The qualities of diamond are heavily influenced by its composition, crystal structure, and mechanical, thermal, and electromagnetic properties. The composition is the most influential factor among these dependencies. The crystal structure represents the repeating pattern of diamond’s composition, and the properties are determined by molecular interaction, which in turn is influenced by composition. Thus, the determination of diamond’s qualities relies primarily on its composition.
Before being discovered, adamantane was referred to as decaterpene by Decker, who named his tricyclic hydrocarbon. Decker believed that decaterpene had a structure similar to that of a diamond lattice and proposed that it was highly structured and free of strain, much like diamond.
In 1924, Decker proposed a substance called decaterpene. However, it was not until 1933 that its existence was confirmed. Decaterpene was found in the petroleum of Hodinin, Czechoslovakia by Landa and Machachaeck. Even though the structure Decker had predicted was discovered, the compound was instead named adamantane, which is the Greek translation of diamond, after its discoverers Landa and Machcahcaeck.
The process of fractional distillation is used to separate crude petroleum into its individual compounds. First, the petroleum sample is heated until it vaporizes, leaving behind any solid impurities. The resulting vapor then enters a fractional distillation column where a temperature gradient has been created. As the vapor rises through the column, the temperature gradually decreases. This decrease in temperature causes the vapor to condense when it reaches its boiling point, and the condensed compound is collected in a specific fraction well. This process allows for the separation of compounds with similar boiling points. An example of this is adamantane, which has a high boiling point and condenses with the kerosene fraction in the 190o C cut.
The fractional distillation method has a limitation in extracting adamantane in large quantities due to its low concentration in petrol. By using the fractional distillation method, the presence of adamantane in petroleum was found to be only 0.0004% of the composition. Additionally, alkylated derivatives of adamantane are also present in the distilled petroleum. To increase the yield of adamantane, the thiourea adduct method can be used on the petroleum. Landa and Hale successfully isolated complexes of adamantane bonded to thiourea from crude petroleum.
Now that the natural product has been discovered, the next logical step would be to formulate the natural process in which the compound was made. As of 1964, the natural method that creates the adamantane compound had not been found. The natural process attempted involved bombarding adamantane-free petroleum with catalysts to try to initiate the formation of adamantane. The resulting mixture was fractioned and analyzed to examine any extra adamantane production. In most instances, the catalysts failed to create any adamantane. Nevertheless, several catalysts yielded derivatives that possessed the ring structure along with additional components attached.5 Among these catalysts, the only one that showed significant production was AlCl3, but not enough for mass production of adamantane. The catalysts that failed included oil-bearing stone from Hodin with and without HF, aluminum silicate, aluminum oxide, concentrated sulfuric acid, zinc chloride, iron(III) chloride, tin(IV) chloride, and antimony(V) chloride.5 It is believed that the reason many of these catalysts did not work, despite their presence in natural petroleum, is that the experimental conditions they were subjected to were not able to reproduce the biogenesis of petroleum necessary for adamantane formation in the lab.2
With the natural mechanism being unknown, researchers looked for a synthetic method to produce the compound in order to study adamantane. This was necessary because Landa had complete control over the limited supply of adamantane, causing its cost to greatly increase. Two methods were examined for creating the structure of natural adamantane: ring closure and isomerization.
Originally, before adamantane was discovered, researchers were working on finding a starting material to synthesize adamantane and its derivatives. In 1922, Meerwein attempted to remove the bridgehead carboxymethoxy group from ring compounds and close the ring structure using diiodomethane(V) and sodium. However, his experiments were unsuccessful because the malonic ester(VI) he created caused the reactant groups to be too far apart for recycling to occur(3,4). Despite his failures, Meerwein’s research with the malonic ester, now known as Meerwein’s ester, inspired further advancements in the search for a pathway to cyclic adamantane(7).
Bottger made significant progress in the research on adamantane synthesis after taking inspiration from Meerwein. Utilizing Meerwein’s ester, Bottger successfully formed a cyclic product by bringing the ring together. This product belonged to the tricyclo-[3.3.1.13,7] decane ring system, which includes adamantane. However, unlike adamantane, Bottger’s product retained external functional groups surrounding the ring instead of solely hydrogen atoms. Therefore, what Bottger synthesized was not adamantane itself but a derivative of it.
In 1937, Prelog and Seiworth successfully synthesized genuine adamantane by building upon Bottger’s research and removing the ring structure via decarboxylation, resulting in the formation of the fundamental ring structure. This process yielded adamantane as the end product, although it was not yet produced in large quantities. Specifically, Prelog and Seiworth’s method generated adamantane at an output rate of 0.16% compared to the input materials.
The advancements made by Prelog and Seiworth were furthered by the research of others, as often occurs in science. New research was conducted by Landa and Stetter in the adamantane research realm. Together, they were able to enhance the efficiency of Prelog and Seiworth’s overall synthesis.
The addition of the Heinsdecker pathway (11%) and the Hoffman reaction (24%) increased the yield of decarboxylation. However, even with these advancements, the synthesis of adamantane by ring closure never achieved an output higher than 6.5% of the reactants.
Nevertheless, Bottger’s process for the synthesis of derivatives remained an efficient method.
