Jeremy E. P. Dahl

Overcoming lability of extremely long alkane carbon-carbon bonds through dispersion forces

Steric effects in chemistry are a consequence of the space required to accommodate the atoms and groups within a molecule, and are often thought to be dominated by repulsive forces arising from overlapping electron densities (Pauli repulsion). An appreciation of attractive interactions such as van der Waals forces (which include London dispersion forces) is necessary to understand chemical bonding and reactivity fully. This is evident from, for example, the strongly debated origin of the higher stability of branched alkanes relative to linear alkanes and the possibility of constructing hydrocarbons with extraordinarily long C–C single bonds through steric crowding. Although empirical bond distance/bond strength relationships have been established for C–C bonds (longer C–C bonds have smaller bond dissociation energies), these have no present theoretical basis. Nevertheless, these empirical considerations are fundamental to structural and energetic evaluations in chemistry, as summarized by Pauling as early as 1960 and confirmed more recently. Here we report the preparation of hydrocarbons with extremely long C–C bonds (up to 1.704u2009Å), the longest such bonds observed so far in alkanes. The prepared compounds are unexpectedly stable—noticeable decomposition occurs only above 200u2009°C. We prepared the alkanes by coupling nanometre-sized, diamond-like, highly rigid structures known as diamondoids. The extraordinary stability of the coupling products is due to overall attractive dispersion interactions between the intramolecular H•••H contact surfaces, as is evident from density functional theory computations with and without inclusion of dispersion corrections.

Synthesis of Higher Diamondoids and Implications for Their Formation in Petroleum

triamantane (3) and so forth, can be prepared by chemical synthesis. Of the higher diamondoids, i.e., those that have isomeric forms, only C2h-symmetric [121]tetramantane (4 a) has been prepared in the laboratory in very low yields. All other higher diamondoids are only accessible from raw petroleum. There are three tetramantanes (4a and [1(2)3]tetramantane, C3v-4b ) including one enantiomeric pair (P)-(+)and (M)-( )-[123]tetramantane (4c), six pentamantanes (with [1(2,3)4]pentamantane being the first exhibiting a diamond {111} surface), 24 hexamantanes (6), 12] nearly one hundred heptamantanes (7), and so forth. Thus far, diamondoids with up to 11 cages have been shown to exist in petroleum, but no other source is known, although recent studies suggest possible interstellar occurrence. The larger nanodiamonds occur as rigid rods (4a, 5c), discs (4b), 12] pyramids (5 a), and helices (4c, 5 f), exhibiting quantum confinement and negative electron affinity. They can be specifically derivatized, 11,17, 18] with electron emission properties superior to any other material making them attractive for molecular electronics. The mechanism for formation of these nanodiamonds for a long time was attributed to thermodynamically controlled carbocation rearrangements. 21] Such mechanisms enable the practical synthesis of 1–3 but they fail in the production of the higher diamondoids. 21, 22] A detailed analysis of the mechanism for adamantane formation from a single starting material shows an amazing 2897 pathways; a more limited analysis of triamantane formation through carbocation pathways indicates at least 300000 potential intermediates. Prospects for higher diamondoid syntheses by these pathways are bleak due to a lack of large polycyclic precursors, problems with intermediates trapped in local energy minima, disproportionation reactions leading to side products, and the exploding numbers of isomers as the size of target higher diamondoid products increases. With the failure of syntheses of higher diamondoids through carbocation rearrangements, attempts at their preparation were abandoned in the 1980s. Since higher diamondoids occur in relatively high concentrations in petroleum that has undergone thermal cracking (i.e., been subjected to very high temperatures due to deep burial), we began to consider that these free-radical cracking reactions might be involved in higher diamondoid formation. The uncatalyzed formation of 1 and 2 from n-alkanes under conditions of cracking was shown recently, presenting evidence that exclusively thermal pathways involving free radicals can readily compete with the typically assumed acidcatalyzed carbocation rearrangements. Such mechanistic proposals underline the notion that diamondoids are thermodynamically the most stable hydrocarbons, i.e., they are more stable than nanographenes (extended polycyclic aromatic hydrocarbons) of comparable molecular weight. Moreover, the relative stabilities of carbocations and alkyl radicals Scheme 1. The family of diamondoids: lower diamondoids 1–3, the three isomers of tetramantane (4), and the six pentamantanes (5). The numbers in brackets refer to the unique Balaban–Schleyer nomenclature.

