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Íons carbônio

Mota,Claudio J. A.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/06/2000 PT
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Carbonium ions are carbocations with a pentacoordinated carbon atom, where the electronic octet is maintained. They possess a three center two electron bond in order to keep the tetravalence of the carbon atom. This paper reviews the concept of carbonium ions, their formation, stability and reactions.

Direct one step preparation and 13C-NMR spectroscopic characterization of alpha-ferrocenyl carbocations derived from ferrocene and carbonyl compounds in trifluoroacetic acid medium1a

Prakash,G.K. Surya; Buchholz,Herwig A.; Vaughan,Julian F.S.; Wang,Qi; Olah,George. A.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/08/1999 EN
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38%
Reaction of aldehydes and ketones with ferrocene, in the presence of trifluoroacetic acid, afforded a series of stable long lived alpha-ferrocenylalkyl carbocations which were characterized by 13C-NMR spectroscopy. When this reaction was attempted using tetraphenylcyclopentadienone, quite unexpectedly the corresponding dihydro derivative 3 was isolated, in very good yield. Formation of this compound may require ferrocene acting as a reducing agent.

A theoretical study of the hydride transfer between 1-Adamantyl cation and isopentane

Oliveira,Patrícia D. de; Rosenbach Jr.,Nilton; Santos,Alex P. A. dos; Mota,Claudio J. A.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2010 EN
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A density functional teory study of the hydride transfer between 1-adamantyl cation and isopentane has been performed at the PBE1PBE/6-31G(d,p) theoretical level. The results indicate that a carbonium ion is involved as intermediate and is more stable than the van der Waals complexes between the carbenium ions and the alkanes. Entropic factors are important and overcome the enthalpic term at room temperature.

Carbocations on zeolites: quo vadis?

Mota,Claudio J. A; Rosenbach Jr,Nilton
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/07/2011 EN
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The nature of carbocations on the zeolite surface is discussed in this account, highlighting experimental and theoretical studies. The adsorption of alkylhalides over metal-exchanged zeolites has been used to study the equilibrium between covalent alkoxides and ionic carbocations. Theoretical calculations indicated that the carbocations are minima (intermediates) on the potential energy surface and stabilized by hydrogen bonds with the framework oxygen atoms. The results indicate that zeolites behave like solid solvents, stabilizing the formation of ionic species.

Interaction of allylic carbocations with benzene: a theoretical model of carbocationic intermediates in terpene biosynthesis

Oliveira,Fernanda G.; Esteves,Pierre M.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/10/2011 EN
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28.15%
Carbocations act in different ways when interacting with aromatic rings. It is interesting that in terpene biosynthesis, the carbocationic intermediates do not alkylate the aromatic side chain of the amino acids present in the enzymatic active site, as would be expected by other carbocations such as the tert-butyl cation. In this study, the interaction between benzene and different allylic carbocations, mimicking terpenoid cations, is analysed in order to better understand how this interaction would occur. Density-functional-theory (DFT) calculations show that for secondary and tertiary allylic carbocations (as found in nature), the non-covalent interaction is energetically favoured with respect to alkylation of the aromatic ring.

Calculated 11B–13C NMR chemical shift relationship in hypercoordinate methonium and boronium ions

Rasul, Golam; Prakash, G. K. Surya; Olah, George A.
Fonte: The National Academy of Sciences Publicador: The National Academy of Sciences
Tipo: Artigo de Revista Científica
Publicado em 23/06/1998 EN
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The boronium-carbonium continuum was extended to include hypercoordinated protonated methanes and their boron analogs. The 11B NMR chemical shifts of the hypercoordinated hydriodo boron compounds and the 13C NMR chemical shifts of the corresponding isoelectronic and isostructural carbocations were calculated by using the GIAO-MP2 method. The data show good linear correlation between 11B and 13C NMR chemical shifts, which indicates that the same factors that determine the chemical shifts of the boron nuclei also govern the chemical shifts of carbon nuclei of these hypercoordinated hydriodo compounds.

Acidic domains around nucleic acids.

Lamm, G; Pack, G R
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /11/1990 EN
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The hydrogen ion concentration in the vicinity of DNA was mapped out within the Poisson-Boltzmann approximation. Experimental conditions were modeled by assuming Na-DNA to be solvated in a buffer solution containing 45 mM Tris and 3 mM Mg cations at pH 7.5. Three regions of high H+ concentration (greater than 10 microM) are predicted: one throughout the minor groove of DNA and two localized in the major groove near N7 of guanine and C5 of cytosine for a G.C base pair. These acidic domains correlate well with the observed covalent binding sites of benzo[a]pyrene epoxide (N2 of guanine) and of aflatoxin B1 epoxide (N7 of guanine), chemical carcinogens that presumably undergo acid catalysis to form highly reactive carbocations that ultimately bind to DNA. It is suggested that these regions of high H+ concentration may also be of concern in understanding interactions involving proteins and noncarcinogenic molecules with or near nucleic acids.

