Ethyl Cation Research Articles

Exploring the potential energy surface of the ethyl cation by new procedures

The MP2/6-31G∗∗ potential energy hypersurface (PES) of the chemically interesting molecule ion, C2H5+, is analyzed by new procedures proposed by us recently. The reaction paths (RPs) start from the minima or saddle points by following the so-called reduced gradient procedure and/or its modification, the tangent search concept (TASC). The minimum energy paths (MEP) obtained by TASC are fine approximations of those gradient extremals, which follow the valley floor (or ridge) along the smallest (absolute) eigenvalue of the Hessian. It is the valley floor gradient extremal which is also termed the streambed of the PES. Tracing the streambed uphill we locate saddle points of the H-scrambling in C2H5+. The potential energy surface of this cation is used to systematically explore the properties of RP definitions with particular focus to RP bifurcations. The intrinsic reaction coordinate (IRC), which is mostly used as MEP in chemistry, is included for the purpose of comparison.

Ab initio study of the structures and stabilities of the dimer of ethyl cation, (C(2)H(+)(5))(2) and related C(4)H(10)(2+) isomers.

Ab initio calculations at the MP4(SDTQ)/6-311G//MP2/6-31G level were performed to study the structures and stabilities of the dimer of ethyl cation, (C(2)H(+)(5))(2), and related C(4)H(10)(2+) isomers. Two doubly hydrogen bridged diborane type trans 1 and cis 2 isomers were located as minima. The trans isomer was found to be more favorable than cis isomer by only 0.6 kcal/mol. Several other minima for C(4)H(10)(2+) were also located. However, the global energy minimum corresponds to C-H (C(4) position) protonated 2-butyl cation 10. Structure 10 was computed to be substantially more stable than 1 by 31.7 kcal/mol. The structure 10 was found to be lower in energy than 2-butyl cation 13 by 34.4 kcal/mol.

Measurement of the Barrier to β-Hydride Elimination in a β-Agostic Palladium−Ethyl Complex: A Model for the Energetics of Chain-Walking in (α-Diimine)PdR+ Olefin Polymerization Catalysts

The cationic Pd(II) ethyl complex [[(2,6-(iPr)2C6H3)NC(An)C(An)N(2,6-(iPr)2C6H3)]Pd(CH2CH2-μ-H)]BAr‘4 (8), which exhibits a β-agostic interaction, has been synthesized via protonation of the corresponding diethyl complex with H(OiPr2)2BAr‘4 (Ar‘ = 3,5-(CF3)2C6H3). The fluxional behavior of this ethyl cation has been studied quantitatively using 1H and 13C NMR spectroscopy. Two dynamic processes have been observed: (1) β-H elimination and reinsertion with opposite regiochemistry and (2) rotation about the C−C bond of the ethyl group. β-H elimination is the lower energy process, with ΔG⧧ = 7.1 kcal/mol at −108 °C. Eyring analysis of the 13C NMR data indicates ΔH⧧ = 6.1 ± 0.2 kcal/mol and ΔS⧧ = −5.2 ± 0.9 eu. Rotation of the agostic methyl group occurs with ΔG⧧ ca. 8.4 kcal/mol at −108 °C. These data provide insight into the energetics of chain walking, which is responsible for the unusual polyolefin microstructures produced using (α-diimine)PdR+ catalysts.

Triplet states of carbenium and silylium cations

Triplet CH 3 +, SiH 3 +, C 2H 5 +, SiCH 5 +, and Si 2H 5 + systems are studied with various correlated quantum mechanical methods. Only the triplet methyl and silyl cations have a single minimum. The other systems have from two to four minima. The best singlet–triplet energy difference predictions for the most stable isomers are: methyl cation 87 kcal/mol, silyl cation 71 kcal/mol, ethyl cation 101 kcal/mol, methylsilyl cation 73 kcal/mol (all CCSD(T)/TZ2PF) and disilyl cation 48 kcal/mol (MP2/TZ2PF). These changes in the singlet–triplet gaps are rationalized by regarding the effect of protonation, methylation and silylation on the energy differences between singly occupied MOs in triplet methylene and triplet silylene.

