Planar Methyl

The orbitals of a planar methyl group, created using the rules of Table 1.7. 

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Consider the planar methyl radical, H3C, which has three geometrically equivalent protons. Their spins combine to produce four distinct values for the local field ... 

The only atomic wave-functions that do not have a node at the nucleus are s-functions. The isotropic coupling constant is thus a measure of the s-character of the wave-function of the unpaired electron at the nucleus in question. The coupling constant for an atomic s-electron can be either measured experimentally or calculated from Hartree-Fock atomic wave-functions so that, to a first approximation, the s-electron density may be calculated from the ratio of the experimental and atomic coupling constants. Should the first-order s-character of the wave-function of the unpaired electron be zero, as for example in the planar methyl radical, then a small isotropic coupling usually arises from second-order spin-polarization effects. The ESR spectra of solutions show only isotropic hyperfine coupling. 

Significant changes are also observed for the proton and halogen constants. Thus, the 11 isotropic coupling constant which is negative in the planar methyl radical (spin polarization, Aiso = -23 G), becomes positive when protons are substituted by fluorines133 (Table 15) and a monotonic increase in the 19F isotropic coupling constant is also caused by the... 

This model shows the electron density for the odd electron of the planar methyl radical. The radical electron is in a p orbital perpendicular to the plane of the atoms. 

Covalent nitrates. Apart from organic nitrates covalent nitrates of non-metals are limited to those of H, F, and Cl. (CINO3 has been prepared from anhydrous HNO3 and CIF as a liquid stable at —40°C in glass or stainless steel vessels. 9 Electron diffraction studies have been made of the explosive gas FN03 and of the (planar) methyl nitrate molecule. A refinement of the crystal structure of pentaerythritol nitrate, C(CH20N02)4, shows that the nitrate group has the same structure, (d), as in nitric acid. 

The symmetry factor a in Eq. (2.10) deserves special consideration. A loose transition state will be characterized by large values of R j and, hence, will have essentially planar methyl radicals. In this case each of the identical fragments has both C2v and C3 axes, resulting in a a = (3 x 2)2 x 2 = 72. The results given in Table I were obtained using this value of a. Results obtained by an... 

Figure 4.3 Mechanism of spin polarization for planar methyl radical.

Figure 3.13 Restricted Hartree-Fock description of the planar methyl radical.

In this procedure, FSGO parameters are obtained from Frost model calculations on molecular fragments chosen to mimic various molecular environments . For example, a planar methyl radical would be used to mimic the environment of an sp carbon atom Fig. 1 a, where the locations of FSGOs containing the inner-shell and bonding electron pairs are indicated by the filled circles on the figure. The pair of spherical Gaussians, gj and g, located above and below the plane of the molecule are combined as... 

To create the group orbitals for planar methyl we used a total of seven atomic orbitals. There must be a conservation of the number of orbitals, meaning that the number of molecular orbitals created must equal the number of atomic orbitals we start with. Yet, in Figure 1.7 we only show five orbitals. Orbital E and the two MOs that we do not show are called virtual orbitals, meaning that they exist but are not t3qjically populated with electrons (see the discussion of adding electrons below). We do not draw two of them because they are not involved in bonding schemes, except with excited electronic states. 

Normal mode Vibration frequencies Tetrahedral Multiplicity methyl in cm" Planar methyl... 

Figure 3-5 Approximate orbital description of the abstraction of a hydrogen atom by a chlorine atom to give a methyl radical and hydrogen chloride. Notice the rehybridization at carbon in the planar methyl radical. The additional three nonbonded electron pairs on chlorine have been omitted. The orbitals are not drawn to scale. The symbol 1 identifies the transition state.

Tye JW, Hartwig IF (2009) Computational studies of the relative rates for migratory insertions of alkcmes into square-planar, methyl, -amido, and -hydroxo complexes of rhodium. J Am Chem Soc 131(41) 14703-14712... 

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