13C NMR chemical shift correlations in application of “tool of increasing electron demand” to stable long-lived carbocations: Comprehensive evaluation*

Olah, George A.; Berrier, Arthur L.; Prakash, G. K. Surya
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /04/1981 EN
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The reliability of 13C NMR chemical shift correlations in the application of the “tool of increasing electron demand” to stable long-lived carbocationic systems is demonstrated by a comprehensive analysis of 22 stable aryl-substituted carbocationic systems. The observation of slopes of less than unity in such chemical shift correlations for several cationic systems has been attributed to additional charge delocalizing mechanisms present in the system (such as homoallylic, cyclopropyl, and π conjugations). The onset of nonclassical σ-delocalization in 2-aryl-2-norbornyl cations with electron withdrawing-substituents previously observed was further verified by using σC+ substituent constants. Difficulties in relating the CαNMR shifts in different carbocationic systems are also discussed.

Structural effects in solvolytic reactions; carbon-13 NMR studies of carbocations†: Effect of increasing electron demand on the carbon-13 NMR shifts in substituted tert-cumyl and 1-aryl-1-cyclopentyl carbocations—correlation of the data by a new set of substituent constants, σC+*†

Brown, Herbert C.; Kelly, David P.; Periasamy, Mariappan
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /12/1980 EN
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27.79%
The cationic carbon substituent chemical shifts (ΔδC+) for nine representative meta-substituted tert-cumyl carbocations are correlated satisfactorily by the σm+ substituent constants (slope ρ+ = -18.18, correlation coefficient r = 0.990). However, the substituent chemical shifts (ΔδC+) for the corresponding para derivatives are not correlated by the σp+ substituent constants. The possibility of developing a set of substituent constants capable of correlating such 13C NMR shifts was examined. The slope of the line defined by the meta substituents (ρ+ = -18.18) was utilized to calculate σC+ constants for both meta and para substituents. The utility of these constants was then tested by their ability to correlate the 13C NMR shifts in the cations for a different system, the 1-aryl-1-cyclopentyl cations. Indeed, these σC+ values correlate very well with the ΔδC+ values, yielding ρC+ = -16.84, r = 0.999.

Carbocations in the synthesis of prostaglandins by the cyclooxygenase of PGH synthase? A radical departure!

Dean, A. M.; Dean, F. M.
Fonte: Cold Spring Harbor Laboratory Press Publicador: Cold Spring Harbor Laboratory Press
Tipo: Artigo de Revista Científica
Publicado em /05/1999 EN
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27.44%
Evidence already available is used to demonstrate that although prostaglandin G/H synthase hydroxylates arachidonic acid through radical intermediates, it effects cyclizations through a carbocation center at C-10. This is produced following migration of H to the initial radical at C-13 and a 1epsilon oxidation. Under orbital symmetry control, the cyclizations can give only the ring size and trans stereochemistry actually observed. After cyclization, the H-shift reverses to take the sequence back into current radical theory for hydroxylation at C-15. Thus 10,10-difluoroarachidonic acid cannot be cyclized, although it can be hydroxylated. Acetylation of Ser516 in the isoform synthase-2 is considered to oppose carbocation formation and/or H-migration and so prevent cyclizations while permitting hydroxylations; the associated inversion of chirality at C-15 can then readily be accommodated without the change in conformation required by other schemes. Suicide inhibition occurs when carbocations form stable bonds upon (thermal) contact with adjacent heteroatoms, etc. Because the cyclooxygenase and peroxidase functions operate simultaneously through the same heme, phenol acts as reducing cosubstrate for the cyclooxygenase, thus enabling it to promote PGG2 production and protect the enzyme from oxidative destruction.

