Comparison of structures and energies of CH ₅ ^2+• with CH ₄ ^+• and their possible role in superacidic methane activation

Abstract

Contrary to previous theoretical studies at the UHF͞6-31G* level, the methonium radical dication CH 5 2؉ is not a C s symmetrical structure with a 2eO3c bond but a C 2v symmetrical structure 1 with two 2eO3c bonds (at the UHF͞6-31G**, UMP2͞6-31G**, and UQCISD(T)͞6-311G** levels). The C s symmetrical structure is not even a minimum at the higher level of calculations. The four hydrogen atoms in 1 are bonded to the carbon atom by two 2eO3c bonds and the fifth hydrogen atom by a 2eO2c bond. The unpaired electron of 1 is located in a formal p-orbital (of the sp 2 -hybridized carbon atom) perpendicular to the plane of the molecule. Hydrogen scrambling in 1 is however extremely facile, as is in other C 1 cations. It is found that the protonation of methane to CH 5 ؉ decreases the energy for subsequent homolytic cleavage resulting in the exothermic (24.1 kcal͞ mol) formation of CH 4 ؉• . Subsequent reaction with neutral methane while reforming CH 5 ؉ gives the methyl radical enabling reaction with excess methane to ethane and H 2 . The overall reaction is endothermic by 11.4 kcal͞mol, but offers under conditions of oxidative removal of H 2 an alternative to the more energetic carbocationic conversion of methane.Activation of alkanes, main components of petroleum and natural gas, is an important area of chemistry. Consequently, knowledge of the parent methane cations and dications are of great significance. There have been many experimental and theoretical studies of methane cations and dications (1-3). The methane radical cation, CH 4 ϩ• is the parent ion in mass spectrometry. In an early PNDO study by Olah and Klopman (4) the structure of CH 4 ϩ• was found to have C 2v symmetry and can be considered as CH 2 ϩ• radical cation complexed with a hydrogen molecule involving a three-center two-electron (2eO3c) bond. The 2eO3c unit is perpendicular to the plane of the molecule. High level ab initio calculations are in agreement with this conclusion (ref. 5 and references therein). Isoelectronic boron analog of CH 4 ϩ• is neutral BH 4• , also has similar C 2v symmetrical structure (6). Methane dication CH 4 2ϩ have also been observed in the gas phase (7-11). Planar C 2v symmetrical structure is preferred for the CH 4 2ϩ as shown by Wong and Radom (12). Earlier calculations predicted (13, 14) a square planar D 4h symmetrical structure for the CH 4 2ϩ dication. The sp 2 -hybridized carbon atom of CH 4 2ϩ contains a 2eO3c bond and an empty p-orbital perpendicular to the plane of the molecule.CH 5 ϩ is considered the parent of nonclassical carbocations containing a five coordinate carbon atom. Recent extensive ab initio calculations by Marx and Parrinello (15) as well as by Müller and Kützelnigg (16) reconfirmed the preferred C s symmetrical structure (17, 18) for the CH 5 ϩ cation with a 2eO3c bond as originally suggested by Olah and coworkers (19,20). The structure can be viewed as a proton inserted into one of the COH bonds of methane to form a 2eO3c bond between carbon and two hydrogen atoms. At the same time ready ...

A theoretical study of three gas-phase reactions involving the production or loss of methane cations

“…[15][16][17] In the terrestrial environment, it occurs in combustion processes because of the high temperature in flames (some ions observed in flames may be produced as a result of the dissociation of CH 4 + ) 18 or during the activation of hydrocarbons in the petroleum and natural gas industry. 19 After ionization, the CH 4 tetrahedral structure is distorted by the Jahn-Teller effect into a C 2v geometry. 20 This effect is observed in photoionization spectra, providing an unexpected and complex vibrational pattern.…”

“…20 This effect is observed in photoionization spectra, providing an unexpected and complex vibrational pattern. 19,[21][22][23][24] Also, the same effect has been invoked to explain the photoelectron spectra of methane: a rigorous theoretical analysis by Mondal and Varandas 22 of the first photoelectron band of methane successfully described the observed vibrational pattern.…”

Subterranean atmospheres may act as daily methane sinks

“…An ion-molecule reaction mechanism is implicated in the formation of these upper homologues of CH 4 , according to 46,47 :…”

Radiolysis via radioactivity is not responsible for rapid methane oxidation in subterranean air

“…However, a study from Spanish caves proposed that rapid CH 4 oxidation may be attributed to non-biological processes via radiolysis and ionization of subterranean air by natural radioactivity that could lead to the oxidation of CH 4 at a sufficiently fast rate to account for appreciable consumption of CH 4 [ 10 ]. It has been proposed that α-radiation (e.g., from 222 Rn) can radiolytically ionize, or generate radicals from, atmospheric components (e.g., H 2 O) including CH 4 [ 16 , 17 ]. The study by Haynes and Kebarle [ 16 ] determined that α-radiation has a slow effect on pure CH 4 and mixed hydrocarbon gas in the absence of air, making it difficult to extrapolate results to CH 4 in air in the presence of ions and radicals from heteromolecules.…”