1,2 → 1,1-Migration of Silyl Group. Formation of 1,1-Bis(dichloromethylsilyl)-2-chloroethane in Catalytic Hydrochlorination of 1,2-Bis(dichloromethylsilyl)ethylene

Hydrochlorination of 1,2-bis(silyl)ethylenes of the general formula MenCl3-nSiCH = CHSiMen· Cl3-n, where n = 0-3, in the presence of FeCl3 was carried out. By NMR spectroscopy it was shown that in the case of the MeCl2Si derivative the major product is 1,1-bis(silyl)chloroethane and the minor product is its isomeric 1,2-bis(silyl)chloroethane (4:1). A reaction scheme is proposed, that includes isomerization of the intermediate bis(silyl)ethyl cation.

The C 2H 7+ potential energy surface: a Fourier transform ion cyclotron resonance investigation of the reaction of methyl cation with methane

Fourier transform ion cyclotron resonance experiments have been carried out to probe the nature of the potential energy surface for the reaction of methyl cation (CH 3 +) with methane (CH 4) to give the ethyl cation (C 2H 5 +). Product distributions for reactions of CH 3 + and CD 3 + with CD 4 and CH 4, respectively, are found to give a near statistical distribution of ethyl cation products, in good agreement with previous work. When the methyl cation is initially coordinated to HF in the form of a methylfluoronium ion, however, the product distributions are decidedly nonstatistical and are indicative of a reaction which is very nearly thermoneutral. Thermochemical data for relevant species suggest that the reaction is very slightly endothermic. All of the experiments support the intermediacy of a C 2H 7 + complex in the reactions. Ab initio calculations, in conjunction with all of the experimental data, reveal that there are likely three different stable forms of C 2H 7 + involved in the potential energy surface for the reaction. One of the forms, a nonclassical C 2H 5 + coordinated to H 2 in a proton bound dimerlike structure, has not previously been considered to play a role in this reaction. The existence of this structure is supported by infrared multiphoton dissociation experiments on C 2H 7 + previously carried out by Lee and co-workers (J. Am. Chem. Soc. 111 (1989) 5597) and the high pressure mass spectrometric experiments of Hiraoka and Kebarle (J. Am. Chem. Soc. 98 (1976) 6119).

Solvent Effect on the Protonation of Acetylene and Ethylene – Continuum Solvent Quantum Chemical Calculations

The protonation of acetylene and ethylene (yielding the vinyl and ethyl cation, respectively) was investigated computationally by ab initio calculations [B3LYP/6–31G(d,p)], in the gas phase and in water, as modeled by the IPCM and SCIPCM continuum methods. The structures and NBO atomic charges were thus determined for the neutral bases and their protonated forms, while the comparison of gas-phase and aqueous basicities afforded the hydration energies of the protonated bases. It was found that the aqueous protonation of acetylene is more endothermic than that of ethylene by 5 kcal/mol, owing to the lower intrinsic basicity of the former (by 7.4 kcal/mol), which is only partly compensated for by the more exothermic hydration (by 3.8 kcal/mol) of the vinyl cation.

Solvent Effect on the Protonation of Acetylene and Ethylene – Continuum Solvent Quantum Chemical Calculations

The protonation of acetylene and ethylene (yielding the vinyl and ethyl cation, respectively) was investigated computationally by ab initio calculations [B3LYP/6–31G(d,p)], in the gas phase and in water, as modeled by the IPCM and SCIPCM continuum methods. The structures and NBO atomic charges were thus determined for the neutral bases and their protonated forms, while the comparison of gas-phase and aqueous basicities afforded the hydration energies of the protonated bases. It was found that the aqueous protonation of acetylene is more endothermic than that of ethylene by 5 kcal/mol, owing to the lower intrinsic basicity of the former (by 7.4 kcal/mol), which is only partly compensated for by the more exothermic hydration (by 3.8 kcal/mol) of the vinyl cation.

Theoretical Study of Bimolecular Reactions between Carbenium Ions and Paraffins: The Proposal of a Common Intermediate for Hydride Transfer, Disproportionation, Dehydrogenation, and Alkylation

The mechanism of the bimolecular reactions of ethyl cation with ethane and propane and of s-propyl cation with ethane, propane, and isopentane is theoretically investigated by means of the B3PW91 density functional method. The study includes complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved, calculation of the reaction enthalpies and activation energies for the different elemental steps, and obtainment of the equilibrium constants and relative reaction rate constants by means of transition state theory. It is found that the interaction of a carbenium ion with a saturated alkane always results in formation of a stable carbonium ion intermediate and that different intramolecular rearrangements of this common intermediate can explain the mechanism of acid-catalyzed hydrocarbon reactions, such as hydride transfer, disproportionation, dehydrogenation, and alkylation.

