Methylene electronic energy levels

Figure 2.14 shows the electron density due to each of the six MOs of methylene with their energy levels calculated at the AMI semiempirical level of theory. Note that the mixing of the s- and AOs on carbon leads to the familiar orbital shapes described by hybrid orbitals in valence bond theory. The OcH-orbital is composed almost exclusively of s-AOs, so that the negative lobe remains very small. The higher orbitals have more p-character on carbon. 

Fig. 2.14 AMI-calculated electron densities and energy levels for the valence molecular orbitals of singlet methylene...

While fewer data are available, the utility of DFT in computing the bond strengths between transition metals and hydrides, methyl groups, and methylene groups has also been demonstrated (Table 8.2). Because of the non-dynamical correlation problem associated with the partially filled metal d orbitals, such binding energies are usually very poorly predicted by MO theory methods, until quite high levels are used to account for electron correlation. 

Now what happens with the carbon atom In the ground state the four L electrons are distributed over a lone pair in the lowest energy state (2s), and one unpaired bonding electron in each of two p levels. Thus one would expect CH2 as a hydrogen compound to have a structure like that of water. Actually methylene exists as a very unstable product formed, among others, in the decomposition of ketene, CH2=C=0, but carbon is nevertheless predominantly quadrivalent. How can this be explained ... 

Theory has been used predominantly to probe the nature of the sites on vanadium clusters and model vanadium oxide surfaces. Cluster and p>eriodic DFT calculations [68,69] have been carried out in order to imderstand the electronic and structural properties of the exposed (100) surface of (VO)2P207. Both cluster and slab calculations reveal that surface vanadium sites can act as both local acid and base sites, thus enhancing the selective activation of n-butane as well as the adsorption of 1-butene. Vanadium accepts electron density from methylene carbon atoms and, thus aids in the subsequent activation of other C-H bonds. Calculations reveal that that the terminal P=0 bonds lie close to the Fermi level and thus present the most nucleophihc oxygen species present at the surface for both the stoichiometric as well as phosphate-terminated surfaces. These sites may be involved in the nucleophilic activation of subsequent CCH bonds necessary in the selective oxidative conversion of butane into maleic anhydride. Full relaxation of the surface, however, tends to lead to a contraction of the terminal P=0 bonds and a lengthening of the P V bonds. This pushes the P V states, initially centered on the oxygen atoms, higher in energy and thus increases their tendency to be involved in nucleophilic attack . 

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