Diamondoid Molecules

Diamondoid molecules are cagelike saturated hydrocarbons. These molecules are ringed compounds, that have a diamondlike structure consisting of a number of six-member carbon rings fused together. They are called "diamondoid" because they can be assumed as repeating units of the diamond. The most famous member of this group, adamantane, is a tricyclic saturated hydrocarbon (tricyclo [3.3.1.1]decane).

Isomers Diamantane
Figure 3.2. Molecular structures of adamantane, diamantane, and trimantane, the smaller diamondoids, with chemical formulas C10H16, C14H20, and C18H24, respectively.

The simplest of these polycyclic diamondoids is adamantane, followed by its homologues diamantane, tria-, tetra-, penta- andhexamantane. In Figure 3.2, adamantane (CioHi6), diamantane (Ci4H20), and triamantane (C18H24), the smaller dia-mondoid molecules, with the general chemical formula C4n+6H4n+12 are reported. These are lower adamantologues, as each has only one isomer. Depending on the spatial arrangement of the adamantane units, higher polymantanes (n > 4) can have numerous isomers and nonisomeric equivalents. There are three possible tetraman-tanes, all of which are isomeric, respectively, iso-, anti-, and skew-tetramantane as depicted in Figure 3.3 Anti- and skew-tetramantanes each possess two quaternary carbon atoms, whereas iso-tetramantane has three. There are seven possible pentamantanes, six being isomeric (C26H32) obeying the molecular formula of the homologous series and one nonisomeric (C25H30). For hexamantane, there are 24 possible structures: among them, 17 are regular cata-condensed isomers with the chemical formula (C30H36), six are irregular cata-condensed isomers with the chemical formula (C29H34), and one is peri-condensed with the chemical formula (C26H30).

Figure 3.3. 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 each possess two quaternary carbon atoms, whereas iso-tetramantane has three quaternary carbon atoms. The number of diamondoid isomers increases appreciably after tetramantane.

Figure 3.3. 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 each possess two quaternary carbon atoms, whereas iso-tetramantane has three quaternary carbon atoms. The number of diamondoid isomers increases appreciably after tetramantane.

When in solid state, diamondoids melt at much higher temperatures than other hydrocarbon molecules with the same number of carbon atoms in their structure. Inasmuch as they also possess low strain energy they are more stable and stiff, and resemble 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 a superior strength-to-weight ratio, as much as 100 to 250 times as strong as titanium, but much lighter in weight. In addition to applications in nanotechnology they are being considered to build stronger, but lighter, rocket and other space components and a variety of other earthbound articles for which the combination of weight and strength is a consideration [13,14].

It has been found that adamantane crystallizes in a face-centered cubic lattice, which is extremely unusual for an organic compound. The molecule therefore should be completely free from both angle and torsional strain, making it an excellent candidate for various nanotechnology applications. At the beginning of growth, crystals of adamantane show only cubic and octahedral faces. The effects of this unusual structure upon physical properties are striking (interested readers are referred to [15] for a further detailed description of thermodynamic and crystalline properties of adamantane). Adamantane is one of the highest melting hydrocarbons known (m.p. ~266-268°C), yet it sublimes easily, even at atmospheric pressure and room temperature. Because of this, it can have interesting applications in nanotechnology such as possibilities for application in molding and cavity formation.

Diamondoids can be divided into two major clusters based upon their size: lower diamondoids (1-2 nm in diameter) and higher diamondoids (>2 nm in diameter).

Adamantane and other light diamondoids are constituents of petroleum and they deposit in natural gas and petroleum crude oil pipelines causing fouling [13,16,17]. Adamantane was originally discovered and isolated from Czechoslo-vakian petroleum in 1933. The isolated substance was named adamantane, from the Greek for diamond. This name was chosen because it has the same structure as the diamond lattice, highly symmetrical and strain free. Actually their carbon atom structure can be superimposed upon a diamond lattice. It is generally accompanied by small amounts of alkylated adamantane: 2-methyl-; 1-ethyl-; and probably 1-methyl-; 1,3-dimethyl; and others.