Structure/Reactivity Relationships in the Benzo[c]phenanthrene Skeleton: Stable Ion and Electrophilic Substitution (Nitration, Bromination) Study of Substituted Analogs; Novel Carbocations and Substituted Derivatives

Brulé, Cédric; Laali, Kenneth K.; Okazaki, Takao; Lakshman, Mahesh K.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
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A series of novel carbocations were generated by low temperature protonation of substituted benzo[c]phenanthrenes B[c]Phs and their charge delocalization pathways were elucidated by NMR based on the magnitude of Δδ13C values. It has been shown that the protonation regioselectivity is strongly controlled by methoxy and hydroxyl substituents, whose directive effects override methyl substitution effects. Regiocontrol by –OMe and –OH substituents, and its stronger influence relative to methyl groups, was also observed in the nitration and bromination reactions. Charge distribution modes in the regioisomeric protonated carbocations via parent B[c]Ph as well as in the benzylic carbocation formed via fjord-region epoxide ring opening were deduced by GIAO-DFT, and from the NPA-derived changes in charges over CHs. These patterns were compared with those derived from NMR experiments in the substituted derivatives. NMR-based charge delocalization mapping provided insight into structure/activity relationships in the methylated and fluorinated B[c]Phs. Regioselectivities observed in the nitration and bromination reactions in representative cases are the same as those via protonations. Among a group of novel nitro and bromo derivatives synthesized in this study are examples where nitro group is introduced into the fjord-region...

Synthetic Efficiency in Enzyme Mechanisms Involving Carbocations: Aristolochene Synthase

Allemann, Rudolf K.; Young, Neil J.; Ma, Shuhua; Truhlar, Donald G.; Gao, Jiali
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
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An intramolecular proton transfer mechanism has been proposed for the carbocationic cyclization of farnesyl pyrophosphate (FPP) to (+)-aristolochene catalyzed by aristolochene synthase. This novel mechanism, which is based on results obtained by high-level ab initio molecular orbital and density functional theory calculations, differs from the previous proposal in the key step of carbocation propagation prior to the formation of the bicyclic carbon skeleton. Previously, germacrene A was proposed to be generated as an intermediate by deprotonation of germacryl cation followed by reprotonation of the C6-C7 double bond to yield eudesmane cation. In the mechanism proposed here the direct intramolecular proton transfer has a computed barrier of about 23 kcal/mol, which is further lowered to 16−20 kcal/mol by aristolochene synthase. An alternative pathway is also possible through a proton shuttle via a pyrophosphate-bound water molecule. The mechanism proposed here is consistent with the observation that germacrene A is not a substrate of aristolochene synthase. Furthermore, the modeled substrate-enzyme complex suggests that Trp 334 and Phe 178 play key roles in positioning the substrate in the reactive orientation in the binding pocket. This is consistent with experimental findings that mutations of either residue lead to pronounced generation of aborted cyclization products.

Electrophilic Chemistry of Thia-PAHs: Stable Carbocations (NMR and DFT), S-alkylated Onium Salts, Model Electrophilic Substitutions (Nitration and Bromination) and Mutagenicity Assay

Laali, Kenneth K.; Chun, Joong-Hyun; Okazaki, Takao; Kumar, Subodh; Borosky, Gabriela L.; Swartz, Carol
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
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First examples of stable carbocations are reported from several classes of thia-PAHs with four fused rings, namely benzo[b]naphtho[2,1-d]thiophene (1), and its 3-methoxy-derivative (2), phenanthro[4,3-b]thiophene (3) and its 7-methoxy- (4), 10-methoxy- (5) and 9-methoxy- (6) derivatives, phenanthro[3,4-b]thiophene (7) and its 7-methoxy- (8) and 9-methoxy- (9) derivative, and 3-methoxybenzo[b]naphtha[1,2-d]thiophene (11). In several cases the resulting carbocations were also studied by GIAO-DFT. Charge delocalization modes in the resulting carbocations were probed. A series of S-alkylated onium tetrafluoroborates namely 1Me+, 1Et+, 2Et+, and 7Me+ (from 1, 2 and 7), 10Me+ and 10Et+ (from benzo[b]naphtha[1,2-d]thiophene 10), 12Me+ and 12Et+ (from phenanthro[3,2-b][1]benzothiophene 12), 13Me+ (from 3-methoxyphenanthro[3,2-b]benzothiophene 13), 14Me+ (from phenanthro[4,3-b][1]benzothiophene 14), and 15Me+ (from 3-methoxyphenanthro[4,3-b][1]benzothiophene 15) were synthesized. PAH-sulfonium salts 1Me+, 1Et+, 10Me+, 10Et+, 12Me+, and 14Me+ proved to be efficient akylating agents towards model nitrogen bases (imidazole and azaindole). Facile transalkylation to model nucleophiles (including guanine) is also supported by favorable reaction energies computed by DFT. Ring opening energies in thia-PAH-epoxides from 1...

