Ethyl Cation Research Articles

Self-Consistent Heats of Formation for the Ethyl Cation, Ethyl Bromide, and Ethyl Iodide from Threshold Photoelectron Photoion Coincidence Spectroscopy

The dissociative photoionization onsets of Br and I loss reactions were measured for C(2)H(5)Br and C(2)H(5)I, respectively, by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy to establish the heats of formation of these two basic ethyl halides. The appearance energy of ethyl cation from ethyl bromide was found to be 1074.2 +/- 0.8 kJ mol(-1), and that from ethyl iodide was found to be 1016.4 +/- 0.8 kJ mol(-1). The heats of formation of ethyl bromide and ethyl iodide are interconnected through the ethyl cation. In establishing the thermochemistry of the ethyl halides, the ethyl cation heat of formation was concluded to be 915.5 +/- 1.3 kJ mol(-1) on the basis of a recent value for ethyl radical heat of formation and the well-established ionization energy and on the basis of ab initio isodesmic calculations using recent PEPICO data. Using this anchor, we obtained the following heats of formation: Delta(f)H(0K) degrees[EtBr] = -40.8 +/- 1.5 kJ mol(-1) and Delta(f)H(0K) degrees[EtI] = 6.3 +/- 1.5 kJ mol(-1). These results are more consistent with the higher EtBr heats of formation values in the literature, contrary to recent findings. For ethyl iodide, the latest calorimetric value does not agree within the claimed accuracy.

The ethyl cation as proton donor for dihydrogen bonds in the (m=1 or 2 and n=1 or 2) complex: A theoretical study

BHandHLYP/6-31G(d,p) calculations and molecular integrations of the quantum theory of atoms in molecules (QTAIM) were applied to study a new class of dihydrogen-bonded complexes (mC2H5+⋯nMgH2 with m = 1 or 2 and n = 1 or 2) based on the assumption that earth alkaline hydride MgH2 can bind with the non-localized hydrogen H+δ of the ethyl cation C2H5+.

Reactivity of C2H5+ with Benzene: Formation of Ethylbenzenium Ions and Implications for Titan’s Ionospheric Chemistry

The reaction of ethyl cation with benzene has been investigated in a combined experimental and theoretical approach. Under single collision conditions, proton transfer affording protonated benzene concomitant with neutral ethene represents the major reaction channel. From pressure-dependent measurements, an absolute cross section of 7 +/- 2 A(2) at hyperthermal energies (about 1.0 eV in the center of mass frame) is derived for this channel, from which a phenomenological rate constant of about 2.9 x 10(-10) cm(3) s(-1) is estimated at low energies. The energy behavior of the cross section as well as several side reactions leading to C-C coupling imply that the reaction of C(2)H(5)(+) with C(6)H(6) proceeds via a long-lived association product, presumably the covalently bound protonated ethylbenzene (ethylbenzenium ion). With regard to chemical processes in the atmosphere of Titan, present results imply that termolecular association of C(2)H(5)(+) with benzene to produce protonated ethylbenzene is very likely to occur. The condensation of alkyl cations with arenes thus provides an alternative route for the growth of larger hydrocarbon molecules.

A B3LYP and QTAIM study of a new proton donor for dihydrogen bonds: the case of the C2H5 +···nBeH2 complexes (n = 1 or 2)

B3LYP/6-311++G(d,p) calculations and molecular integrations from the quantum theory of atoms in molecules (QTAIM) were performed for the purposes of studying a new class of dihydrogen-bonded hyperconjugation complexes formed by C2H5 +···n(BeH2), when n = 1 (bimolecular) or n = 2 (trimolecular). Whether bimolecular or trimolecular, when the hyperconjugation on the ethyl cation (C2H5 +) is taken into account, this enables the earth alkaline hydride, BeH2, to interact efficiently with the nonlocalized hydrogen (H+) of the C2H 5 . In addition to computation of QTAIM topological parameters, analysis of the infrared harmonic spectrum at the B3LYP/6-311++G(d,p) level of theory revealed the existence of red-shifts on BeH2, and this effect is explained by means of the atomic charges derived from the ChelpG approach.

