Comparison and Search for CH53+ and CH64+ and Their Isoelectronic Boron Analogues BH52+ and BH63+ 1

Oláh, George A.; Rasul, Golam

doi:10.1021/ja961597e

Cited by 22 publications

(12 citation statements)

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“…We recently reported (21) that the planar D 5h symmetric structure 5 is an energy minimum for the CH 5 3ϩ trication. On the other hand, our calculations showed that the energy-minimum structure of isoelectronic boron analog, the BH 5 2ϩ dication has the planar C 2v symmetric structure 6, with two 2eO3c bonds (21).…”

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confidence: 75%

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: 75%

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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“…There is a consistent pattern to be found in examining the geometries of the BH 2 Table 3) are also noted. This is not unlike the situation found in geometry optimized [13] and ‫ם‬ CH 5 [14], entities containing two hydrogens that CH BH 3 3 cule. This is in contrast with the protonated benzene, C 6 [12] (which is thought to be involved as an in-‫ם‬ H 7 termediate in H/H proton exchange in AlCl 3 /HCl promoted reactions); the CH 2 group has rather more standard C-H distances (Ϸ1.1 Å ) and an H • • • H distance (Ϸ1.7 Å ) suggesting very little H-H bonding.…”

Section: Mechanism Of D/h Exchangementioning

confidence: 70%

Deuteration of closo‐1,2‐ and 1,7‐C2B10H12 using C6D6/AlCl3: Mechanistic considerations

et al. 1998

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“…Thus, the calculated structure for the protonated carborane species could well imply that the boron of the BH 2 group is sharing a pair of H-H r electrons, just as it could be implied [14] for the special bonding situation in "stable species" of and .…”

Section: Figurementioning

confidence: 86%

Deuteration of closo-1,2- and 1,7-C2B10H12 using C6D6/AlCl3: Mechanistic considerations

et al. 1998

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Regioselective deuteration of 1‐X‐C2B10H12 (X = 2, 7) cage systems with C6D6/AlCl3 is correlated to ab initio calculational results on a [C2B10H13]+ intermediate. Full geometry optimizations of pertinent [C2B10H13]+ isomers, derived from each of the two 1‐X‐C2B10H12 carborane isomers, result in cage geometries not unlike the (nearly) icosahedral starting carborane. Each isomer contains a BH2 group having an acute H‐B‐H angle, long B–H bonds, and a very short H · · · H distance, hinting that the pertinent boron shares the electrons of a hydrogen molecule σ pair. It is suggested that the structural differences between the BH2 group of [C2B10H13]+ and the CH2 group of the benzenium ion, [C6H7]+ (the intermediate strongly intimated upon protonation of benzene), can be explained, in part, by (a) the availability of the π‐ring electrons for bonding to the (extra) proton in the latter and (b) the unavailability of π electrons from the carborane. Thus, the C2B10H12 cage is most probably very reluctant to give up a cage electron pair in order to assist in bonding to an (externally bound) proton. Instead, it is more probable that “hydridic” B–H sigma electrons could very well play the important role in providing bonding to the attacking proton. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:95–102, 1998

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Comparison and Search for CH₅³⁺ and CH₆⁴⁺ and Their Isoelectronic Boron Analogues BH₅²⁺ and BH₆³⁺ ¹

Cited by 22 publications

References 22 publications

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

Deuteration of closo‐1,2‐ and 1,7‐C2B10H12 using C6D6/AlCl3: Mechanistic considerations

Deuteration of closo-1,2- and 1,7-C2B10H12 using C6D6/AlCl3: Mechanistic considerations

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Comparison and Search for CH53+ and CH64+ and Their Isoelectronic Boron Analogues BH52+ and BH63+ 1

Cited by 22 publications

References 22 publications

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

Deuteration of closo‐1,2‐ and 1,7‐C2B10H12 using C6D6/AlCl3: Mechanistic considerations

Deuteration of closo-1,2- and 1,7-C2B10H12 using C6D6/AlCl3: Mechanistic considerations

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Comparison and Search for CH₅³⁺ and CH₆⁴⁺ and Their Isoelectronic Boron Analogues BH₅²⁺ and BH₆³⁺ ¹

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