Methyl groups symmetry

Figure 7.18. The lowest tunneling multiplet of two coupled methyl groups. Symmetry labels and in-tramultiplet transition frequencies are indicated.

Sometimes no simple reaction coordinate exists. In the rare cases in which the transition state is symmetric, say hydrogen abstraction from methane by a methyl group, symmetry constraints can be applied. 

The effect of the substituents on chain mobility and chain packing has been related to the gas transport properties [209]. Role of symmetry of methyl group placement on bisphenol rings in PES shows the permeability coefficients in the following order ... 

The C2 symmetry of sulfide 13 means that a single diastereomer is formed upon alkylation (Scheme 1.10). Attack from the Si face of the ylide is preferred as the Re face is shielded by the methyl group cis to the benzylidene group (28). Metzner postulates that this methyl group also controls the conformation of the ylide, as a steric clash in 27b renders 27a more favorable [16]. However, computational studies by Goodman revealed that 27a was not particularly favored over 27b, but it was substantially more reactive, thus providing the high enantioselectivity observed... 

Another early example is nitromethane which is special in two respects. First, the barrier has sixfold symmetry because of the threefold character of the methyl group and the twofold character of the nitro group. Secondly, the barrier turns out to be extremely low, only about 5 small calories. For such a low barrier, it is convenient to treat the coupling between free internal rotation and overall rotation exactly and consider the barrier as a small perturbation. 

The methylene group carries an empty p orbital. Howr will the methyl group interact w ith this empty orbital Clearly the 7r-type orbitals of the methyl group (ttu in E, tt. in B) have the appropriate symmetry to mix with the p orbital. A typical orbital interaction diagram (for E) is shown in Fig. 37. Several conclusions emerge immediately from this diagram ... 

Because of its high structural symmetry, tetrakis[(methyldichlorosilyl)ethyl -p-xylene was easily purified by recrystallization from THF. The less alkylated products, monoalkylated to irialkylated compounds, were also formed along with many isomers due to reorientation of the methyl group. The yields of peralkylated and less alkylated products including isomers are summarized in Table Vll. 

A simpler way to draw the conclusion would be to recognize that the molecule has an internal plane of symmetry that chops through the center of one of the methyl groups ... 

B2) 888 (Aj) methyl groups in transposition assignment acc. to local symmetry C21,. Compounds with higher alkyl substance are known, too (43)... 

Here N represents the order of the rotation axis, i.e, N = 3 for the hindered rotation of a methyl group about its C3 symmetry axis (see Chapter 9). 

The methyl group responds to the difference in the three-dimensional electron density distribution about the two nearest ring CC bonds, and the natural bond orders most simply quantifies the key difference in a unified manner across many molecules. At one extreme, 2-methylpropene has essentially localized single and double bonds (03-0b = 1) and a 1010 cm-1 barrier. At the other extreme, when the geometry of the ring has good local C2v symmetry, as in the S0 state of toluene, m-fluorotoluene, p-fluorotoluene, 3,5-difluorotoluene, and 2,6-difluorotoluene, 03 - Ob and the barrier is invariably very small, even for nominal threefold cases. We interpret this equality of bond orders as indicative of essentially equal contributions of the two dominant resonance structures at all a. 

Propene and cis-butene-2 possess the molecular symmetries C2 and C2 v (28, 29), respectively. (Fig. 4 the geometry parameters given are derived from force field calculations (19, 30) and agree well with experiment.) The barriers // observed for the rotation of the methyl groups amount to 1.98 kcal mole-1 for propene (28), and 0.75 kcal mole-1 for cis-butene-2 (29). The torsional force constants K for the methyl groups... 

---