Group theory and reaction mechanisms: An extension of the mciver-stanton rules

McIver and Stanton's rules concerning the symmetry of transition states have a counterpart in rules concerning the permutation symmetry of single steps in degenerate rearrangements, derivable with the aid of Longuet-Higgins group theory. Examples in organometallic fluxional systems and carbonium ions illustrate the rules; the analysis leads to prediction of hitherto unexplored pathways for rearrangements in ethyl and homotetrahedryl cations.

Uncommon hydrogen bonds between a non-classical ethyl cation and π hydrocarbons: a preliminary study

This study undertakes a theoretical investigation into uncommon hydrogen bonds between the ethyl cation (C2H5 +) and π hydrocarbons. Firstly, it considers the hyperconjugation effect of the ethyl cation, in which the non-localized hydrogen (H+) is taken to be a pseudoatom bound to the carbons of the methyl groups. The goal of the research is to use this electronic phenomenon to gain a better understanding of the (H+···π) and (H+···p-π) hydrogen bonds, which are considered uncommon because they are formed through the interaction of the H+ of the ethyl cation with the π bonds of the acetylene (C2H2) and ethene (C2H4), as well as with the pseudo-π bond of the cyclopropane (C3H6). In view of this, B3LYP/6-311++G(d,p) calculations were used to determine the geometries of the C2H5 +···C2H2, C2H5 +···C2H4, and C2H5 +···C3H6 hydrogen-bonded complexes. Deformations of the bond lengths and bond angles of these systems were analyzed geometrically. Examination of the stretch frequencies and absorption intensities of the (H+···π) and (H+···p-π) hydrogen bonds has revealed red-shifts in π and p-π bonds. After structural modeling and vibrational characterization, analysis of the charge transfer following the ChelpG approach and subsequently quantification of the hydrogen bond energies (basis sets superpostition error and zero point vibrational energies being considered) were used to predict the strength of the (H+···π) and (H+···p-π) hydrogen bonds. In addition, the molecular topography was estimated using the quantum theory of atoms in molecules (QTAIM). QTAIM was chosen because of a desire to understand the (H+···π) and (H+···p-π) hydrogen bonds chemically on the basis of the quantity of charge density and interpretation of Laplacian fields.

Molecular modelling simulations to predict density and solubility parameters of ionic liquids

Molecular modelling simulations are one of the most convenient tools to predict solvent properties of ionic liquids, which are otherwise difficult to predict or measure by experimental means. Ionic liquids have emerged as effective and desirable alternative solvents during the last decade. Their use is greatly favoured because of their ‘green’ characteristics: low vapour pressure, non-flammability and chemical stability. An enormous variety of ionic liquids are potentially available and thus the design of task-specific ionic liquids is gaining importance. Traditional solvents are often evaluated based on their multi-component solubility parameters to assess their compatibility with different solutes. The analogous parameter values for ionic liquids are also useful, though their availability is often limited. Consequently, solvent design and pre-screening of candidates by molecular modelling tools are attractive alternatives. In this work, the density and two-component solubility parameter of a variety of ionic liquids were investigated using atomistic level molecular modelling and the commercially available Materials Studio® software package. The ionic liquids chosen consisted of ethyl, butyl and hexyl imidazolium cations in conjunction with four anions (Cl− , [(CN)2N] − , [CF3CO2]− and [Tf2N]− ). The goal was to develop appropriate computational protocols and carry out calculations, which are able to predict densities and two-component solubility parameters for these ionic liquids, as well as to evaluate the usefulness and accuracy of the currently available commercial molecular modelling software for these applications. It was found that the density of ionic liquids comprised of all possible combinations of these cations and anions could be calculated with satisfactory accuracy with the exception of systems containing the Cl− anion. A Hess cycle-based protocol was developed to correct cohesive energy densities and solubility parameters as calculated in Materials Studio to take into account ion pair interactions to successfully reproduce the experimental results. It was found that both the cation and the anion of the ionic liquid had a strong impact on the components of the solubility parameter.