Stereoselective biodegradation of tricyclic terpanes in heavy oils from the Bolivar Coastal Fields, Venezuela

Gas chromatography‐mass spectrometry (GC‐MS) and GC‐MS‐MS analyses of heavy oils from Bolivar Coastal Fields (Lagunillas Field) show a complete set of demethylated tricyclic terpanes. As is the case for the 25-norhopanes, the demethylated tricyclics are probably formed in reservoirs by microbially-mediated removal of the methyl group from the C-10 position, generating putative 17-nor-tricyclic terpanes. Diastereomeric pairs of tricyclic terpanes are resolved above C24 due to resolution of 22S and 22R epimers, but the elution order of the 22S and 22R epimers is unknown. Early-eluting diastereomers (EE) predominate over late-eluting diastereomers (LE) (C25‐C29) in the heavily degraded oils, indicating a stereoselective preference for the LE stereoisomers during biodegradation. Conversely, the LE diastereomers predominate over the EE diastereomers in the 17-nor tricyclic series (C24‐C28), indicating that tricyclic terpanes and 17-nor-tricyclic terpanes are directly linked as precursors and products, respectively. A good correlation exists between the destruction of steranes and the demethylation of hopanes and tricyclic terpanes. This suggests that terpane demethylation occurs during sterane destruction and hopane demethylation, although the rate is slower, indicating that tricyclic terpanes are more resistant to biodegradation. # 2001 Elsevier Science Ltd. All rights reserved.

Evidence of Diamond Nanowires Formed inside Carbon Nanotubes from Diamantane Dicarboxylic Acid

to1Ddiamondnano-wires inside CNTs. The bisapical diamondoid diacid is morereactive than the pristine diamondoid, requiring milderreaction conditions. Unlike in 3D space, the diamantanedicarboxylic acid molecules are pulled inside a CNT by aneffective “capillary force” that originates in the stabilizationof the molecule inside the surrounding nanotube. Thiscapillary force, in turn, compresses the enclosed moleculararray of 1 axially and aligns the molecules favorably. In thisspecial surrounding environment, 1 may react in an unusualway to create an extended diamondoid cage during a mech-anistically complex polymerization process. Hence, the fusionof 1 under the confinement of CNTs may be a promisingchoice to yield diamond nanowires.Compound 1 was sublimed and self-assembled into thequasi 1D space of double-wall CNTs (DWCNTs) by a vaporphase reaction.

Synthesis and Transformation of Linear Adamantane Assemblies inside Carbon Nanotubes

We report the assembly and thermal transformation of linear diamondoid assemblies inside carbon nanotubes. Our calculations and observations indicate that these molecules undergo selective reactions within the narrow confining space of a carbon nanotube. Upon vacuum annealing of adamantane molecules encapsulated in a carbon nanotube, we observe a sharp Raman feature at 1857 cm(-1), which we interpret as a stretching mode of carbon chains formed by thermal conversion of adamantane inside a carbon nanotube. Introduction of pure hydrogen during thermal annealing, however, suppresses the formation of carbon chains and seems to keep adamantane intact.

Experimental and theoretical study of the absorption properties of thiolated diamondoids

Nanoscale hybrid systems are a new class of molecular aggregates that offer numerous new possibilities in materials design. Diamondoid thiols are promising nanoscale building blocks for such hybrid systems. They allow the incorporation of functional groups and the investigation of their effects on the unique materials properties of diamondoids. Here we combine experimental data with ab initio theory to explore the optical properties of diamondoid thiols and their dependence on size and shape. Agreement between theoretically and experimentally obtained absorption spectra allows the identification of the nature of the optical transitions that are responsible for some photophysical and photochemical processes. We show that the optical properties of diamondoid thiols in the deep UV regime depend on the functionalization site but are largely size independent. Our findings provide an explanation for the disappearance of diamondoid UV photoluminescence upon thiolation for smaller diamondoids. However, our theoretical results indicate that for larger diamondoid thiols beyond the critical size of six diamondoid cages the lowest energy transitions are characterized by diamondoidlike states suggesting that UV luminescence may be regained.

Template Synthesis of Linear-Chain Nanodiamonds Inside Carbon Nanotubes from Bridgehead-Halogenated Diamantane Precursors.

