Methane valence state

It was pointed out in Section 4-2 that the configuration sp8, which has promotion energy about 200 kcal/mole relative to the ground configuration 2s22p2, is the basis of the quadrivalent state of the carbon atom and is shown by quantum-mechanical calculations for methane to contribute about 49 percent to this valence state. Now let us consider the iron atom, for which spectroscopic energy levels are shown on... 

Consider the methane molecule, CH . The ground stale of a carbon atom is iP corresponding to the electron configuration of Is Zs lp. Carbon in (his state would be divalent because Only two unpaired electrons are available for bonding in the and py orbitals. Although divalent carbon is well known in methylene and carbene intermediates in organic chemistry, stable carbon compounds are tetravalent. fn order for four bonds to form, the carbon atom must be raised to its valence state. This requires... 

In decomposition reactions of dimethyl-metal complexes of palladium(II) and nickel (II) one finds the formation of only traces of methane [49] which may also attributed to an a-elimination process. In regard to the valence state, note that, formally, the alkylidene ligand is considered as a neutral ligand and therefore, in the tantalum-alkylidene complex in Fig. 4.29, tantalum is trivalent. The electronic structure of the alkylidene is of course reminiscent of the corresponding oxide CpTa(Cl)20, which we would definitely call pentavalent. All that matters is that there should be a sufficient number of electrons for the multiple bonds which we draw. 

Methane, CH4. The hydrogen atoms are at the corners of a regular tetrahedron, with the carbon at the centre. A carbon valence state with four identical, singly occupied hybrid AO s can be set up by allowing the mixing of 2s and all the 2p AO s. When they arc denoted by h, hg, hg, and h4, the carbon valence state is C(ls2 h hg hg h4 ). The... 

The electronic energy, AL/ i, is of interest mainly in connection with bonding theory since it is not an experimentally accessible quantity. A second quantity of this type is the intrinsic bond energy, the difference in energy between the atoms in the molecule and the separated atoms in the valence stafe, i.e., with all of the atoms in the same condition (with respect to spin and hybridization) as in the molecule. It is a measure of the strength of the bond after all other factors except the bringing together of valence state atoms have been eliminated (cf. the discussion of methane, Chapter 5 and McWeeny, R. Coulson s Valence, 3rd ed. Oxford University Oxford, 1979 Chapter 7). 

The explanation for this situation seems to He 1n hypothesizing that oxygen can diffuse 1n and out of the bulk (sub-surface atomic layers) of the catalyst, presumably oxidizing the metal to a higher valence state, during the a1r-flow part of the cycle. Then during the methane-flow part of the cycle, the oxygen can diffuse out to the the catalyst surface and react with... 

Ethane has a similar structure, each carbon being sp3 hybridised, there being a central link between the two carbon atoms by means of one sp3 orbital of one atom overlapping with the other. The three remaining sp3 hybrid orbitals on each carbon atom are used to overlap with three hydrogen Is orbitals. Both methane and ethane are typical saturated hydrocarbons, with the carbon exhibiting a full valency state of four. The bonds all result from end-on overlap between the orbitals, and are [Pg.39]

In our discussions of methane above and water below, we use patterns in IPs to support the results of an ab initio calculation. In fact, the connection can be made more quantitatively. Koopmans theorem states that the valence state IPs of a molecule can be directly related to the MO energies obtained from a HF calculation on the system ... 

In order to form bonds effectively in these directions, the nitrogen atom needs to use tetrahedral hybrid AOs (cf. methane. Section 1.10). We therefore need to promote the nitrogen atom into a valence state where its five valence electrons occupy tetrahedral hybrid AHs, each one-fourth s and three-fourths p (Fig. 3.27b). The effective occupation of the 2s and 2p AOs in such a state is then five-fourths 2s and fifteen-fourths 2p. To get there from the ground state involves in effect the promotion of three-fourths of an electron from the 2s AO to the 2p. The energy required to do this is supplied by the... 

The proper valence state for carbon in methane differs from that in ethylene, which in turn differs from that in acetylene. Generally, this is ignored in EHMO calculations and a cointromise set of VSlEs is selected for use over the whole range of molecules. 

Carbon occurs in the biosphere either reduced (methane, fatty acid, carbohydrate) or oxidized (alcohol, aldehyde, carbonic acid, carbon dioxide). The valence state 0 is found only in coal (a compound of biological origin). 

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