IR Spectrum of the Ethyl Cation: Evidence for the Nonclassical Structure

IR Spectrum of the Ethyl Cation: Evidence for the Nonclassical Structure

IR Spectrum of the Ethyl Cation: Evidence for the Nonclassical Structure

Durch Markierung mit Argon konnte das IR-Spektrum des Ethylkations (C2H5+) mithilfe von resonanter IR-Photodissoziationsspektroskopie schwach gebundener C2H5+⋅Ar-Komplexe abgeleitet werden (siehe Bild). Das experimentelle Spektrum ähnelt stark den theoretischen Spektren von 1 und 1⋅Ar, weicht jedoch deutlich vom Spektrum von 2 ab; damit wird der erste direkte spektroskopische Nachweis für die nichtklassische Struktur 1 des protonierten Ethens erbracht.

Energy-Resolved Photoionization of Alkylperoxy Radicals and the Stability of Their Cations

The photoionization of alkylperoxy radicals has been investigated using a newly developed experimental apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. Methylperoxy (CH(3)OO) and ethylperoxy (C(2)H(5)OO) radicals are produced by the reaction of pulsed, photolytically produced alkyl radicals with molecular oxygen, and the mass spectrum of the reacting mixture is monitored in time by using synchrotron-photoionization with a double-focusing mass spectrometer. The kinetics of product formation is used to confirm the origins and assignments of ionized species. The photoionization efficiency curve for CH(3)OO has been measured, and an adiabatic ionization energy of (10.33 +/- 0.05) eV was determined with the aid of Franck-Condon spectral simulations, including ionization to the lowest triplet and singlet cation states. Using the appearance energy of CH(3)(+) from CH(3)OO, an enthalpy of formation for CH(3)OO of Delta(f) (CH(3)OO) = (22.4 +/- 5) kJ mol(-1) is derived. The enthalpy of formation of CH(3)OO(+) is derived as Delta(f) = (1019 +/- 7) kJ mol(-1) and the CH(3)(+)-OO bond energy as (CH(3)(+) - O(2)) = (80 +/- 7) kJ mol(-1). The C(2)H(5)OO(+) signal is not detectable; however, the time profile of the ethyl cation signal suggests its formation from dissociative ionization of C(2)H(5)OO. Electronic structure calculations suggest that hyperconjugation reduces the stability of the ethylperoxy cation, making the C(2)H(5)OO(+) ground state only slightly bound with respect to the ground-state products, C(2)H(5)(+) and O(2). The value of the measured appearance energy of C(2)H(5)(+) is consistent with dissociative ionization of C(2)H(5)OO via the Franck-Condon favored ionization to the ã (1)A' state of C(2)H(5)OO(+).

Vapour phase reaction of ethylbenzene with diethyl carbonate over Al-MCM-41 supported phosphotungstic acid

The vapour phase ethylation of ethylbenzene with diethyl carbonate was studied over Al-MCM-41 and 10, 20 and 40 wt% PW loaded Al-MCM-41 catalysts. Based on the conversion 20 wt% PW/Al-MCM-41 was found to be more active than others. Diethylbenzene isomers were found to be more predominant than tri-, tetra-, penta- and hexa- alkylated products. The formation of hexaethylbenzene suggested high density of ethyl cations on the catalyst surface. The feed ratio of 2:1 (ethylbenzene:diethyl carbonate) was observed to yield 100 % selectivity of diethylbenzene isomers. Although ortho, meta and para diethylbenzene isomers were observed, the selectivity to para isomer was found to be higher than other isomers.