Insights into the C(4)H(8)(+.) potential energy surface: fourier transform ion cyclotron resonance studies of ion-molecule reactions of D-labeled partners

Reactions of [ethylene](+.) with ethylene and of [acetylene](+.) with ethane were studied by Fourier transform ion cyclotron resonance spectrometry using labeled reactants. The results confirm and clarify the different steps of the mechanism proposed previously and elaborated with other methods. The [[acetylene](+.), ethane] system can either dissociate to give the ethyl cation product, or isomerize into [[ethylene](+.), ethylene]. The latter system can either dissociate to yield ionized ethylene or convert into ionized but-2-ene, which undergoes a complete H-exchange prior to dissociation, leading to methyl radical, hydrogen radical and ethylene losses. The transfers of labeled atoms and the existence of H-exchange prior to formation of the products were used as a probe to check the different steps of the mechanism. The influence of the initial energy of the system on the reaction pathway is discussed. Copyright 1999 John Wiley & Sons, Ltd.

Insights into the C4H8+· potential energy surface: Fourier transform ion cyclotron resonance studies of ion–molecule reactions of D-labeled partners

Reactions of [ethylene]+· with ethylene and of [acetylene]+· with ethane were studied by Fourier transform ion cyclotron resonance spectrometry using labeled reactants. The results confirm and clarify the different steps of the mechanism proposed previously and elaborated with other methods. The [[acetylene]+·, ethane] system can either dissociate to give the ethyl cation product, or isomerize into [[ethylene]+·, ethylene]. The latter system can either dissociate to yield ionized ethylene or convert into ionized but-2-ene, which undergoes a complete H-exchange prior to dissociation, leading to methyl radical, hydrogen radical and ethylene losses. The transfers of labeled atoms and the existence of H-exchange prior to formation of the products were used as a probe to check the different steps of the mechanism. The influence of the initial energy of the system on the reaction pathway is discussed. Copyright © 1999 John Wiley & Sons, Ltd.

Effect of acid structure on deprotection of poly(2-trimethylsilyl-2-propyl methacrylate)

Deprotection of poly(2-trimethylsilyl-2-propyl methacrylate) was studied for various photogenerated acids. Deprotection of 2-trimethylsilyl-2-propyl (TMSP) ester did not occur in a catalytic manner when the photogenerated acid possessing a fluoride ion source such as SbF 6 − or AsF 6 − was used. Because the cleaved product, α-methyl- α-(trimethylsilyl)ethyl cation, undergoes rearrangement through methyl migration from silicon to carbon with nucleophilic attack on the silicon by the fluoride ion, the acid was consumed in the reaction. Fortunately, when the counterion of the acid does not provide a fluoride ion, e.g. sulfonate, the carbonium ion undergoes elimination to produce 2,2,3-trimethyl-2-silabut-3-ene, and regenerates another acid. The thermal stability of TMSP ester was very similar to t-butyl ester, and acid catalyzed cleavage of the TMSP begins at 80°C.

Theoretical studies of pentene cracking on zeolites: C–C β-scission processes

The C–C bond-breaking step of pentene adsorbed on a model zeolite cluster is examined using ab initio and density functional theory (DFT/B3LYP) electronic structure techniques as an example of the β-scission process that arises in cracking of alkanes and alkenes. After pentene has been protonated by the acid site, the reactant for the cracking process corresponds to a pentyl cation covalently bound to the oxygen of the zeolite, ZO −–C 5H + 11. The product of the C–C bond-breaking process is propene plus an ethyl cation bound to a neighboring oxygen. The energy of the transition state relative from B3LYP calculations is 60 kcal/mol relative to the pentyl cationic reactant. For the case of the branched olefin methyl pentene, the transition state energy is slightly lower (55 kcal/mol), but the overall reaction energy is essentially the same as for pentene. The results are compared to the case of the gas phase pentyl carbenium ion.

