Diamondoid

Figure 1, Molecular structures of adamantane, diamantane, and triamantane, the smaller diamondoids, with chemical formulas C10H16, C14H20, and CigH24, respectively.

Figure 2. There are three possible tetramantanes, all of which are isomeric, respectively, from left to right as anti-, iso-, and skew-tetramantane. Anti- and skew-tetramantanes possess two quaternary carbon atoms, whereas iso-tetramantane has three quaternary carbon atoms. The number of diamondoid isomers increases appreciably after tetramantane.

Diamondoids, when in the solid state, melt at much higher temperatures than other hydrocarbon molecules with the same number of carbon atoms in their structures. Since they also possess low strain energy, they are more stable and stiff, resembling diamond in a broad sense. They contain dense, three-dimensional networks of covalent bonds, formed chiefly from first and second row atoms with a valence of three or more. Many of the diamondoids possess structures rich in tetrahedrally coordinated carbon. They are materials with superior strength-to-weight ratio. 

Many of the diamondoids can be brought to macroscopic crystalline forms with some special properties. For example, in its crystalline lattice, the pyramidal-shaped [l(2,3)4]pentamantane (see Table I) has a large void in comparison to similar crystals. Although it has a diamond-like macroscopic structure, it possesses the weak, noncovalent, intermolecular van der Waals... 

Diamondoid Chemical Formula Molecular Structure MW MP (°C) aBP (°C) p (g/ec) Crystal Structures... 

The presence of chirality is another important feature in many derivatives of diamondoids. It should be pointed out that tetramantane[123] is the smallest of the lower diamondoids to possess chirality [6] (see Table I for chemical structures). 

Diamondoids show unique properties due to their exceptional atomic arrangements. Adamantane consists of cyclohexane rings in chair conformation. The name adamantine is derived from the Greek word for diamond since its chemical structure is like the three-dimensional diamond subunit, as shown in Fig. 5. 

Later, the name diamondoids was chosen for all the higher cage hydrocarbon compounds of this series because they have the same structure as the diamond lattice highly symmetrical and strain-free so that their carbon atom structure can be superimposed on a diamond lattice, as shown in Fig. 5 for adamantane, diamantane, and triamantane. These compounds are also known as adamanto-logs and polymantanes. 

These compounds are chemically and thermally stable and strain-free. These characteristics cause high melting points (m.p.) in comparison to other hydrocarbons. For instance, the m.p. of adamantane is estimated to be 269 °C, yet it sublimes easily, even at atmospheric pressure and room temperature. The melting point of diamantane is about 236.5 °C and the melting point of triamantane is estimated to be 221.5 °C. The available melting point data for diamondoids are reported in Table I. 

Limited amounts of other chemical and physical property data have been reported in the literature for diamondoids [5, 9-30]. What is available is mostly for low molecular weight diamondoids. In what follows we report and analyze a selection of the available property data for diamondoids. 

Diamondoid Phase Transition Kind Temperature Range In P (kPa) = (K) Reference... 

Diamantane (CAS No 2292-79-7) pentacyclo[7.3.1.1 -. 0 -. 0 ]tetrade-cane, also known as decahydro-3,5,l,7-[1.2.3.4]-butanetetraylnaphtalene, can exist in gas, liquid, and three different solid crystallme states. Higher diamondoids possess two or more solid crystalline states [5],... 

For higher diamondoids very limited data are available in the literature. For triamantane, for example, the enthalpy and entropy of transition from crystalline phase II to crystalline phase I are reported [31] as follows ... 

Diamondoid (kJ/gmol) soUd (kJ/gmol) A TJO sublimation (kJ/gmol) A combustion (kJ/gmol) (solid phase) References... 

Little data is reported on tetra-, penta-, and hexamantane and other higher diamondoids. What is available is compiled by ChevronTexaco scientists as reported in Table I. This is possibly due to the fact that of these compounds only anti-tetramantane has been successfully synthesized in the laboratory in small quantities [32, 33]. 

Trends of solubility enhancement for each diamondoid follow regular behavior like other heavy hydrocarbon solutes in supercritical solvents with respect to variations in pressure and density [38, 39]. Supercritical solubilities of... 

The supercritical fluid and liquid solubilities reported in Figs. 9-12 suggest that diamondoids will preferentially partition themselves into the high-pressure, high-temperature, and rather low-boiling fraction of any mixture including crude oil. 

Schleyer s Lewis acid-catalyzed rearrangement method, which is based on diamondoid thermodynamic stability during carbocation rearrangements, has had little or no success in synthesizing diamondoids beyond triamantane. In recent years, outstanding successes have been achieved in the synthesis of adamantane and other lower molecular weight diamondoids [42 9]. Some new methods have been developed and the yield has been increased to 60%. 

Adamantane and other diamondoids are constituents of petroleum, gas condensate (also called NGL or natural gas liquid), and natural gas reservoirs [52-56]. Adamantane was originally discovered [40] and isolated from petroleum fractions of the Hodonin oil fields in Czechoslovakia in 1933. 

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