Perfluorinated Alkoxyaluminate Salts of Cationic Brønsted Acids: Synthesis, Structure, and Characterization of [H(OEt2)2][Al{OC(CF3)3}4] and[H(THF)2][Al{OC(CF3)3}4

AbstractThe strong cationic Brønsted acids [H(OEt2)2][Al{OC(CF3)3}4] (1) and [H(THF)2][Al{OC(CF3)3}4] (2) were prepared in up to 95 % yield by reacting stoichiometric amounts of diethyl ether/THF solutions of Li[Al{OC(CF3)3}4] and HX (X = Cl, Br), removing the solvent, and extracting the residue with CH2Cl2. Compounds 1 and 2 were characterized by multinuclear NMR and IR spectroscopy as well as by their single crystal X‐ray structures and additional quantum chemical calculations. The cation in 1 probably contains an unsymmetrical O–H···O hydrogen bond, while in 2 the protic hydrogen atom was not found in the difference Fourier map. From an analysis of all published [H(OEt2)2]+ structures, we show that for an unsymmetrical H‐bridge further rearrangement of the structure occurs leading to the structural type III that may formally be viewed as ethanol coordinating to an ethyl cation. This point of view has hitherto been neglected in the discussion of the nature of short, strong, low‐barrier H‐bonds, although it needs to be accounted for. Both 1 and 2 are suitable starting materials to introduce the weakly coordinating and chemically very robust [Al{OC(CF3)3}4]– anion by protonolysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Positive and negative ion chemistry of the anesthetic halothane (1‐bromo‐1‐chloro‐2,2,2‐trifluoroethane) in air plasma at atmospheric pressure

The ion chemistry of 1-bromo-1-chloro-2,2,2-trifluoroethane (the common anesthetic halothane) in air plasma at atmospheric pressure was investigated by atmospheric pressure chemical ionization mass spectrometry (APCI-MS). The major positive ion observed at low declustering (API interface) energies is the ionized dimer, M(+.)M, an unexpectedly abundant species which possibly is stabilized by two H-bonding interactions. At higher energies [M--HF](+.) and [M--Br](+) prevail; the former, corresponding to ionized olefin [ClBrC=CF(2)](+.), appears to originate from M(+.)M and is quite stable towards fragmentation. The latter fragment ion ([M--Br](+)) and its analogue, [M--Cl](+), which is also observed though at much lower abundance, are originally ethyl cations (+)CHX--CF(3) (X = Br, Cl) which, upon collisional activation, rearrange and fragment to CHFX(+) via elimination of CF(2). All of the above described ions are also observed in humid air: in addition, the oxygenated ion [ClBrC=CFOH](+.) also forms in humid air via water addition to [ClBrC=CF(2)](+.) and HF elimination, as observed earlier for ionized trichloroethene. In contrast with similar chloro- and fluoro-substituted ethanes, halothane does not react with H(3)O(+) in the APCI plasma, a result confirmed by selected ion APCI triple-quadrupole (TQ) experiments. Major negative ions formed from halothane in the air plasma are Br(-) and, to a lesser extent, Cl(-), and their complexes with neutral halothane. APCI-TQ experiments indicated that Br(-) and Cl(-) are formed via reaction of halothane with O(2) (-.), O(2) (-.)(H(2)O) and O(3) (-.), possibly via dissociative electron transfer or nucleophilic substitution. Competing proton transfer was also observed in the reaction with O(2) (-.) and, at high halothane pressure, also with O(2) (-.)(H(2)O); at lower pressures the molecular anion M(-.) was observed instead. The other minor anions of the air plasma, NO(2) (-), N(2)O(2) (-.) and NO(3) (-), were found to be unreactive towards halothane.

