Methyl group orbitals

The origin of the preference for the eclipsed conformation of propene can be explained in MO terms by focusing attention on the interaction between the double bond and the n component of the orbitals associated with the methyl group. The dominant interaction is a repulsive one between the filled methyl group orbitals and the filled n orbital of the double bond. This repulsive interaction is greater in the bisected conformation than in the eclipsed conformation. ... 

Localized orbitals, see bond orbitals and group orbitals Lone pairs, 9, 39, 42, 49 cyclopropanone, 37 methyl fluoride, 42 pyrazine, 28 water, 42... 

Corresponding to this valence bond view is a molecular orbital picture. The three cr-orbitals of a CH3 group are regarded as a basis from which three group orbitals may be constructed. One of the possible combinations of the tr-orbitals has the same local symmetry as the vacant p-orbital on the cationic centre, and hence may overlap with it. Therefore, a withdrawal of electrons from the methyl group can take place. The orbital from which electron density... 

In many instances, the interaction of a neighboring methylene group or methyl group influences the characteristics of a functional group. The appropriate group orbitals of —CH2— and —CH3 are shown in Figures 3.19 and 3.20, respectively. 

Figure 3.20. Group orbitals for a tetracoordinated atom as a substituent with a tetrahedral arrangement of a bonds. A methyl group is illustrated.

As discussed at the end of Chapter 3, one group orbital of a methyl or methylene group will always have the correct nodal characteristics to interact with an adjacent n orbital or with an adjacent spn orbital in fashion. The degree of interaction may be inferred from the energies of the orbitals, which may in turn be obtained by measurements of ionization potentials and application of Koopmans theorem. Thus, the methyl groups adjacent to the n bond in (Z)-2-butene (ionization potential IP = 9.12 eY [63]) raise the energy of the n orbital by 1.39 eV relative to that of ethylene (IP = 10.51 eV [87]). A similar effect is observed in cyclohexene [64]. 

Figure 10.9 Interaction of the methyl group orbital 4 with the p orbital of the cationic carbon. 4", 4 moves down, stabilizing the structure. Its perturbed form shows delocalization of the electron pair into the cation p orbital. Orbital p moves up, but it is unoccupied and so does not alter the energy.

Such two-center orbitals may take part in molecular orbitals of sigma or pi symmetry. For example, the methyl group in propene contains three C-H bonds, each of which is of local sigma symmetry (i.e., without a nodal plane including the internuclear axis), but these three sigma bonds can in turn be combined to form a set of group orbitals, one of which has pi symmetry with respect to the principal molecular plane and can accordingly interact with the two-center orbital of pi symmetry (pi bond) of the double-bonded carbon atoms to form a molecular orbital of pi symmetry. 

Theoretically derived values were compared to some which could be derived from experimental data. The calculated values agreed quite closely with the experimental values where both were available. We present here a list of the calculated ATf values for the above reaction as G, ATT (kJ/mol), and refer the reader to the original reference for a detailed discussion [208] H, -43 CH3, -39 NH2, +77 OH, +93 F, +67 SiHs, -53 PH2, -16 SH, +23 Cl, +28 CN, -46 CH=CH2, -2 C=CH, -23 and CF3, -52. Thus, electropositive groups (SiH3) and both Z (CN, CF3) and C (CH=CH2, C=CH) substituents prefer attachment to methyl rather than carbonyl, possibly because more stabilization is available to them from 7r-type (hyperconjugative) interactions with the methyl group orbitals than with the carbonyl group orbitals. 

The orbital hybridization model of covalent bonding is readily extended to carbon-carbon bonds. As Fignre 1.23 illustrates, ethane is described in terms of a carbon-carbon O bond joining two CH3 (methyl) groups. Each methyl group consists of an -hybridized carbon attached to three hydrogens by o bonds. Overlap of the... 

The ethyl cation is the prototype system for demonstrating the effect of hyperconjugation. Consider classical CH3CH2 as a combination of a methyl group and a -CH2 centre. The group orbitals of the methyl group (equivalent to the MOs of NH3) include the 7CcH3-orbital shown schemat-... 

Figure 1 An interaction diagram for the Px and py orbitals of a carbon atom and the methyl group orbitals.

---