Quantum Chemical Calculations on α-Substituted Ethyl Cations: A Comparison between B3LYP and Post-HF Methods

Alkyl cations of the form CH3C(+)HR have been investigated using high-level quantum chemical methods to study the influence of α-substituents R (R = H, CH3, CHCH2, C⋮CH, F, and Cl) on cation geometries and relative energies with respect to the neutral precursors CH3CH2R. The results of density-functional B3LYP computations with a variety of basis sets were compared with MP2, MP4, QCISD(T), and CBS-Q model chemistry results and benchmarked against experimental data. The results show that geometrical features are already accurately described using B3LYP/6-311G(d,p) or MP2/6-311G(d,p). For a systematic study of the energetics of α-substitution on alkyl cations, B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) computations form a useful compromise between accuracy (average deviation within 1 kcal/mol of the experimental error) and computational efficiency. The electronic structures of these species and their precursors CH3CH2R were studied using both natural bond orbital (NBO) and Atoms-in-molecules (AIM) analyses. These analyses clearly show that the electron-donating power of α-substituents at (partially) positively charged carbon atoms does not correlate well with the thermochemical stabilization of cations provided by such substituents.

Butyl Cations and Their Gas-Phase Dissociation Chemistry: Uniting Experiments with Theory

The results of extensive mass spectrometric experiments have been interpreted with the aid of recent high-level ab initio theoretical calculations. The dissociative ionization of species containing formal n-C4H9 and i-C4H9 groups leads only to mixtures of s-C4H9+ and t-C4H9+, irrespective of the precursors' internal energy. Experimental distinction between s-C4H9+ and t-C4H9+ ions rests upon the much larger kinetic energy release observed when s-C4H9+ are collisionally excited by O2 to lose CH3• (500 meV) relative to t-C4H9+ (90 meV). This distinction is lost when He is the target gas. Target gas behavior was shown to most strongly depend on its polarizability, and it is proposed that, in contrast to vertical excitation in the case of collision with He, small polarizable targets allow structural change during the collisional excitation. The C3H6•+ fragment ions produced from collisionally activated C4H9+ ions only have the [propene]•+ structure. The C3H5+ fragment ion resulting from the metastable loss of CH4, a process common to all C4H9+ isomers, has the 2-propenyl cation structure, not allyl as earlier concluded. Metastable C4H9+ ions also lose C2H4 yielding the nonclassical ethyl cation. The structure leading to this dissociation is proposed to be a proton-bridged tetramethylene ion, shown by theory to occupy a potential energy well.

Gas-Phase Reactivity of Ethyl Cations: Protonation and Alkylation of N-Methylpyrrole and Thiophene

The gas-phase reactivity, in terms of deuteration versus alkylation, of radiolytically formed ethyl cations C2D5+ toward N-methylpyrrole and thiophene has been studied in CH4−C2D6 systems at 760 and 1520 Torr and in the presence of variable concentrations of triethylamine (TEA) with the aim of obtaining quantitative data on their competitive behavior. The substrate intramolecular selectivity has also been determined in both reaction channels. The gas-phase behavior of ethyl cations under high-pressure conditions demonstrates their predominant Bronsted versus Lewis acid reactivity toward the selected heteroaromatics, whose branching ratios, at a total pressure of 760 Torr, are 2.34 for N-methylpyrrole and 2.08 for thiophene. Intramolecular selectivity results indicate a substrate alkylation β:α = 58:42 for N-methylpyrrole and β:α = 46:54 for thiophene. The substrate deuteration, determined by 2H NMR, is β:α = 50:50 for N-methylpyrrole and β:α = 36:64 for thiophene.

Zirconium−Alkyl Isomerizations in Zirconocene-Catalyzed Olefin Polymerization: A Density Functional Study

The migration of a Zr center between adjacent carbon atoms of a Zr-bound alkyl group is investigated by a density functional study of alternative reaction paths available to a (C5H5)2Zr−alkyl cation. This migration is found to occur by the classical reaction route, i.e. by β-H transfer, olefin rotation, and reinsertion into the Zr−H bond without loss of the olefin ligand, rather than by a concerted Zr/H exchange. The activation barrier is determined to decrease from 75 kJ/mol for the degenerate isomerization of a zirconocene ethyl cation to 49 kJ/mol for the isomerization of a primary to a secondary zirconocene propyl cation and 41 kJ/mol for its back-reaction. It is further reduced to 40 kJ/mol for the isomerization of a primary to a tertiary zirconocene isobutyl cation and 31 kJ/mol for its reverse, which model the isomerization process competing with stereoregular chain growth in zirconocene-based catalysts. The facility with which the zirconocene isobutyl cation transfers its β-H atom is connected wit...

Reversible β-hydrogen transfer between Fe(C 2H 5) + and HFe(C 2H 4) +: case of two-state reactivity?

