Methyl cation molecular orbitals

Further examination of Table 1.10 reveals that the lowest unoccupied molecular orbital, i/ 4, is a pure p orbital, localized on carbon, since the coefficients are zero for all but the 2p orbital. The MO picture is in agreement with the usual qualitative hybridization picture for the methyl cation. 

In view of the clear correlation of local ring geometry with methyl rotor barrier height in the S0 and D0 states, the strong effects of S, <— S0 excitation on rotor potentials seem to indicate substantial distortion of the ring away from hexagonal symmetry in the S, state as well. There is little clear evidence of this from molecular spectroscopy. We have speculated that such a n-molecular orbital orientation effect in the S j state (similar to that in the cation) might explain the observed characteristic... 

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... 

Hitherto, thio ether formation has clearly been proved only in the case of the ji-donor substituted 4-nitrosophenetol and the electron-rich l-methyl-2-nitrosoimidazole. The low yields of this adduct (about 2% at 1 1- and about 10% at 1 5-stoichiometry for 4-nitrosophenetol reacting with GSH56) may be the reason for its rare discovery. However, other nitrosoarenes should yield this family, too. Semiempirical molecular orbital calculations (MNDO) indicate a similar positive charge at the exposition of the N-(methylthiol-S -yl)-aniline cation and -4-anisole cation as well (Scheme 6). Furthermore, formation of l-(glutathion-S -yl)-2-naphthylamine was reported to occur in mixtures of 2-nitrosonaphthalene and GSH12. 

AMI molecular orbital studies of gas-phase deprotonations of A-methyl-4-(Y-phcnylsulfonylmcthyl)pyridinium cations and A-(Y-benzyl)-4-(phenylsulfonylmethyl)-pyridinium cations promoted by NH3 reveal negative transition state imbalances, I (= a — (1) < 0 (where a = 0.17-0.18 and /i = 0.53), which are a consequence of the difference in distance between the substituents and the anionic charge centre in the transition state and products.149... 

Figure 5.50 shows three related molecules, the 7-methyl substituted (the visual orbital progression explained here is not quite as smooth for the unsubstituted molecules) derivatives of the 7-norbomyl cation (a), the neutral alkene norbomene (b), and the 7-norbomenyl cation (c). For each species an orbital is shown as a 3D region of space, rather than mapping it onto a surface as was done in Fig. 5.49. In (a) we see the LUMO, which is as expected essentially an empty p atomic orbital on C7, and in (b) the HOMO, which is, as expected, largely the n molecular orbital of the double bond. The interesting conclusion from (c) is that in this ion the HOMO of the double bond has donated electron density into the vacant orbital on C7 forming a three-center, two-electron bond. Two n electrons may be cyclically delocalized, making the cation a bishomo (meaning expansion by two carbons) analogue of the aromatic cyclopropenyl cation [326], This delocalized bishomocyclopropenyl structure for 7-norbomenyl cations has been controversial, but is supported by NMR studies [327].

As these rearrangements are a new type of reaction, we should just spend a moment looking at the molecular orbitals that are involved. For the first step, migration of the methyl group, the LUMO must clearly be the empty p orbital of the cation, and the HOMO is the C-C o bond, which is about... 

Another test of orbital symmetry comes from the reaction of methyl cations with ketones. ICR experiments have shown at least 10 different pathways for reaction of CD with acetone. One of the minor pathways produces CsHf cation. Thermodynamically it is plausible that the neutral products are formaldehyde and molecular hydrogen, as equation 10 depicts for the orbital-symmetry allowed decomposition of the... 

Table III. Highest Occupied Molecular Orbital (HOMO) Energies, Mulliken Charges, g0> and Calculated and Experimental Proton and Methyl Cation Affinities of Some Oxyanions...

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