Benzyl cation calculated structure

The described superelectrophilic activation and fluorene-cyclization is thought to involve a lowered energy of the LUMO and concomitant delocalization of positive charge into the aryl ring(s).32b Calculations at the 4-31G//STO-3G level on a model system (Figure 2) have shown that the amount of positive charge in the phenyl ring increases upon formation of the dication (67) when compared to the monocation (66) and the benzyl cation (calculations are based on fully planar structures). It is well known... 

The Yukawa-Tsuno r values have been measured for the solvolysis reactions fonning benzyl cations and several a-substituted derivatives, 6-3IG charges and bond orders have been calculated for the presumed cationic intermediates. Analyze the data for relationships between r and the structural parameters. (Hint. Plot r versus the bond orders and the charges at C-1, C-2, C-3, and C-4.)... 

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroantimonate salt at —124 °C.31 It is nearly planar (8 ° twist), with a short bond between the C+ and the ring (1.41 A), consistent with benzylic delocalization. The Me—C+ bonds are also shortened, indicative of hyperconjugative interaction.31 However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5).32 The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation.32 The heats of formation of some a-mcthylbcnzyl cations indicate that the primary stabilization in these species comes from the a-substitucnts, and that the stabilization provided by the aromatic ring is secondary.33... 

Calculated structures for the benzyl radical and benzyl cation are presented in Fig. 15.6. These structures show the presence at their ortho and para carbons of unpaired electron density in the radical and positive charge in the cation, consistent with the resonance structures above. 

FIGURE 15.6 The gray lobes in the calculated structure for the benzyl radical (left) show the location of density from the unpaired electron. This model indicates that the unpaired electron resides primarily at the benzylic, ortho, and para carbons, which is consistent with the resonance model for the benzylic radical discussed earlier. The calculated electrostatic potential map for the bonding electrons in the benzyl cation (right) indicates that positive charge (blue regions) resides primarily at the benzylic, ortho, and para carbons, which is consistent with the resonance model for the benzylic cation. The van der Waals surface of both structures is represented by the wire mesh. 

Part Three. The benzyl (and allyl) halides are a special case they have resonance. To see how the charge is delocalized in the benzyl carbocation, request two plots the electrostatic potential mapped onto a density surface and the LUMO mapped onto a density surface. Submit these for calculation at the AMI semiempirical level. On a piece of paper, draw the resonance-contributing structures for the benzyl cation. Do the computational results agree with the conclusions you draw from your resonance hybrid ... 

Figure 1.21 presents the MOs of benzene, both in symbolic form and as produced by an accurate quantum mechanical calculation. Also shown is the "HOMO" of benzyl—-that is, the singly occupied orbital of benzyl radical, the empty orbital of benzyl cation, and the doubly occupied HOMO of benzyl anion. To a good approximation, the MO has the same form for all three structures. Here, we show the orbitals from the top, so that the p orbitals that make up the molecular orbitals appear only as spheres. 

We present in Fig. 35 the structure (C symmetry) optimized at the MP2(full)/6-31G(d) level (this work). Recently, a large computational study has been published on benzyl and larger carbocations. It uses both the optimized force field MMP2 extended to carbocations and MP4sdq/6-31G(d)//MP2(full)/6-31G(d) calculations. It provides, inter alia, valuable estimates of standard heats of formation. In this case, the difference in stability between phenethyl and 4-methylbenzyl cations (1.7 by ab initio computations and 2.9 kcal mol by MMP2) is in good agreement with the experimental results in Ref. 51. Our own G2(MP2) results are presented in Table 9. 

Pyridine and methyl imidazole were chosen for the validation calculations because they exhibit some structural similarity to the imidazolium cation, present in the ionic liquids under investigation. Carbon disulfide was chosen as a limiting case because it has a negligible electrostatic component, whereas formamide was chosen because it has both large dispersive and electrostatic components. Benzyl alcohol exhibits an average behavior. 

---