Methyl and Ethyl Cation Affinities of Rare Gas Atoms and N2

Gas-phase methyl cation affinities (MCAs) for rare gases Ne, Kr, and Xe were measured with a pulsed electron-beam high-pressure mass spectrometer. The MCAs for Ne and Kr were determined to be 1.2 ± 0.3 and 19.8 ± 2.0 kcal/mol, respectively, by the observation of the clustering reaction, CH3+ + Rg = CH3+(Rg) (Rg = Ne and Kr). The MCA of Xe was measured to be 2.0 ± 0.6 kcal/mol larger than that of N2 by the observation of the substitution reaction CH3+(N2) + Xe = CH3+(Xe) + N2. Based on the MCA of N2 of 44.1 kcal/mol proposed by McMahon et al., the MCA of Xe is determined to be 46.1 ± 0.6 kcal/mol. Molecular orbital calculations at six different levels consistently gave almost identical MCA values for each of the rare gases. At QCISD(T)(full)/6-311++G(2df,p)//B3LYP/6-311++G(d,p), the calculated values (all in kcal/mol) are as follows: He, 0.6; Ne, 2.2; Ar, 15.9; Kr, 24.1; and N2, 43.2. For Xe at B3LYP/DZVP//B3LYP/DZVP, the calculated MCA is 39.0 kcal/mol. The ethyl cation affinities of Ar, Kr, and Xe were ...

A Topological Study of the Transition States of the Hydrogen Exchange and Dehydrogenation Reactions of Ethane on a Zeolite Cluster

The transition states of the hydrogen exchange and dehydrogenation reactions of ethane on a zeolite acid site are calculated using the density functional theory and analyzed with the atoms-in-molecules method. The transition state for the hydrogen exchange reaction is characterized by a slightly ionic interaction between a distorted H-ethonium structure, C2H7+, and a negatively charged zeolite cluster. No free carbocation is found. The dehydrogenation reaction, on the other hand, shows a transition state with three well-defined fragments, namely, an ethyl cation, C2H5+, a negatively charged zeolite cluster, and a H2 pseudomolecule, in which the H−H bond is somewhat larger than the equilibrium value in H2. The interaction between those fragments can be described as a closed-shell one, typical of rather ionic systems.

Persistent carbocations from 4,4′-dimethylaminodiphenylethylenes within CaY zeolite: intrazeolite-water controls the structure of the carbocation

Inclusion of 4,4′-dimethylaminodiphenylethylene and related olefins within activated CaY results in the formation of persistent monomer carbocations. The structure of the cation, consequently the color of the zeolite, is controlled by water molecules present within zeolites. Under ‘dry’ conditions the zeolite is light yellow and under ‘wet’ conditions it is blue colored. Blue color results from the quininoidal structure adopted by 1,1-bis-(4-dimethylaminophenyl)ethyl cation through participation of the lone pair electrons present on the amino substituent at the para position.

Enthalpy Barriers for Asymmetric SN2 Alkyl Cation Transfer Reactions between Neutral and Protonated Alcohols

Enthalpy barriers for gas phase asymmetric alkyl cation transfer reactions between neutral and protonated alcohols in mixed alcohol systems have been calculated by B3LYP, MP2, and G3(MP2) computational methods. As expected, on the basis of the proton affinities of the alcohols, the enthalpy barrier for transfer of the larger alkyl cation is calculated to be lower than that for transfer of the smaller alkyl cation. A linear relationship between the difference in gas basicity (GB) between the alcohols and the enthalpy barrier is observed for the less favorable alkyl cation transfer reactions. The slope of the regression line is 0.55, and the intercept (at ΔGB = 0) is found to be −22.5 kJ mol-1, which is extremely close to the enthalpy barriers for symmetric alkyl cation transfer. This relationship is discussed in the context of the Hammett relationship. Experiments conducted on the reaction of protonated ethanol with 18O-labeled methanol established that about 7.8% of the protonated ethyl methyl ether formed was via the less favorable methyl cation transfer. This experimental value agrees well with the 8.5% predicted based upon the calculated difference in energy barriers between ethyl and methyl cation transfer (6.1 kJ mol-1). This work shows that in the gas phase, the protonated ether products in mixed alcohol systems are formed via two competing SN2 reactions and that the efficiency of the less favorable alkyl cation transfer reaction increases as the difference in proton affinity of the two alcohols decreases.