Research on adamantane was initially hindered due to the lack of economical synthesis methods. Instead, scientists turned to studying derivatives of adamantane to gather information. However, in 1957, Paul von R. Schleyer accidentally synthesized adamantane while researching the reversible endo-exo isomerization of tetrahydrodicyclopentadiene. During this experiment, Schleyer noticed the formation of a white crystalline compound as a side product. This compound was set aside and examined at a later time. Eventually, it was discovered that this mysterious compound had a melting point that matched the experimental melting point of adamantane. Further investigation confirmed that this compound possessed other characteristics similar to adamantane, thus establishing it as a match.
Schleyer’s discovery caused a major impact among scientists as it offered a more efficient way to produce adamantane. By subjecting tetrahydrodicyclopentadiene to an AlCl3-HCl mixture under 40 atms. of hydrogen pressure and using an HF-BF3 catalyst, Schleyer was able to increase the yield of adamantane synthesis to 30% and 40%.7
When Schleyer concentrated on his method for obtaining adamantane, he discovered that the synthesis was abundant using the common compound dicyclopentadiene as the starting reactant.3 Research then proceeded vigorously to examine every minute detail of the enigmatic compound. Their findings confirmed their previous claims that adamantane was unlike any carbohydrate known to mankind.
A three fused chairs of cyclohexane rings bound only to hydrogen atoms was identified as the carbohydrate. The crystallized structure of adamantane was extensively studied using X-ray diffraction. This technique involves the interaction of photons emitted from an excited metal atom with the crystal form of a compound, resulting in the creation of an X-ray diffraction pattern. The photons either bypass the crystal atoms or are altered by them. While most photons do not hit the atoms, those that are deflected follow a consistent pattern due to the repetitive nature of crystals. This pattern can be recorded using photographic film or a two-dimensional array detector to obtain a physical copy of the deflection pattern. Through analysis of this diffraction pattern, valuable information such as the crystalline lattice type, distance between atoms, and number of atoms per unit cell can be determined. Interestingly, the crystal orientation observed was entirely distinct from any known carbohydrate crystal orientation and followed a face centered cubic lattice structure. In adamantane, which had a tetragonal space group with four molecules per unit cell, the values for vector quantities were measured as a = 6.60A and c = 8.81A. Additionally, carbon bond lengths were found to be 1.54 ± 0.01A and bond angles were measured to be 109.5 ± 1.5o. These results provided evidence of adamantane’s stability, while further physical properties were yet to be determined.
The melting point of adamantane was determined to be 269oC, which is the highest melting point for a carbohydrate when exposed to the atmosphere. It is unexpected for such a happening, but adamantane continues to surprise. The boiling point of adamantane cannot be accurately determined as it can only be reached through mixture with other carbohydrates, with a boiling point of 190oC. This characteristic allowed adamantane to be discovered through fractionalization. The enigmatic nature of adamantane is further exemplified by its high melting and boiling points, while also being capable of sublimation at room temperature and atmospheric pressure.
The crystal structure and physical properties of adamantane are now known. However, there is still a need for technology to uncover the molecular interactions of the compound. Adamantane underwent NMR and IR testing (Fig 1,2) which produced unique results compared to any carbohydrate tested under the same conditions. The unique results are likely due to the symmetrical nature of adamantane. In fact, adamantane has an unprecedented symmetry number of twelve, unlike any carbohydrate. This means that throughout its structure, there are twelve planes and axes of symmetry or identity. While many compounds are symmetrical in one or two dimensions, few possess the three-dimensional symmetry found in adamantane.
The use of Nuclear Magnetic Resonance (NMR) takes advantage of the magnetic properties of atom nuclei. By placing a compound within a magnetic field, the nuclei can be excited by radiofrequency radiation. The absorption of radiofrequency by the nuclei is influenced by their environment, including neighboring nuclei and bonded nuclei. In the case of adamantane, a proton NMR revealed a sharp doublet with a spacing of 0.95 ppm, indicating that all protons are identical and the structure is symmetric. This demonstrates that each proton in adamantane shares electrons equally, leading to a strong resonance dependence. The singularity of the NMR result becomes an important tool for assessing the purity of adamantane. Any substitution on the ring would disrupt the resonance and be detected by the NMR as additional peaks, indicating an adamantane derivative, as long as the doublet remains present.
IR results, similar to NMR results, are affected by impurities. Adamantane itself produces a clear result in IR, but the presence of impurities clouds the results. Figure 2 shows that adamantane produces a prominent doublet at 2926 cm-1 with a transmittance of 0.8983, along with other peaks. This indicates the presence of methyl groups around the adamantane compound that have a similar nature and environment. As a result, all bonds in the compound absorb the same wavelengths, indicating identical motion such as stretching or scissoring. Any variation in a functional group would cause a change in the absorbance wavelength. Consequently, an increase in the number of peaks and a decrease in the intensity of existing peaks would be observed due to changes in the bonding pattern, which can limit or expand the possible motions of the bonds. Since different types of bond motion absorb different wavelengths in IR, any changes in bond types will alter the absorbances. The IR measures the amount of transmitted light per wavelength and converts it into an electrical signal through Fourier transform to generate an IR spectrogram.10 Absorbance and transmittance are inversely related, so any changes in absorbance will affect transmittance and the resulting spectrogram values.
The symmetrical and stable nature of adamantane makes it an ideal subject for numerous studies and research. Its versatility allows it to be used for various purposes such as studying structure reactivity relationships, developing empirical force field methods, creating orientation disorder probe models, and serving as a structural basis for drugs. The simplicity of adamantane’s nature and structure enables it to possess a one-of-a-kind strength and uniqueness, opening endless possibilities for its utilization.
Resources
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