Pyrolysis of C2H7+ and other Ion–Molecule Reactions in Methane containing Traces of Ethane

French, Margaret; Kebarle, P.

doi:10.1139/v75-318

Cited by 23 publications

(8 citation statements)

References 4 publications

(7 reference statements)

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“…The proton transfer process used by Bohme was reaction 17, undertaken at room temperature. As expected, the C2H7+ formed at room temperature by (17) is identical with the C2H7+(b).…”

Section: Resultssupporting

confidence: 81%

Stabilities and energetics of pentacoordinated carbonium ions. The isomeric protonated ethane ions and some higher analogs: protonated propane and protonated butane

Hiraoka¹,

Kebarle²

1976

J. Am. Chem. Soc.

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143

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The kinetics and equilibria of reaction 1 were studied in H2 gas at 1-5 Torr over the temperature range -160 to 200 °C. The experiments were done in a pulsed electron beam high pressure ion source mass spectrometer. At the lowest temperatures (-160 to -130 °C) reaction 1 reached equilibrium. The forward rate of (1) decreased with increasing temperature. This means that the C2H7+(a) formed at this temperature was a result of an exothermic third body dependent reaction without activation energy. Measurement of the temperature dependence of the equilibrium led to AH]a = -4.0 kcal/mol and A//KC2H7+(a)) = 215 kcal/mol. At temperatures above -130 °C, where C2H7+(a) was unstable, little C2H7+ could be observed. Between -100 and 40 °C a new species C2H7+(b) was formed by a second-order process (reaction lb). The rate constant /fcib for this reaction increased with temperature. The activation energy was E\b = 1.2 kcal/mol. At temperatures between 40 and 200 °C, C2H7+(b) began to decompose to C2H5+ + H2 and an equilibrium (reaction lb) could be measured giving AHib = -11.8 kcal/mol and AEff(C2H7+(b)) = 207 kcal/mol. The C2H7(a) is identified with the C-H protonated ethane structure A which, however, is very close to an undisturbed ethyl ion and an H2 molecule. The more stable structure C2H7+(b) is identified as B, the C-C protonated structure of ethane. The energy barrier between species (a) and (b) is E\b -AHia = 5.2 kcal/mol. Earlier results on the species C3H9"1" are combined with new measurements to establish the energetics of two experimental species CsHg+fa) which corresponds essentially to iec-C3H7+ interacting very weakly with H2 and the C3H9+(b) species of a structure similar to B. The heats of formation are A//f(C3H9+(a)) = 189.5 kcal/mol and A//f(C3H9+(b)) = 194.5 kcal/mol. Thus for the propanes the (a) species has the lower heat of formation. However, the (b) species is more stable with respect to dissociation. The potential energy barrier between C3H7+(a) and C3H7+(b) is ~14 kcal/mol. Two analogous structures for C4Hn+ are also investigated. Proton affinities for protonatioh to the respective isomers are also derived.The pentacoordinated methonium ion CH5"1" was among the first ion-molecule reaction products observed with mass spectrometers.1 The next higher analogue C2H7+ was also observed relatively early.2 However, until recently no clear experimental evidence for the existence and stability of higher protonated alkanes was available.The bonding in CH5"1" was considered in several theoretical papers3™8 which ultimately agreed on the three center-bonded structure with Cs symmetry shown below as being the most stable. Ab initio calculations4 (STO-3G) for the C2H7+ ion predicted two stable isomers, the C-H protonated structure (A) and the C-C protonated structure (B) shown below. The ° .structure (B) was found4 more stable by some 10-12 kcal/mol. However, more recently results obtained with the simpler semiempirical9 MINDO/3 predicted structure (A) to be more stable. More accurate ab initio calculations wit...