Carbocations from Oxidized Metabolites of Benzo[a]anthracene. A Computational Study of Their Methylated and Fluorinated Derivatives and Guanine Adducts

Borosky, Gabriela L.; Laali, Kenneth K.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /07/2006 EN
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28%
Structure-reactivity relationships and substituent effects on carbocation stability in benzo[a] anthracene (BA) derivatives have been studied computationally at the B3LYP/6-31G* and MP2/6-31G** levels. Bay-region carbocations are formed by O-protonation of the 1,2-epoxides in barrierless processes. This process is energetically more favored as compared to carbocation generation via zwitterion formation/O-protonation, via single electron oxidation to generate a radical cation, or via benzylic hydroxylation. Relative carbocation stabilities were determined in the gas phase and in water as solvent (PCM method). Charge delocalization mode in the BA carbocation framework was deduced from NPA-derived changes in charges, and substitution by methyl or fluorine was studied at different positions selected on basis of the carbocation charge density. A bay-region methyl group produces structural distortion with consequent deviation from planarity of the aromatic system, which destabilizes the epoxide, favoring ring opening. Whereas fluorine substitution at sites bearing significant positive charge leads to carbocation stabilization by fluorine p-π back-bonding, a fluorine atom at a ring position which presented negative charge density leads to inductive destabilization. Methylated derivatives are less sensitive to substituent effects as compared to the fluorinated analogues. Although the solvent decreases the exothermicity of the epoxide ring opening reactions due to greater stabilization of the reactants...

A Marcus Treatment of Rate Constants for Protonation of Ring-Substituted α-Methoxystyrenes: Intrinsic Reaction Barriers and the Shape of the Reaction Coordinate

Richard, John P.; Williams, Kathleen B.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
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Rate and equilibrium constants were determined for protonation of ring-substituted α-methoxystyrenes by hydronium ion and by carboxylic acids to form the corresponding ring-substituted α-methyl α-methoxybenzyl carbocations at 25 ° C and I = 1.0 (KCl). The thermodynamic barrier to carbocation formation increases by14.5 kcal/mol as the phenyl ring substituent(s) is changed from 4-MeO- to 3,5-di-NO2-, and as the carboxylic acid is changed from dichloroacetic to acetic acid. The Brønsted coefficient α for protonation by carboxylic acids increases from 0.67 to 0.77 over this range of phenyl ring substituents and the Brønsted coefficient β for proton transfer increases from 0.63 to 0.69 as the carboxylic acid is changed from dichloroacetic to acetic acid. The change in these Brønsted coefficients with changing reaction driving force, ∂α∕∂ΔGavo=∂β∕∂ΔGavo=1∕8Λ=0.011 is used to calculate a Marcus intrinsic reaction barrier of Λ = 11 kcal/mol which is close to the barrier of 13 kcal/mol for thermoneutral proton transfer between this series of acids and bases. The value of α = 0.66 for thermoneutral proton transfer is greater than α = 0.50 required by a reaction that follows the Marcus equation. This elevated value of β may be due to an asymmetry in the reaction coordinate that arises from the difference in the intrinsic barriers for proton transfer at the oxygen acid reactant and resonance stabilized carbon acid product.

Synthesis of β–Heteroaryl Propionates via Trapping of Carbocations with π-Nucleophiles

Fu, Tsung-hao; Bonaparte, Amy; Martin, Stephen F.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 01/07/2009 EN
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A variety of heterocyclic alcohols and acetates were coupled with silyl ketene acetals and other π-nucleophiles in the presence of trimethylsilyl trifluoromethanesulfonate to provide an array of substituted β-heteroaryl propionates, including those with contiguous quaternary centers, as well as vinylogs thereof. This reaction also proceeds with high diastereoselectivity when the π-nucleophile bears a chiral auxiliary.

A Greatly Under-Appreciated Fundamental Principle of Physical Organic Chemistry

Cox, Robin A.
Fonte: Molecular Diversity Preservation International (MDPI) Publicador: Molecular Diversity Preservation International (MDPI)
Tipo: Artigo de Revista Científica
Publicado em 28/11/2011 EN
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17.44%
If a species does not have a finite lifetime in the reaction medium, it cannot be a mechanistic intermediate. This principle was first enunciated by Jencks, as the concept of an enforced mechanism. For instance, neither primary nor secondary carbocations have long enough lifetimes to exist in an aqueous medium, so SN1 reactions involving these substrates are not possible, and an SN2 mechanism is enforced. Only tertiary carbocations and those stabilized by resonance (benzyl cations, acylium ions) are stable enough to be reaction intermediates. More importantly, it is now known that neither H3O+ nor HO− exist as such in dilute aqueous solution. Several recent high-level calculations on large proton clusters are unable to localize the positive charge; it is found to be simply “on the cluster” as a whole. The lifetime of any ionized water species is exceedingly short, a few molecular vibrations at most; the best experimental study, using modern IR instrumentation, has the most probable hydrated proton structure as H13O6+, but only an estimated quarter of the protons are present even in this form at any given instant. Thanks to the Grotthuss mechanism of chain transfer along hydrogen bonds, in reality a proton or a hydroxide ion is simply instantly available anywhere it is needed for reaction. Important mechanistic consequences result. Any charged oxygen species (e.g....

