Diprotonated methane, CH62+, and diprotonated ethane, C2H82+

Lammertsma, Koop; Oláh, George A.; Barzaghi, Mario; Simonetta, M.

doi:10.1021/ja00388a095

Cited by 50 publications

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“…Hydrogen scrambling in 1 therefore is extremely facile. This type of facile hydrogen scrambling was also found for CH 4 2ϩ (12), CH 5 ϩ (16-18), CH 6 2ϩ (25,26), and CH 7 3ϩ (27). Thus, it is clear that the most of the C 1 carbocations containing one or more 2eO3c bond can undergo very readily low energy bond to bond rearrangements as postulated in case of CH 5 ϩ by Olah et al as early as 1969 (19,20).…”

supporting

confidence: 56%

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

Rasul

Prakash²,

Oláh³

1997

Proc. Natl. Acad. Sci. U.S.A.

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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 ...

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confidence: 56%

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

Rasul

Prakash²,

Oláh³

1997

Proc. Natl. Acad. Sci. U.S.A.

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“…The calculated parent six-coordinate carbocation, diprotonated methane (CH 6 2ϩ ), has two twoelectron, three-center (2e-3c) bonding interactions in its minimum-energy structure 2a (C 2v ) (23,24). The calculated ␦ 13 C value of the dication 2a is Ϫ25.7, 14.2 ppm more shielded than that of five-coordinate monocation CH 5 ϩ 1a.…”

Section: Chmentioning

confidence: 99%

Calculated ¹¹ B– ¹³ C NMR chemical shift relationship in hypercoordinate methonium and boronium ions

Rasul

Prakash

Oláh

1998

Proc. Natl. Acad. Sci. U.S.A.

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The boronium-carbonium continuum was extended to include hypercoordinated protonated methanes and their boron analogs. RESULTS AND DISCUSSIONThe structures of the hypercoordinate protonated methanes and their boron analogs have been optimized at the MP2͞6-31G** level by using the (8) package of programs. 13 C and 11 B NMR chemical shifts were calculated by the correlated GIAO-MP2 method (9, 10) (tzp͞dz basis set; refs. 9 and 10) using the ACES II programs (11). CH 5 ؉ and BH 5 . CH 5 ϩ (protonated methane) is considered the parent of nonclassical carbocations containing a fivecoordinate carbon atom. The C s symmetrical structure 1a with a three-center, two-electron (3c-2e) bond is the preferred ground-state structure for the CH 5 ϩ (12-16). The isoelectronic boron analog of CH 5 ϩ is neutral BH 5 and is also C s symmetrical 1b based on high-level ab initio calculations (17, 18).The 13 C and 11 B NMR chemical shifts of 1a and 1b were calculated by using the GIAO-MP2 method (9, 10) ( Table 1) C of methane is Ϫ2.3. The shielding effect in 1a, however, is expected for such a hypercoordinate (nonclassical) carbocation (6, 7). Evidence for CH 5 ϩ in the condensed phase came from hydrogen-deuterium exchange on methane in deuteriated superacids (21,22) and ESCA studies (G.A.O., unpublished results). Direct NMR spectroscopic observation of the ion was not yet achieved and is difficult because of its expected low concentration and fast exchange in superacids.The calculated ␦ 11B of 1b is Ϫ28.8. We also estimated the ␦ 11 B values in 1b by applying Eq. 2 and by using the ␦ 13 C value of 1a. This yielded a ␦ 11

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“…It appears that C H t k retains in the 1 : 1 complex the C, structure of the isolated ion, and in particular the geometry of the characteristic 3c-2e bond is slightly affected by the interaction with methane: HI-H2 been predicted by refined HF calculations performed on different carbo and dicarbo cations, such as CH,", C2H62t, and C z H T (20,21). The presence of a two 3c-2e bond structure, suggested by Hiraoka and Kebarle (12) by analogy with the Hjt structure, was accurately checked.…”

Section: Ch'ch4mentioning

confidence: 99%

A theoretical investigation of energetics and structures of CH₅⁺(CH₄)_n clusters for n = 1–4

Fois¹,

Gamba²,

Simonetta³

1985

Can. J. Chem.

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Diprotonated methane, CH62+, and diprotonated ethane, C2H82+

Cited by 50 publications

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

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

Calculated ¹¹ B– ¹³ C NMR chemical shift relationship in hypercoordinate methonium and boronium ions

A theoretical investigation of energetics and structures of CH₅⁺(CH₄)_n clusters for n = 1–4

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Diprotonated methane, CH62+, and diprotonated ethane, C2H82+

Cited by 50 publications

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

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

Calculated 11 B– 13 C NMR chemical shift relationship in hypercoordinate methonium and boronium ions

A theoretical investigation of energetics and structures of CH5+(CH4)n clusters for n = 1–4

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

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

Calculated ¹¹ B– ¹³ C NMR chemical shift relationship in hypercoordinate methonium and boronium ions

A theoretical investigation of energetics and structures of CH₅⁺(CH₄)_n clusters for n = 1–4