Benzyl cations calculations

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.)... 

Experimental observations,23 supported by high-level ab initio calculations, 24 indicate that two extreme resonance forms contribute to the general energy of the benzyl cation the aromatic form A, in which the positive charge is concentrated at the methylene group, and the nonaromatic, methylene arenium form B with a sp2 ipso-carbon atom and ring-localized charge (Scheme 3.13). Unlike benzyl cations of the form A, which were isolated and studied, especially by Olah and coworkers,23 compounds represented by the form B remained elusive. Thus, metal complexation... 

The thermodynamic stabilities of phenonium ions relative to the parent have been determined in the gas phase by measuring the position of the equilibrium between (46) and (47)7 The results followed a Yukawa-Tsuno relationship with a p value of -12.6 and an r+ value of 0.62, the general behaviour being more like benzenium ions than benzyl cations, with tt-delocalization less effective than in benzyl cations. A theoretical study of the elimination of molecular H2 from the benzenium ion C6H7+ shows that the barrier to this process appears to be very small.The gas-phase Friedel-Crafts alkylation reaction of CF3C6L6+ (L = H or D) with C2L4 is accompanied by isotopic scrambling, which has been used to elucidate the mechanism of this process. A theoretical calculation shows that the lifetime of triplet phenyl cation must be very short. ... 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

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... 

From the relative solvolysis rates of 213 and 215, Shimizu and coworkers calculated a stabilization by 7 kcalmoD1 of the benzyl cation by the /1-Si—Si bond compared with a /1-C—Si bond. Due to the antagonistic effects of the a- and the /)-silyl groups, the net effect of the disilanyl group is relatively small86. Thus, 213 is only 12 times as reactive as a-methylbenzyl bromide 219, X = Br (Table 7 in 30% acetone the relative rate is decreased to 0.66). This might be compared with the rate acceleration of 1.05 x 105 by... 

What can we say about a benzyl cation From the ionization potential (179 kcal) of the benzyl free radical, we can calculate (Sec. 5.18) that the benzyl cation is 69 kcal more stable than the methyl cation, and Just about as stable as the /er/-butyl cation. We can now expand our sequence of Sec. 8.21 to include the benzyl cation ... 

Using semiempirical calculations, Shevchenko et al. [100] studied the impact of conformation on charge and frontier molecular orbitals in quinone methides and benzyl cations. Charges were sensitive to conformation, while the frontier molecular orbitals were not. These data were used to interpret the stereoselectivity of quinone methides toward nucleophiles. Additional related work addressed the Z-E conformation about the C-l-C-a double bond in quinone methides. Konshcin et al. 

It is known that for SnI ewaction (substitution-nucleophilic-unimolecular) [140-141] a key intermediate is a carbocation, therefore the more reactive substrate will be the one that can produce the most stable carbocation. The reactivity of methyl, ethyl, 2-propyl, and 2-methyl-2-propyl tosylates under SnI reaction conditions is inversely proportional to the calculated hydride affinity of the corresponding carbocations. The calculated values were in agreement with the experimental findings which were obtained through solvolysis rate measurement of these tosylates under SnI conditions [142, 143]. Correlation of the cation stability-hydride and affinity-solvolytic rate of the reaction under Sn 1 reaction conditions was observed for the allyl cation (allyl, 3-penten-2-yl, and 2-methyl-3-butene-2-yl cations)[144] and the benzyl cation (benzyl, 1-phenylethyl, and 2-phenyl-2-propyl cations) [145] series. The most reactive substrates were the ones that formed the carbocations with the lowest hydride affinity. 

Electron impact ionization of phenylsilane has provided evidence for the silatropyhum (1) and silabenzyl (2) cations <92JA3573) the relative stabilities of the two cations have been determined by ab initio molecular orbital calculations <94JA9769>. The calculations indicated that (2) is more stable than (1) by 38 kJ mol , which is the contrary to the situation for the tropylium and benzyl cations. 

We know (see p. 60) that an alternant hydrocarbon (AH) has a self-consistent field so that = 0 at all atoms therefore if we remove an electron from the NBMO to get a benzyl cation, the pasitive charge will be distributed sole over those atoms whose orbital coefficients are not zero for the NBMO. The same will be true if we add an electron to the radical and make the benzyl anion. The NBMO coefficients are clearly of signal importance since their values determine the calculated distribution of the odd electron in the radical and the charges in the cation and anion. For the benzyl radical the NBMO may be rendered schematically as follows ... 

HOMO (left) and LUMO (right) of benzyl cation (PM3 calculation). 

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. 

---