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

Abstract

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...

The Protonation of Cubane Revisited

“…[34] This product forms through trapping of the exo-4-substituted tetracyclo[3.3.0.0 2,8 .0 3,6 ]oct-7-yl cationic structure with the reagent.…”

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Protonation of saturated hydrocarbons is the simplest and practically most important electrophilic reaction.[1] Small alkanes like methane, ethane, and isobutane [2] form welldefined protonated species, and their experimental proton affinities (PAs) are excellently reproduced computationally. [3][4][5] However, the current data on the protonations of strained hydrocarbons are rather controversial, and facile rearrangements are thought to account for the difficulties encountered in measuring the PAs.

[6] Cyclopropane (strain energy(E str ) = 27.2 kcal mol À1 ) is an exception because the number of energetic downhill paths is very limited and the barrier for the ring opening of corner-protonated C 3 H 7 + to the 2-propyl cation is high.

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

“…High-pressure mass spectrometric studies on size-selected C 2 H 5 + ·L n clusters (e.g., L = OCS, CO 2 , N 2 O, CH 4 , N 2 , H 2 , noble gas) [16] demonstrated mainly formation of weakly bound aggregates without reaction, with binding enthalpies between about 7 (L = Ar) and about 50 kJ mol À1 (L = N 2 O). [17] However, no structural information on the C 2 H 5 + ion core could be obtained, although accompanying quantum chemical calculations suggested that the geometry of C 2 H 5 + sensitively depends on the type of solvent and the degree of solvation.…”

Proton Transfer from Alkane Radical Cations to Alkanes

“…A useful experimental approach consists in the determination of the dissociation energy of the carbonium ions by ion-equilibrium measurements, as performed by Kebarle and Hiraoka using high-pressure ion source mass spectrometry [14]. The thermodynamics of the protonation of alkanes and the energetics of pentacoordinated alkyl carbonium ions are not easily accessible experimentally, but they are linked through the dissociation energy of the carbonium ions to the energetics of the corresponding carbenium ions that can more readily be studied experimentally.…”