Phenyl cation calculations

Calculate energies for reaction of 2-methyl-2-propyl chloride (to 2-methyl-2-propyl cation), 2-phenyl-2-propyl chloride (to 2-phenyl-2-propyl cation) mdphenyl chloride (to phenyl cation). (The energy of chloride is given at right.) Assuming that reaction (1) is normal , what is the effect of a benzene ring Does it facilitate or hinder loss of chloride ... 

Furthermore, we have to keep in mind that differences in thermodynamic stability of reagent(s) and product(s) do not include a kinetic parameter, the activation energy. The assumption made by Vincent and Radom, as well as by Brint et al., that the addition of N2 to the phenyl cation is a reaction with zero activation energy may be correct for the gas phase, but perhaps not for reaction in solution. One must therefore add an activation energy barrier to the calculated thermodynamic stability mentioned above for the reverse reaction (C6HJ + N2 — C6H5NJ). 

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. ... 

Theoretical calculations predict that the highest occupied Walsh orbital of the cyclopropene ring in 1 should stabilize a positive charge at C2, ° and this prediction was experimentally confirmed. Appearance energy measurements for the loss of Br from ionized 2-bromobenzocyclopropene (202 X = Br) suggest that the benzo-cyclopropen-2-ylium ion 291 is stabilized by more than 27.6 kcal/mol over the phenyl cation 292. Calculations (3-2IG ) give a stabilization energy of 23 and 22 kcal/mol of 291 over 292 and over the m-isomer 293, respectively. ... 

The phenyl cation has been the subject of high-level ab initio calculations.116 The ground state was shown to be a singlet, and the singlet-triplet gap was estimated.116 The gas-phase transfers of H+, H-, and H2O to Ph+ have been studied.117 Arenium... 

Silyl substituents have also distinct effects on the stability of aryl cations. For example, the disilyl (23) and trisilyl-substituted (24) phenyl cations are calculated to be more stable than the parent phenyl cation 25 by 25 kcalmol-1 (HF3-21G)11 and by 22.4 kcalmol-1 (HF6-31G(d))12, respectively. 

The phenyl cation (134) firstpostulated by Waters335 is a highly reactive species oflow stability and plays a fundamental role in organic chemistry—for example, in the chemistry of diazonium ions. According to gas-phase studies and calculations, its stability is between that of the ethyl cation and the vinyl cation.336 Since it is an extremely electrophilic and short-lived species, it could not be isolated or observed directly in the condensed phase. For example, solvolytic and dediazoniation studies under superacidic conditions by Faali et al.337,338 failed to find evidence of the intermediacy of the phenyl cation. Hyperconjugative stabilization via orf/zo-Me3Si or... 

Protonation of fluorobenzene in the gas phase has been studied by infrared photodissociation (IRPD) spectroscopy by Solca and Dopfer.351 F-protonated fluorobenzene was formed in significant amount when protonation was carried out with CH5+. It was found to be the most stable isomer in the gas phase by quantum mechanical calculations [B3LYP/6-311G(2df,2pd) level] separated by a large energy barrier from the four Wheland intermediates. F-protonated fluorobenzene is best described as a weakly bound ion-dipole complex between the phenyl cation and HF. 

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... 

A linear bond-length/bond-order correlation has been proposed for the S—S bond in 1,2-dithiolium cations. Calculated 7r-bond orders and experimental sulfur-sulfur bond lengths for 3- and 4-phenyl-1,2-dithiolium cations are in good agreement with this correlation. ... 

The Group 14 metallocenes are the most widely known series of isolated main group sandwich compounds. Schleyer calculated the model compounds T -CpC and T -CpSi and found that the pentahapto carbon compound is less stable than the single-bonded fulvenyl cation or the phenyl cation (36). On the other hand, Tj -CpSi has been predicted to be the most stable one among possible isomers. A mass spectrometry study of Group 14 half-sandwich metallocenes has been made and their atomization energies were calculated (57). It was concluded that nido-cluster is the most stable stmcture and that the stability of these metallocenes increases from CpPb to CpC+. 

Constrained geometries have been used extensively in the context of mechano-chemistry, with applications including studies of bond mpture [82-86], reactivity of disulfide bonds [87], unfolding of supramolecular polymers [88], mechanochemical synthesis of phenyl cations [89], extraction of gold nanowires [90], evaluation of restoring forces in force probes [14, 91], and calculation of free energy barriers [92-98]. Examples of selected applications are described below. 

Theory predicts that the 1-napthyl cation is a ground-state triplet. The gas-phase IR spectrum of the cation matched the spectrum calculated for the triplet and not the singlet. Phenyl cations substituted with ortho groups such as CHjOH, CH=0, CH=CH2, NO2 were investigated by DPT methods. In most cases, these cations undergo a ring-closure reaction with the substituent. 

There exists the possibility of an electron transfer from the tt system, leading to structure D, which would have two unpaired electrons. A molecular orbital calculation at the 4-3IG level has indicated that the singlet structure C is more stable than the triplet structure by about 20 kcal/mol, but it is highly distorted with the C-C-C bond angle at the divalent carbon calculated to be 145°." " Phenyl cations are formed by the thermal decomposition of aryldiazonium ions." The relatively high stability of arylidiazonium ions, even in comparison with primary aliphatic diazonium ions, reflects the instability of the phenyl cation. The cation is also extremely reactive. 

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