Solvated CH5+ in Liquid Superacid

Abstract

The transition states for methane activation in liquid superacid have been studied by experimentally determined secondary kinetic deuterium isotope effects (SKIEs) and computational chemistry. For the first time, the SKIEs on hydrogen/deuterium exchange of methane have been measured by using the methane isotopologues in homogeneous liquid superacid (2HF/SbF5). To achieve high accuracy of the SKIEs, the rate constants for pairs of methane isotopologues were simultaneously measured in the same superacid solution by using NMR spectroscopy. Density functional theory (DFT) and high-level ab initio methods have been employed to model possible intermediates and transition states, assuming that the superacids involved in the exchange reactions are H2F+ ions solvated by HF. Only the unsolvated superacid H2F+ is found to be strong enough to protonate methane, yielding the methonium ion solvated by HF as a potential energy minimum. In contrast, the (HF)x-solvated H2F+ superacids (x = 1-4) do not appear to be strong enough to yield stable solvated methonium ions. However, such ions show up as parts of the transition states of the exchange in which the methonium ions are solvated by (HF)x. The calculated DFT activation barrier is in good agreement with that experimentally observed.

Methane Activation and Oxidation in Sulfuric Acid

“…However between 270 and 315 8C the observed rate constant of deuterium incorporation into methane increased steadily ( Figure 2) going from 4.21 Â10 À6 s À1 at 270 8C to 8.41 Â10 À5 s À1 at higher than the values obtained previously in the very strong superacid H(D)F/SbF 5 . In a recent study of methane reactivity in DF/SbF 5 (15 mol % SbF 5 ) at À 20 8C, we [26] found a free energy of activation of 78.6 kJ mol À1 for this reaction. In the late 1960s Hogeveen and co-workers [8] reported an activation energy of 75 kJ mol À1 for the inverse reaction of dedeuteration of CH 3 D in HF/SbF 5 at high pressure (7 atm) over a temperature interval from À 10 8C to 258C.…”

“…The hydrogen exchange between methane and other acidic species, such as H 3 O , [31] H 2 F (HF) n , [26] SbF 5 /HF, [32] Sb 2 F 11 / HF, [33] and zeolites, [29] have previously been investigated theoretically (Figure 7). The protonating species are both neutral and cationic species and the charge influences the structure of the exchange transition state.…”

“…The concentration of CH 4 decreases gradually with time, whereas the concentration of deuterated products increases. Similar features were observed not only in DF/SbF 5 [26] but also on deuterated solid acids such as zeolites [9] and sulfated zirconias, [27] which are capable of exchanging protons in methane, albeit at higher temperature (300 ± 500 8C).…”

“…Previously investigated transition states for hydrogen exchange in methane: a) zeolites, [29] b) SbF 5 / HF, [32] c) H 3 O , [31] d) H 3 F 2 . [26] ing the MS parent peak of CO 2 (m/z 44) with the peak m/z 40 of argon used as internal standard. This rate was found to be the same as the rate of decrease of the methane peak (m/z 12 to 20 considering all the isotopologues and their fragmentation).…”

Properties of C−C Bonds inn-Alkanes: Relevance to Cracking Mechanisms

“…The carbonium mechanism, in particular, is seeing increased research interest, with new experiments [11][12][13][14][15][16][17] but particularly with computational chemistry methods. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Carbonium ions, or protonated alkanes, were first detected in mass spectrometry experiments 36 and have been spectroscopically detected only in the gas phase.…”