CARBON VAPOR MOLECULES

At high temperature, carbon vaporizes to form a gas. This gas is a mixture of single carbon atoms and d/atom/cand polyatomic molecules, that is, molecules containing two, three, four, or more carbon atoms. These gaseous constituents are usually designated as C, C2, C3, etc. 

The understanding of the composition and behavior of these carbon vapors, the accurate measurement of their heat of formation, and the precise determination of their ratio are essential to calculate the heat of formation of organic compounds, i.e., the energies of all bonds involving carbon. 

The vaporization of carbon is a major factor in the ablation of carbon. This ablation is the basic phenomena that controls the performance of rocket-nozzle throats, reentry nose cones and other components exposed to extremely high temperatures (see Ch. 9). The rate of ablation is related to the composition of the carbon vapor formed during ablation and to the heat of formation of the various carbon-vapor species and their evaporation coefficient. 

Recent and accurate mass-spectrographic measurements of the energy required to vaporize graphite to the monoatomic gas give a value of 710.51 kJ mol (171.51 kcal mol ) Values for the heat of formation 

and particularly C3 are the dominant species in the equilibrium vapor in the temperature range of 2400 - 2700 K as shown in the Arrhenius plot of the partial pressure of these species in Fig. 2.19.f l[ l The contribution of Cg and Ictrger molecules to the vapor pressure is small and generally of no practical import. The general structure of these carbon molecule is believed to consist of double carbon bonds, C=C C=C (the so-called cumulene structure) which have delocalized bondings and an axial symmetry. Larger molecules, i.e., the fuiierenes, are reviewed below. 

---

FIGURE 1.3 The UV-vis spectrum of carbon vapor molecules trapped in argon, deposited at 10 K (bottom). In the other spectra the matrix was annealed to the indicated temperature. Notice the decay of the initially intense C3 absorption at 410 nm with increasing annealing. In the final spectrum the linear species C are produced and are indicated by the number of atoms. 

When a candle is burned, a gas is produced—a gas containing carbon dioxide and water vapor. It is useful to describe such a gas as a collection of molecules, each molecule containing smaller units called atoms. Each carbon dioxide molecule contains one carbon atom and two oxygen atoms. Each water molecule contains one oxygen atom and two hydrogen atoms. Where did these atoms come from Were they present in the candle before it burned ... 

The actual adsorption of vapor molecules takes place mainly on the surface of internal passages within the adsorbent particles, since that is where most of the available surface exists. The adsorption process may be either physical or chemical in nature. Physical adsorption is a readily reversible process that occurs as a result of the physical attraction between the gas molecules and the molecules of the solid surface. If the gas-solid intermolecular attraction is greater than the intermolecular attractions in the gas phase, the gas will condense on the solid surface, even though its pressure is lower than its vapor pressure at the prevailing temperature. For example, the equilibrium adsorption pressure of acetone on activated carbon may, under some conditions, be as little as 150 to 1,100 of the equilibrium vapor pressure at... 

The principal methods of gas activation are thermal and electrical much less common are chemical and photochemical activation. In the most commonly used thermal activation technique - the hot filament technique - a W or Ta wire is arranged in the immediate vicinity of the substrate to be coated by diamond (Fig. 1). The wire is heated until it reaches the temperature when H2 molecules dissociate readily. The gas phase is a mixture of a carbon-containing gas (e.g. methane, acetone or methanol vapor), at a concentration of a few per cent, and hydrogen. Upon the contact of the gas with the activator surface, excited carbon-containing molecules and radicals are produced, in addition to the hydrogen atoms. They are transferred to the substrate surface, where deposition occurs. Table 2 gives an indication of the hot-filament deposition process parameters. 

Existence. The existence of C13 was first proved by King and Birge2 in 1929 with a remarkable spectrogram of carbon vapor obtained by heating carbon up to 2,500°C in an evacuated furnace. A banded spectrum was observed with its head at 4,737.0 A. This is the so-called Swan band which is prominent in all emission spectra of carbon flames and which is due to the radical C2. Just at the side of this band King and Birge found a much weaker band identical in pattern but with its head at 4,744.5 A. They interpreted the band properly as being due to the diatomic carbon molecule Cl2-C13. 

Heath, J.R., Zhang, Q., Obrien, S.C. et al. (1987) The formation of long carbon chain molecules during laser vaporization of graphite. Journal of the American Chemical Society, 109, 359-363. 

When carbon vaporizes at extremely high temperatures, among the species present in the vapor is the diatomic molecule C2. Write a Lewis formula for C2. Does your Lewis formula of C2 obey the octet rule (C2 does not contain a quadruple bond.) Does C2 contain a single, a double, or a triple bond Is it paramagnetic or diamagnetic Show how molecular orbital theory can be used to predict the answers to questions left unanswered by valence bond theory. 

In 1985 chemists at Rice University in Texas used a high-powered laser to vaporize graphite in an effort to create unusual molecules believed to exist in interstellar space. Mass spectrometry revealed that one of the products was an unknown species with the formula Ceo- Because of its size and the fact that it is pure carbon, this molecule has an exotic shape, which the researchers worked out using paper, scissors, and tape. Subsequent spectroscopic and X-ray measurements confirmed that Ceo is shaped like a hollow sphere with a carbon atom at each of the 60 vertices. Geometrically, buckyball (short for buckminsterfullerene ) is the most symmetrical molecule known. In spite of its unique features, however, its bonding scheme is straightforward. Each carbon is xp -hybridized, and there are extensive delocalized molecular orbitals over the entire structure. 

A molecular dynamic simulation of the collision of a carbon atom and a fullerene molecule was performed recently to demonstrate a new method of producting a fullerene molecule with defects [46]. Such simulations can be considered both a model of an artificial process of atomic implantation and a model of collisions which can occur in the hot carbon vapors. 

---