Carbon atom, ground state

Here, the bonding between carbon atoms is briefly reviewed fuller accounts can be found in many standard chemistry textbooks, e.g., [1]. The carbon atom [ground state electronic configuration (ls )(2s 2px2py)] can form sp sp and sp hybrid bonds as a result of promotion and hybridisation. There are four equivalent 2sp hybrid orbitals that are tetrahedrally oriented about the carbon atom and can form four equivalent tetrahedral a bonds by overlap with orbitals of other atoms. An example is the molecule ethane, CjH, where a Csp -Csp (or C-C) a bond is formed between two C atoms by overlap of sp orbitals, and three Csp -Hls a bonds are formed on each C atom. Fig. 1, Al. 

Methane. The carbon-atom ground-state electron configuration ls 2s 2p has two unpaired electrons and would seem to indicate a valence of 2. To get the well-known tetravalence of carbon, we assume that a 2s electron is promoted to the vacant 2p orbital, giving the configuration ls 2s2p. If we then assume one bond is formed with the 2s electron and three bonds are formed with the 2p electrons, the bonds are not all equivalent as they are known to be. Hence Pauling proposed that the 2s and 2p functions be linearly combined to form hybridized sp atomic orbitals, of the form... 

A further major deficiency becomes apparent as soon as we apply these arguments to carbon. The ground state valence configuration of a carbon atom is 2s 2pi2py, which suggests that it should be capable of forming only two bonds, not the four bonds that are so characteristic of this element. 

The meso carbon atom should present a carbenium structure with a low TT electron density in the ground state, in the excited state this carbon possesses the carbeniate structure (C ) with a high tt electron density (119). An electron-donating group in such a position should stabilize the ground state and rise the excited state to the highest level hypsochromic shift results as a whole. 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

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