Methyl rotations

From all of this, we can deduce that this transition structure connects two structurally-equivalent minima, and that the path between them corresponds to J methyl rotation. This is not a very interesting transition structure. 

Methyl rotors pose relatively simple, fundamental questions about the nature of noncovalent interactions within molecules. The discovery in the late 1930s1 of the 1025 cm-1 potential energy barrier to internal rotation in ethane was surprising, since no covalent chemical bonds are formed or broken as methyl rotates. By now it is clear that the methyl torsional potential depends sensitively on the local chemical environment. The barrier is 690 cm-1 in propene,2 comparable to ethane,... 

Sixfold barriers to internal rotation occur in molecules such as toluene andp-fluoro-toluene whose molecular frame has C2v symmetry about the rotor axis. The simplest spectroscopic model of internal methyl rotation assumes a rigid, threefold symmetric methyl rotor attached to a rigid molecular frame with the C2 axis coincident with the rotor top axis.25 The effective one-dimensional sixfold torsional potential takes the traditional form,... 

Each complex in Fig. 5.64 is shown with the methyl rotor in the pseudo-r/.v (PC) conformation the corresponding pseudo-trans (PT) conformers have similar appearance, except for the 60° methyl rotation. The displayed PC conformer represents a top-of-barrier transition state for complexes (a)-(i) in Fig. 5.64, but is the stable equilibrium geometry for complexes (j) and (k).97... 

Table 5.27. Methyl rotation barriers Ai+b for various H-bonded andprotonated acetamide X complexes (cf. Fig. 5.64), with comparison NRT bond orders bco and bcs and bond lengths Rco and Rq n of the amide moiety in each complex...

The intimate connection between methyl torsional stiffening and the variation in amide CO/CN bond orders is illustrated in Fig. 5.65. This plot shows that the methyl rotation barrier A /s,b varies roughly linearly with the difference Ab in CO/CN bond orders,... 

Figure 5.65 The dependence of the acetamide methyl-rotation barrier (AT ) on NRT bond-order differences in the amide group (Ab = bco - cn) for various H-bonded complexes of the pseudo-cA (occH(in) = 0°) rotamer (see Table 5.27).

Fig. 4. Integrated intensity of spectra for a methyl rotation with the three site jump as a function of jump rate k.

In CH3CH2=X molecules, the methyl rotational barrier is the energy difference between the eclipsed and staggered conformations with the former being the energy minimum and the latter being the energy maximum. 

This is due to the small coefficient on the oxygen atom in the it MO. The methyl rotational barrier forCH3CH=0 should be less than for CH3C=CH2... 

Experimentally, acetaldehyde is known to exist in the eclipsed conformation. It has a methyl rotational barrier of 1.16 kcal/mol94-96 as contrasted with a barrier of 2.00 kcal/mol in the case of propene. 

Experimentally, the methyl rotational barrier is found to be smaller for the cis isomer relative to the tram isomer. The experimental methyl rotational barrier as well as ab initio values are shown in Table 12. 

Table 12. Methyl rotational barriers in cw- and trans-2-butene...

The theoretical approach used above to elucidate the conformational preferences of CH3-CO-X molecules can also be applied to a discussion of the methyl rotational barrier in these systems. The methyl rotational barrier corresponds to the energy difference between the eclipsed and staggered conformations with the eclipsed conformation being an energy minimum and the staggered conformation being an energy... 

The above analysis can also be used in connection with the problem of the methyl rotational barrier in double rotor molecules, e. g. dimethyl ether, relative to... 

Table 16. Methyl rotational barrier difference between double rotor (CH3-rotor (CH3-X-H) molecules -X-CH3) and single...

Finally, experimental measurement as well as ab initio computation show that the methyl rotational barrier is also higher in the cis than the tram conformation. These results are shown in Table 17. 

Table 17. cis - trans energy differences and methyl rotational barriers for methyl-vinyl-ether... 

As X becomes increasingly electronegative along a row of the Periodic Table, the conformational minimum (cis) will be increasingly stabilized relative to the maximum (trans) by Och-ffcx conjugative interactions. Consequently, the methyl rotational barrier should increase as X becomes more electronegative. 

Partial experimental support for this idea is found in the methyl rotational barrier of acetyl fluoride and acetic acid. Specifically, the methyl rotational barrier increases by 561 cal/mol when -OH is replaced by F131 132) 1371. 

Figure 3. Librational OH modes in hydrogen bonded alcohol clusters may be correlated with overall rotation (bottom left) and torsion (top left) of the monomer (illustrated for methanol), but methyl rotation is actually decoupled from OH torsion by hydrogen bonding. Note that the wavenumbers of monomer rotation (fa 4 cm-1) and torsion (fa 280 cm-1) are much lower than that of the cluster libration (fa 600cm ) [93].

In contrast, the chemical shifts of the axial methyl groups in 53 are nearly independent of X (slope = -0.7) (Figure 4). From that finding these authors (164) concluded tht electronegativity information is transmitted to a y-gauche carbon atom only through a rigid pathway of parallel orbitals and that methyl rotation protects this carbon atom from electronic interaction. 

A rate process was observed for the methyl rotation in solid hexamethylbenzene (45) and this was interpreted as a sure case of... 

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