level, it may be above the peaks in this torsional potential. At some point, the torsion of the... 

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