Formation and decomposition of radical anions. A theoretical study

Villar, Hugo O.; Castro, Eduardo A.; Rossi, Roberto A.

doi:10.1139/v82-362

Cited by 26 publications

(3 citation statements)

References 5 publications

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“…It is very surprising that despite the importance of the processes that involve the cleavage of formal three-electron bonds in aromatic derivatives, very few theoretical studies have been performed on them. A series of well-established empirical rules, and the qualitative use of the “reactive mixed-valence approach” are the customary tools in predicting the organic reactivity in this particular field. ,4c,, Theoretical molecular orbital (MO) ab initio investigation of the reductive C−Cl bond cleavage in nonsubstituted benzyl derivatives led to the conclusion that the corresponding radical anions show a dissociative behavior 18a. However, recent density functional theory (DFT) calculations reveal the formation of radical anions in these systems 18b.…”

Section: Introductionmentioning

confidence: 99%

Thermodynamics, Kinetics, and Dynamics of the Two Alternative Aniomesolytic Fragmentations of C−O Bonds: An Electrochemical and Theoretical Study

et al. 2002

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Fragmentation reactions of radical anions (mesolytic cleavages) of cyanobenzyl alkyl ethers (intramolecular dissociative electron transfer, heterolytic cleavages) have been studied electrochemically. The intrinsic barriers for the processes have been established from the experimental thermodynamic and kinetic parameters. These values are more than 3 kcal/mol lower as an average than the related homolytic mesolytic fragmentations of radical anions of 4-cyanophenyl ethers. In the particular case of isomers 4-cyanobenzyl phenyl ether and 4-cyanophenyl benzyl ether, the difference in intrinsic barriers amounts to 5.5 kcal/mol, and this produces an energetic crossing where the thermodynamically more favorable process (homolytic) is the kinetically slower one. The fundamental reasons for this behavior have been established by means of theoretical calculations within the density functional theory framework, showing that, in this case, the factors that determine the kinetics are clearly different (mainly present in the transition state) from those that determine the thermodynamics and they are not related to the regioconservation of the spin density ("spin regioconservation principle"). Our theoretical results reproduce quite well the experimental energetic difference of barriers and demonstrate the main structural origin of the difference.

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Section: Introductionmentioning

confidence: 99%

Thermodynamics, Kinetics, and Dynamics of the Two Alternative Aniomesolytic Fragmentations of C−O Bonds: An Electrochemical and Theoretical Study

et al. 2002

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“…Cleavage processes in organic radical ions have been examined in some ab initio and semiempirical − theoretical studies, while other papers have dealt with this subject from both an experimental and a theoretical point of view. , In the first study carried out in this laboratory, the nature of the C−C or C−Si σ bond cleavage process, which follows the generation of a radical cation or anion from neutral model precursors, was examined . Four rigid and symmetric ( C s ) methyl or silyl-cyclopropenyl model radical cations and anions were studied (Scheme a, Y = CH 3 or SiH 3 ).…”

Section: Introductionmentioning

confidence: 99%

Methyl and Silyl Mesolytic Dissociations in the Radical Cations and Radical Anions of But-1-ene, Allylsilane, Hexa-1,3-diene, and Penta-2,4-dienylsilane. CAS−MCSCF and Coupled Cluster Theoretical Study

2003

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Methyl or silyl dissociation in the CH(2)=CHCH(2)-XH(3) (a-XH(3)(*)(+)) and CH(2)=CHCH=CHCH(2)-XH(3) (p-XH(3)(*) (+)) radical cations (X = C, Si) yields a(+) or p(+) and XH(3)(*). Similarly, the radical anions a-CH(3)(*) (-) and p-CH(3)(*) (-) give the pi-delocalized anion and CH(3)(*) preferentially. In contrast, a-SiH(3)(*) (-) and p-SiH(3)(*-) prefer to dissociate into the pi-delocalized radical and silide. All reactions are endoergic: by 43-50 kcal mol(-)(1) in the radical cations, and easier to some extent in the radical anions, that require 29-33 (X = C) and 13-14 kcal mol(-)(1) (X = Si). The fragmentation energy profiles do not present significant barriers for the backward process in the case of the radical cations. All radical anions exhibit an energy maximum along the dissociation pathway, but the barrier is lower than the dissociation limit. Fragmentation is "activated" more in the anions than in the cations with respect to homolysis in the corresponding neutrals (that requires 72-81 kcal mol(-)(1)). Wave function analysis indicates that the C-X bond cleavage in the hydrocarbon radical ions, although formally comparable to a homolytic process, is at variance with this model, due to the spin recoupling of one of the two C-X bond electrons with the originally unpaired electron. This is basically true also for the silyl-substituted radical anions, in which the initial more delocalized charge distribution might suggest some heterolytic character of the bond cleavage.