Reactivities of Stable Rotamers: XLIV. Ring-Opening Reactions of 1-(9-Fluorenyl)-2-(2-methyl-2-oxiranyl)naphthalene Rotamers With Acids and the Structure of Oxiranes

X-ray crystallography of the ap-form of the 1-(9-fluorenyl)-2-(2-methyl-2-oxiranyl)naphthalene has revealed that the carbon atom of the oxirane which is connected to the naphthyl group in this compound is almost planar. The specific structural features of the epoxy ring in this compound are caused by sterical effects and by the presence of a π-system in the immediate vicinity of the oxirane ring. Certain differences have been found also in reactivity of rotational isomers of 2-X-substituted 1-[1-(9-fluorenyl)-2-naphthyl]ethyl cations (X = O, S, Se). At X = O arose more deprotonation product from the α-position of the oxygen than in reactions of sulfur and selenium-containing analogs. Reactions of epoxides with zinc chloride almost exclusively gave the corresponding aldehydes.

Protonated benzofuran, anthracene, naphthalene, benzene, ethene, and ethyne: measurements and estimates of pK(a) and pK(R).

Aqueous solvolyses of acyl derivatives of hydrates (water adducts) of anthracene and benzofuran yield carbocations which undergo competitive deprotonation to form the aromatic molecules and nucleophilic reaction with water to give the aromatic hydrates. Trapping experiments with azide ions yield rate constants k(p) for the deprotonation and k(H2O) for the nucleophilic reaction based on the "azide clock". Combining these with rate constants for (a) the H(+)-catalyzed reaction of the hydrate to form the carbocation and (b) hydrogen isotope exchange of the aromatic molecule (from the literature) yields pK(R) = -6.0 and -9.4 and pK(a) = -13.5 and -16.3 for the protonated anthracene and protonated benzofuran, respectively. These pK values may be compared with pK(R) = -6.7 for naphthalene hydrate (1-hydroxy-1,2-dihydronaphthalene), extrapolated to water from measurements by Pirinccioglu and Thibblin for acetonitrile-water mixtures, and pK(a) = -20.4 for the 2-protonated naphthalene from combining k(p) with an exchange rate constant. The differences between pK(R) and pK(a) correspond to pK(H2O), the equilibrium constant for hydration of the aromatic molecule (pK(H2O) = pK(R) - pK(a)). For naphthalene and anthracene values of pK(H2O) = +13.7 and +7.5 compare with independent estimates of +14.2 and +7.4. For benzene, pK(a) = -24.3 is derived from an exchange rate constant and an assigned value for the reverse rate constant close to the limit for solvent relaxation. Combining this pK(a) with calculated values of pK(H2O) gives pK(R) = -2.4 and -2.1 for protonated benzenes forming 1,2- and 1,4-hydrates, respectively. Coincidentally, the rate constant for protonation of benzene is similar to those for protonation of ethylene and acetylene (Lucchini, V.; Modena, G. J. Am. Chem Soc. 1990, 112, 6291). Values of pK(a) for the ethyl and vinyl cations (-24.8) may thus be derived in the same way as that for the benzenonium ion. Combining these with appropriate values of pK(H2O) then yields pK(R) = -39.8 and -29.6 for the vinyl and ethyl cations, respectively.

Experimental Determination of Activation Energies for Gas-Phase Ethyl and n-Propyl Cation Transfer Reactions

Alkyl cation transfer reactions between ethanol and protonated ethanol and ethanol and protonated propionitrile, as well as between n-propanol and protonated n-propanol have been investigated experimentally by low-pressure Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The two ethyl cation transfer reactions were found to be the dominant reaction channels with association being only a minor reaction pathway. The n-propyl cation transfer reaction was found to compete with the association pathway resulting in an approximately 50:50 mixture of protonated di-n-propyl ether and the proton-bound dimer of n-propanol, depending on temperature and pressure. The enthalpies of activation were determined to be −16.1 ± 0.8, −17.5 ± 0.8, and −15.7 ± 0.9 kJ mol-1 for the ethanol/protonated ethanol, ethanol/protonated propionitrile, and n-propanol/protonated n-propanol alkyl cation transfer reactions, respectively. The entropies of activation were found to be essentially the same, −121 ± 28 J K-1 m...