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“…The proton transfer process used by Bohme was reaction 17, undertaken at room temperature. As expected, the C2H7+ formed at room temperature by (17) is identical with the C2H7+(b).…”

Section: Resultssupporting

confidence: 81%

Stabilities and energetics of pentacoordinated carbonium ions. The isomeric protonated ethane ions and some higher analogs: protonated propane and protonated butane

Hiraoka¹,

Kebarle²

1976

J. Am. Chem. Soc.

Self Cite

143

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“…Small alkonium ions, such as CH 5 + and C 2 H 7 + , have been originally detected in high-pressure mass spectrometry experiments ,, and later on they have been studied on a quantitative basis. The first reference to the three-center−two-electron (3c−2e) bond in CH 5 + was by Dyczmons, who interpreted its character on the basis of Boys' localization, while Muller et al stated when considering the C 2 v structure of the CH 5 + ion “in the C 2 v structure the proton interacts equally with two CH bonds, such that a 5-center 4-electron bond is formed” (the two orthogonal hybrid atomic orbitals (AOs) of carbon atom are considered as two centers).…”

Section: Introductionmentioning

confidence: 99%

Three-Center−Two-Electron and Four-Center−Four-Electron Bonds. A Study by Electron Charge Density over the Structure of Methonium Cations

2006

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We study the electronic density charge topology of CH(5)(+) species 1 (C(s)()), 2 (C(s)()), and 3 (C(2)(v)) at ab initio level using the theory of atoms in molecules developed by Bader. Despite the reports of previous studies concerning carbocationic species, the methane molecule is protonated at the carbon atom, which clearly shows its pentacoordination. In addition to the fact that hydrogen atoms in the methonium molecule behave in a very fluxional fashion and that the energy difference among the species 1, 2, and 3 are very low, is important to point out that two different topological situations can be defined on the basis of our study of the topology of the electronic charge density. Then, the species 1 and 2 present a three-center-two-electron (3c-2e) bond of singular characteristics as compared with other carbocationic species, but in the species 3, the absence of a 3c-2e bond is noteworthy. This structure can be characterized through the three bond critical points found, corresponding to saddle points on the path bonds between the C-H(2,3,5) that lie in the same plane. These nuclei define a four-center interaction where the electronic delocalization produced among the sigma(C-H) bonds provide a stabilization of the three C-H bonds involved in this interaction (the remaining two C-H bonds are similar to those belonging to the nonprotonated species). Our results show that bonding situations with a higher number of atom arrays are possible in protonated hydrocarbons.

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“…While these species have a lifetime too short to allow direct observation in superacid media by NMR, hydrogen exchange, and cleavage reactions have been described . Small alkonium ions, such as CH 5 + and C 2 H 7 + , have been detected first in high-pressure mass spectrometry experiments and later studied on a quantitative basis . In 1989, Lee and co-workers reported the IR spectrum of C 2 H 7 + , indicating the presence of two isomeric ions (C−H or C−C protonated form) …”

Section: Introductionmentioning

confidence: 99%

Reversible Protonation of Isobutane in Liquid Superacids in Competition with Protolytic Ionization

Sommer¹,

Bukala²,

Hachoumy³

et al. 1997

J. Am. Chem. Soc.

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The deuterium distribution observed in isobutane recovered after short contact times with the DF−SbF5 superacid at 0 °C shows that a very fast reversible protonation of all C−H bonds occurs before ionization of the alkane, in accord with the Olah σ-basicity concept. Comparison of the amounts of hydrogen with the amount of tert-butyl ions generated during ionization shows that the reaction is purely protolytic in HF containing up to 20 mol % SbF5, but becomes oxidative at higher concentrations.

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Pyrolysis of C₂H₇⁺ and other Ion–Molecule Reactions in Methane containing Traces of Ethane

Cited by 23 publications

References 4 publications

Stabilities and energetics of pentacoordinated carbonium ions. The isomeric protonated ethane ions and some higher analogs: protonated propane and protonated butane

Stabilities and energetics of pentacoordinated carbonium ions. The isomeric protonated ethane ions and some higher analogs: protonated propane and protonated butane

Three-Center−Two-Electron and Four-Center−Four-Electron Bonds. A Study by Electron Charge Density over the Structure of Methonium Cations

Reversible Protonation of Isobutane in Liquid Superacids in Competition with Protolytic Ionization

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