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“…The following systems have been dealt with. The PhX radical anions, generated by Ph • + X - coupling (X = Me, F - , Cl - ), that have been studied using the INDO and CNDO/2 methods . Then the C−C bond cleavage in PhEt •+ to give a benzyl cation and a methyl radical was studied by Hartree−Fock (HF) 5a and AM1 5b methods; in the latter paper the bibenzyl radical cation was also studied.…”

Section: Introductionmentioning

confidence: 99%

Nature of Methyl and Silyl Mesolytic Dissociations in Substituted Cyclopropenyl Radical Cations and Anions. A CAS-MCSCF and CCSD(T) Theoretical Study

1999

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Methyl-and silyl-cyclopropenyl radical charged systems are chosen to model the dissociative behavior of rigid and symmetric species. Dissociation of the radical cations in two fragments yields c-C 3 H 3 + and XH 3 • moieties (X ) C, Si), while, in the radical anions c-C 3 H 3 • and XH 3 -fragments are produced. CAS-MCSCF C s energy profiles show the presence of C-X bond cleavage saddle points in all four cases, separated from the resulting products by energy minima corresponding to electrostatic complexes. These features are retained in the coupled cluster C s energy profiles, obtained by series of single-point calculations on CAS-MCSCF geometries, optimized at fixed C-X distances. However, at this theory level, the radical cation reactions are significantly more endoergic.The methyl system has a less unfavorable reaction energy than the silyl (16 vs 20 kcal mol -1 ), and both saddle points prove to be slightly lower in energy than the dissociation limits (by ca. -4 and -2.5 kcal mol -1 , respectively). For the radical anions, a more pronounced endoergicity in the carbon case and a less unfavorable process for silicon are found (54 vs 39 kcal mol -1 ). Moreover, while the C s saddle point is lower in energy than the dissociation limit in the carbon case, it is higher for silicon (ca. -7 and +2 kcal mol -1 , respectively). It has to be pointed out, however, that even in the more endoergic radical anion fragmentations the process is easier than homolysis in the neutral parent molecules. The calculations carried out on C s radical anions show the possible occurrence, in rigid systems, of real surface crossings, which open in principle the possibility of obtaining excited fragment products. However, it is clear that for more flexible systems a deformation of the structure along the dissociation pathway could generate a conical intersection. In this case the radical anions could certainly follow a lower-energy C 1 pathway in correspondence of an avoided crossing and bypass the real crossing.

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Formation and decomposition of radical anions. A theoretical study

Cited by 26 publications

References 5 publications

Thermodynamics, Kinetics, and Dynamics of the Two Alternative Aniomesolytic Fragmentations of C−O Bonds: An Electrochemical and Theoretical Study

Thermodynamics, Kinetics, and Dynamics of the Two Alternative Aniomesolytic Fragmentations of C−O Bonds: An Electrochemical and Theoretical Study

Methyl and Silyl Mesolytic Dissociations in the Radical Cations and Radical Anions of But-1-ene, Allylsilane, Hexa-1,3-diene, and Penta-2,4-dienylsilane. CAS−MCSCF and Coupled Cluster Theoretical Study

Nature of Methyl and Silyl Mesolytic Dissociations in Substituted Cyclopropenyl Radical Cations and Anions. A CAS-MCSCF and CCSD(T) Theoretical Study

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