The iron ethyl cation, Fe(C 5H 5) +, and its tautomer, the ethene complex of the iron hydride cation HFe(C 2H 4) +, have been examined computationally using a hybrid of density functional theory and the Hartree-Fock approach ( Becke3LYP). The quintet Fe(C 2H 5) +( 5A′) corresponds to the global minimum of the [Fe,C 2,H 5] + potential energy hypersurface. Fe(C 2H 5) + can interconvert via β-hydrogen transfer into HFe(C 2H 4) + ( 5A′), which is ca. 13 kcal mol −1 less stable. The transition structure (TS) associated with their mutual interconversion on the quintet surface requires 36 kcal mol −1 relative to Fc(C 2H 5) +. However, this barrier may be circumvented by a reaction path on the energetically low-lying triplet surface in which the corresponding transition structure for β-H transfer is 8 kcal mol −1 lower in energy than the quintet TS. Thus, the path of minimal energy requirement connects the quintet species Fe(C 2H 5) + and HFe(C 2H 4) + via the triplet surface such that spin inversion is part of the reaction coordinate. Agostic interaction, which is only possible in the low-spin system, constitutes an essential factor for this unprecedented reaction mechanism. Further support to this interpretation is providedby mass spectrontetric experiments which demonstrate that the interconversion Fe(C 2H 5) + ⇄ HFe(C 2H 4) + is facile and occurs well below the respective dissociation asymptotes.

Theoretical Study of Ion Pair SN2 Reactions: Ethyl vs Methyl Reactivities and Extension to Higher Alkyls

Identity SN2 reactions of methyl fluoride and chloride with fluoride and chloride ions are compared computationally with the ion pair displacement reactions of lithium fluoride and chloride and of sodium fluoride. The results with the RHF MP2 and B3LYP methods and the 6-31+G* basis set essentially confirm some previous results obtained by RHF and MPn methods. The ion pair reactions proceed via an initial dipole−dipole complex and a cyclic transition structure with highly bent X−C−X bonds that resemble a collection of two cations and two anions. Extension of the calculations to the corresponding ethyl systems gives lower ion pair barriers than methyl, completely different from the corresponding ionic barriers. The bending away of the halogens in the ion pair transition structures reduces steric effects. Analysis of charge distributions shows that in the ionic transition structures the nearby halogens electrostatically inhibit polarization and delocalization of charge. The nonlinear X−C−X bond angle in the ion pair transition structures coupled with the electrostatic influence of the cation permits greater charge polarization and delocalization within the organic moiety and allows the greater stability of ethyl cation compared to methyl to manifest itself. n-Propyl, isopropyl, and neopentyl chlorides have lower barriers than ethyl in reaction with lithium chloride but isopropyl and neopentyl chlorides have higher barriers than ethyl toward chloride ion.

Gas-Phase Substituent Effects in Highly Electron-Deficient Systems. I. Intrinsic Stabilities of 1-Aryl-1-(trifluoromethyl)ethyl Cations

Abstract The relative stabilities of 1-aryl-1-(trifluoromethyl)ethyl cations were determined by measuring the proton-transfer equilibria of 1-aryl-1-(trifluoromethyl)ethylenes or the chloride-transfer equilibria of 1-aryl-1-(trifluoromethyl)ethyl chlorides in the gas phase. The stability of 1-phenyl-1-(trifluoromethyl)ethyl cation was found to be 16 kcal mol−1 lower than that of the α-cumyl (1-phenyl-1-methylethyl) cation. The substituent effect on the stability of this cation can be correlated in terms of the Yukawa–Tsuno equation, giving an r+ of 1.41 and a ρ of −10.0 (in log K/Ko−1 unit). While the ρ value is nearly identical to that of the α-cumyl cation series, the r+ value is remarkably higher than the value of unity for the α-cumyl cation, indicating that such a highly electron-deficient carbocation system should be characterized by an extremely large r+ value compared with that of the stable α-cumyl cation. In addition, this r+ value agrees with that for the SN1 solvolysis of 1-aryl-1-(trifluoromethyl)ethyl tosylates. Such agreement of the r+ values between the gas phase and solvolysis reactions has been generally observed for the benzylic carbocation systems. It is concluded that the enhanced r+ value for the solvolysis of 1-aryl-1-(trifluoromethyl)ethyl tosylates must reflect the intrinsic resonance demand characteristic of the parent 1-phenyl-1-(trifluoromethyl)ethyl cation, itself, and that the extremely large ρ+ values given by a simple correlation with σ+ (r+ = 1) are an artifact due to an improper analysis of underestimating the resonance demand for such highly deactivated substrates.