The unique structure of adamantane is reflected in its highly unusual physical and chemical properties, which can have many applications in nanotechnology, as do the diamond nanosized crystals, with a number of differences. The carbon skeleton of adamantane comprises a cage structure, which may be used for encapsulation of other compounds, such as drugs for drug delivery. Because of this, adamantane and other diamondoids are commonly known as cage hydrocarbons. In a broader sense they may be described as saturated, polycyclic, cagelike hydrocarbons.

Diamantane, triamantane, and their alkyl-substituted compounds, just as adamantane, are also present in certain petroleum crude oils. Their concentrations in crude oils are generally lower than that of adamantane and its alkyl-substituted compounds. In rare cases, tetra-, penta-, and hexamantanes are also found in petroleum crude oils. Their diamondlike rigidity and strength make them ideal for molecular building blocks. Research along this line could have a significant impact on practical applications of diamondoids in nanotechnology. Vasquez et al. [13,16] succeeded in identifying diamondoids in petroleum crude oils, measuring their concentrations and separating them from petroleum. Recently, Dahl et al. have identified higher diamondoids, from n = 4-11, and their isomers in certain petroleum fluids [12].

Diamondoids show unique properties due to their exceptional atomic arrangements. These compounds are chemically and thermally stable and strain free. These characteristics make them have a high melting point in comparison with other hydrocarbons. For instance, the m.p. of adamantane is estimated to be in the range of 266-268°C and of diamantane in the range of 241-243°C. Such high melting points of diamondoids have caused fouling in oil wells, transport pipelines, and processing equipment during production, transportation, and processing of diamondoid-containing petroleum crude oil and natural gas [13,16].

One may exploit the large differences in melting points of diamondoids and other petroleum fractions for isolation of diamondoids from petroleum [14]. Many of the diamondoids can be brought to macroscopic crystalline forms with some special properties. For example, in its crystalline lattice, pyramidal [1(2,3)4]pen-tamantane has a large void in comparison with similar crystals. Although it has a diamondlike macroscopic structure, it possesses weak noncovalent intermolecular van der Waals attractive forces involved in forming a crystalline lattice [12,18].

The crystalline structure of 1,3,5,7-tetracarboxy adamantane is formed via car-boxyl hydrogen bonds of each molecule with four tetrahedral nearest neighbors. The similar structure in 1,3,5,7-tetraiodoadamantane crystal would be formed by I... I interactions. In 1,3,5,7-tetrahydroxyadamantane, the hydrogen bonds of hy-droxyl groups produce a crystalline structure similar to inorganic compounds, such as a CsCl, lattice [19] (see Figure 3.4).

Presence of chirality is another important feature in many derivatives of diamondoids. Such chirality among the unsubstituted diamondoids occurs first of all in tetramantane [12].

The vast number of structural isomers and stereoisomers is another property of diamondoids. For instance, octamantane possesses hundreds of isomers in five molecular weight classes. The octamantane class with formula C34H38 and molecular weight 446 has 18 chiral and achiral isomeric structures. Furthermore, there is unique and great geometric diversity with these isomers. For example, rod-shaped diamondoids (the shortest one of which is 1.0 nm), disc-shaped diamondoids, and screw-shaped ones (with different helical pitches and diameters) have been recognized [12].

Diamondoids possess great capability for derivatization. This matter is of importance in reaching suitable molecular geometries needed for molecular building blocks of nanotechnology. These molecules are substantially hydrophobic and their solubility in organic solvents is a function of that (adamantane solubility

Figure 3.4. The quasi-cubic units of a crystalline network for 1,3,5,7-tetrahydroxyadamantane. Molecules have been shown as gray spheres and hydrogen bonds as solid linking lines

Figure 3.4. The quasi-cubic units of a crystalline network for 1,3,5,7-tetrahydroxyadamantane. Molecules have been shown as gray spheres and hydrogen bonds as solid linking lines

in THF is higher than in other organic solvents [20]). Using derivatization, it is possible to alter their solubility. Functionalization by different groups can produce appropriate reactants for desired reactions.

The strain-free structures of diamondoids give them high molecular rigidity, which is quite important for a MBB. High density, low surface energy, and oxidation stability are some other preferred diamondoid properties as MBBs.

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment