Protonation of methane

Protoiiation (and protolysis) of alkanes is readily achieved with superacids. The protonation of methane itself to CH5, as discussed earlier, takes place readily. 

Figure 4. Graph of the GIAO-calculated shielding increment (Ag) of a proton of methane (oriented as in Figure 3) vs. distance above the center of ethene.

The effect of basis set superposition error (BSSE) was estimated for this series using the counterpoise method of Boys and Bemardi (16), The isotropic shielding value of methane by itself was calculated to be 31.8276 ppm. Isotropic shielding values corrected for BSSE for the proximal proton of methane were calculated for each distance from ethene (Table I). The difference between the isotropic shielding value calculated for methane alone and the isotropic shielding values at each geometry for the proximal proton of methane obtained for the methane-ethene pair in a counterpoise calculation including the basis functions of ethene (but no electrons) is... 

Table I. The isotropic shielding values (in ppm) calculated by GIAO for the proximal proton of methane over ethene as shown in Figure 3 with correction for basis set superposition error (BSSE).

An input file of the molecular geometry indicated in Figure 5 was created as described above. In this geometry two protons of methane are 2.0 A above the plane of ethene one is directly over the center of the carbon-carbon double bond, whereas the other is in a plane normal to the carbon-carbon double bond. The other two protons of methane are more distant from the plane of ethene. After creating the merged file, multiple copies of the file were made. Coordinates of the methane portion of the input file were manipulated in these copies so as to keep ethene (in the XY plane) stationary while the methane molecule was moved over the face of the ethene molecule incrementally in the X and Y directions, keeping the Z distance above the plane of ethene constant. The symmetry of ethene was employed to limit the number of geometries to be calculated. Only one quadrant over one face of... 

Figure 8. Pictorial representations of the HOMO of ethene (wiremesh) superimposed with the HOMO of ethene together with methane (solid) oriented with one proton of methane 2.0 A above the center of the carbon-carbon double bond and directed normal to the plane of ethene. (Reproduced with permission from ref 9. Copyright 1998 Plenum Press.)...

As calculations showed, protonation of BH, to form BI 1 is highly exothermic by 154.4kcalmol , which is even more exothermic by 26.4kcalmoT than the protonation of methane. These data suggest that in a previous study, when Olah et al. investigated the protonation of BIB to obtain evidence... 

Structure 3 has C2V symmetry and corresponds to protonation of methane along an HCH bisector. At the MP4(SDQ)/6-311G(d,p)//MP2/6-31G(d) level of theory, 3 is l.lkcalmol" above It has two pairs of equivalent hydrogens and the closeness in energies of 1 and 3 indicates that the barrier for scrambling of hydrogens in is very low. 

In the same way as protonation of a neutral metal-hydride complex yields a cationic H2 complex, protonation of methane yielding transient CHs+ may be considered as protonation of a C-H bond leading to a transient H2 complex of the carbocation CH3+, a strong Lewis acid. 

Lower alkanes such as methane and ethane have been polycondensed ia superacid solutions at 50°C, yielding higher Hquid alkanes (73). The proposed mechanism for the oligocondensation of methane requires the involvement of protonated alkanes (pentacoordinated carbonium ions) and oxidative removal of hydrogen by the superacid system. 

Electron Nuclear Dynamics (48) departs from a variational form where the state vector is both explicitly and implicitly time-dependent. A coherent state formulation for electron and nuclear motion is given and the relevant parameters are determined as functions of time from the Euler equations that define the stationary point of the functional. Yngve and his group have currently implemented the method for a determinantal electronic wave function and products of wave packets for the nuclei in the limit of zero width, a "classical" limit. Results are coming forth protons on methane (49), diatoms in laser fields (50), protons on water (51), and charge transfer (52) between oxygen and protons. 

Fig. 4. Proposed catalytic cycle for the hydroxylation of methane by MMO. The reductase and B components may interact with the hydroxylase in one or more steps of the cycle. Protons are shown in the step in which intermediate Q is generated, but other possibilities exist (see Fig. 3 and the text). The curved line represents a bridging glutamate carboxylate ligand.

Reaction of 63 with S(N Bu)3 followed by protonation with [ BuNI 13]C1 yields H2C[S(NtBu)2(NHtBu)]2, an imido analogue of methane disulfonic acid.178... 

The reactivity of the closely related system TpMe2PtMeH2 toward electrophiles in arene solvents has also been reported recently (68). The boron-based Lewis acid B(C6F5)3 induced elimination of methane and formation of an aryl(dihydrido) platinum(IV) complex via arene C-H activation (Scheme 17, A -> C). The active acid may be either B(C6F5)3 or alternatively a proton generated from B(C6F5)3 and trace water. It was proposed that the acid coordinates to a pyrazole nitrogen (shown in Scheme 17, B) forming an intermediate five-coordinate platinum(IV) complex, which readily eliminates methane. 

The observation of stable Pt(IV) alkyl hydrides upon protonation of Pt(II) alkyls has provided support for the idea that the methane which had been observed in earlier studies (89-92) of protonation of Pt(II) methyls could be produced via a reductive elimination reaction from Pt(IV). An extensive study of protonation of Pt(II) methyl complexes was carried out in 1996 (56) and an excellent summary of these results appeared in a recent review article (14). Strong evidence was presented to support the involvement of both Pt(IV) methyl hydrides and Pt(II) cr-methane complexes as intermediates in the rapid protonolysis reactions of Pt(II) methyls to generate methane. The principle of microscopic... 

Fig. 1. Unified scheme (similar to that presented in Ref. (56)) for protonation of platinum(II) methyl compounds and for methane activation. L is a general two-electron donor ligand. The ligands L on Pt need not be identical, and charges are not shown.

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

---