Hyperconjugation in Carbocations, a BLW Study with DFT approximation

Alamiddine, Zakaria; Humbel, Stéphane
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
Publicado em 07/01/2014 EN
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27.79%
The geometry of ethyl cation is discussed, and the hyperconjugation effect in carbocations is evaluated at the B3LYP/6-311G(d) level. The Block Localized Wavefunction (BLW) method is used for all evaluations of the hyperconjugation, considered as the energy gained by the delocalization onto the C+ atom. This energy is defined as the energy difference between the delocalized (standard) calculation, where the electrons are freely delocalized, and a localized form where the positive charge sits on the carbon center. It is evaluated for 18 carbocations, including conjugated systems. In these cases we were particularly interested in the additional stabilization brought by hyperconjugative effects. Among other effects, the β-silicon effect is computed. Hyperconjugation amounts in several cases to an energy similar to conjugation effects.

Índice de aromaticidade baseado na deslocalização, degenerescência e densidade (D3BIA) para acenos e moléculas homoaromáticas

Costa, Tamires Ferreira da
Fonte: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Química; Físico-Química; Química Publicador: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Química; Físico-Química; Química
Tipo: Dissertação Formato: application/pdf
POR
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The aromaticity index is an important tool for the investigation of aromatic molecules. This work consists on new applications of the aromaticity index developed by teacher Caio Lima Firme, so-called D3BIA (density, delocalization, degeneracy-based index of aromaticity). It was investigated its correlation with other well-known aromaticity indexes, such as HOMA (harmonic oscillator model of aromaticity), NICS (nucleus independent chemical shielding), PDI (para-delocalization index), magnetic susceptibility (), and energetic factor in the study of aromaticity of acenes and homoaromatic species based on bisnoradamantanyl cage. The density functional theory (DFT) was used for optimization calculations and for obtaining energetic factors associated with aromaticity and indexes HOMA and NICS. From quantum theory of atoms in molecules (QTAIM) it was obtained the indexes D3BIA, PDI and . For acenes, when the over-mentioned indexes were applied it was observed no correlation except for D3BIA and HOMA (R2=0.752). For bisnoradamantenyl dication and its derivatives, it was obtained a good correlation between D3BIA and NICS. Moreover, it was evaluated solely one of the factors used on D3BIA calculation, the delocalization index uniformity (DIU)...

α-sulfur or α-fluorine ̶ Which is more stabilizing for a carbocation? A computational study of electrophilic addition to HFC=CH(SMe) and FC(R1)=CR2(SMe) and related model systems

Laali, Kenneth K.; Borosky, Gabriela Leonor
Fonte: Elsevier Publicador: Elsevier
Tipo: info:eu-repo/semantics/article; info:ar-repo/semantics/artículo; info:eu-repo/semantics/publishedVersion Formato: application/pdf
ENG
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Carbocations derived from protonation, methylation, and bromine addition to HFC55CH(SMe) (1) were studied by DFT [B3LYP/6-31G*, B3LYP/6-311G(3df,p), and B3LYP/aug-cc-pVTZ] and by MP2/6-31G* to examine relative carbocation stabilizing effects of a-SMe versus a-fluorine. The a-SMe carbocations 1aE+ and the unsymmetrical thiiranium ions 1cE+ were found to be considerably more stable than the corresponding a-fluorocarbenium ions 1bE+ . Study of protonation and methylation of FC(R1)55CR2(SMe) [R1 = H, R2 = SMe (2); R1 = F, R2 = H (3); R1 = F, R2 = SMe (4); R1 = Me, R2 = H (5); R1 = Me, R2 = SMe (6)] by B3LYP/6-31G* identified the corresponding unsymmetrical thiiranium cations as lowest energy minima, followed closely by the a-thiocarbenium ions. With 2, 4, and 6, skeletally rearranged a-thiocarbenium ions are formed by SMe migration. The a-SMe and the thiiranium cations are also favored relative to a- fluorocarbenium ions in protonation and methylation of the cyclic analogs 8 and 9. Computed NPA charges and the GIAO-derived 13C and 19F NMR chemical shifts underscore the significance of athiocarbocations and thiiranium ions in electrophilic addition to FC(R1)55CR2(SMe).; Fil: Laali, Kenneth K.. University of North Florida. Department of Chemistry; Estados Unidos de América;; Fil: Borosky...