sp2 Hybridization
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Table 2622. Comparison of C–C and C–H bonds in some materials.

Substance Bond Bond strength (kJ/mol) Bond length (Å)
Methane, CH4 (sp3) C–H 439 1.09
Ethane, CH3CH3 (sp3) C–C (sp3) 377 1.54
(sp3) C–H 420 1.09
Ethylene, H2C=CH2 (sp2) C=C (sp2) 728 1.34
(sp2) C–H 464 1.09
Acetylene, HC≡CH (sp) C≡C (sp) 965 1.20
(sp) C–H 558 1.06

The configuration of the valence electrons in carbon is a combination (called hybrids) of s and p orbitals. Graphite is one example with an sp2 type, while diamond is of an sp3 form.

For graphite, two 2p and one 2s orbitals hybridize to form sp2 atomic orbitals at a lower energy level, while a single p orbital remains at the original 2p energy level. This 2p orbital forms the π and π* molecular orbitals.

The sp2 bonded solids have both σ/σ* and π/π* states available to the electrons, while for sp3 bonded solids only the σ/ σ* states present. For the diamond and graphite, electron transitions to these states generate many of the characteristic features in EEL spectra. For instance, in core loss spectra from diamond, the excitation of the 1s electrons to the σ* states generates the carbon k edge peak. For graphite, the transitions of the inner shell electrons into unoccupied π* states give a peak prior to the edge onset. In the valence band, valence electron transitions into the π* states also produce a peak at around 6 eV.

EELS technique can be used to quantify the sp3 fraction versus sp2, e.g. in amorphous diamond.





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