A simple method for the synthesis of linear-chain diamond-like nanomaterials, so-called diamantane polymers, is described. This synthetic approach is primarily based on a template reaction of dihalogen-substituted diamantane precursors in the hollow cavities of carbon nanotubes. Under high vacuum and in the presence of Fe nanocatalyst particles, the dehalogenated radical intermediates spontaneously form linear polymer chains within the carbon nanotubes. Transmission electron microscopy reveals the formation of well-aligned linear polymers. We expect that the present template-based approach will enable the synthesis of a diverse range of linear-chain polymers by choosing various precursor molecules. The present technique may offer a new strategy for the design and synthesis of one-dimensional nanomaterials.

Photoluminescence of diamondoid crystals

The photoluminescence of diamondoids in the solid state is examined. All of the diamondoids are found to photoluminesce readily, with initial excitation wavelengths ranging from 233u2009nm to 240u2009nm (5.3u2009eV). These excitation energies are more than 1u2009eV lower than any previously studied saturated hydrocarbon material. The emission is found to be heavily shifted from the absorption, with emission wavelengths of roughly 295u2009nm (4.2u2009eV) in all cases. In the dissolved state, however, no fluorescence is observed for excitation wavelengths as short as 200u2009nm. We also discuss predictions and measurements of the quantum yield. Our predictions indicate that the maximum yield may be as high as 25%. Our measurement of one species, diamantane, gives a yield of 11%, the highest ever reported for a saturated hydrocarbon, even though it was likely not at the optimal excitation wavelength.

Host–Guest Complexes of Cyclodextrins and Nanodiamonds as a Strong Non-Covalent Binding Motif for Self-Assembled Nanomaterials

We report the inclusion of carboxy- and amine-substituted molecular nanodiamonds (NDs) adamantane, diamantane, and triamantane by β-cyclodextrin and γ-cyclodextrin (β-CD and γ-CD), which have particularly well-suited hydrophobicity and symmetry for an optimal fit of the host and guest molecules. We studied the host-guest interactions in detail and generally observed 1:1 association of the NDs with the larger γ-CD cavity, but observed 1:2 association for the largest ND in the series (triamantane) with β-CD. We found higher binding affinities for carboxy-substituted NDs than for amine-substituted NDs. Additionally, cyclodextrin vesicles (CDVs) were decorated with d-mannose by using adamantane, diamantane, and triamantane as non-covalent anchors, and the resulting vesicles were compared with the lectin concanavalinu2005A in agglutination experiments. Agglutination was directly correlated to the host-guest association: adamantane showed lower agglutination than di- or triamantane with β-CDV and almost no agglutination with γ-CDV, whereas high agglutination was observed for di- and triamantane with γ-CDV.

NMR spectral properties of the tetramantanes - nanometer-sized diamondoids.

Tetramantanes, and all diamondoid hydrocarbons, possess carbon frameworks that are superimposable upon the cubic diamond lattice. This characteristic is invaluable in assigning their 1H and 13C NMR spectra because it translates into repeating structural features, such as diamond‐cage isobutyl moieties with distinctively complex methine to methylene signatures in COSY and HMBC data, connected to variable, but systematic linkages of methine and quaternary carbons. In all tetramantane C22H28 isomers, diamond‐lattice structures result in long‐range 4JHH, W‐coupling in COSY data, except where negated by symmetry; there are two highly symmetrical and one chiral tetramantane (showing seven 4JHH). Isobutyl‐cage methines of lower diamondoids and tetramantanes are the most shielded resonances in their 13C spectra (<29.5u2009ppm). The isobutyl methylenes are bonded to additional methines and at least one quaternary carbon in the tetramantanes. W‐couplings between these methines and methylenes clarify spin‐network interconnections and detailed surface hydrogen stereochemistry. Vicinal couplings of the isobutyl methylenes reveal positions of the quaternary carbons: HMBC data then tie the more remote spin systems together. Diamondoid 13C NMR chemical shifts are largely determined by α and β effects, however γ‐shielding effects are important in [123]tetramantane. 1H NMR chemical shifts generally correlate with numbers of 1,3‐diaxial H–H interactions. Tight van der Waals contacts within [123]tetramantanes molecular groove, however, form improper hydrogen bonds, deshielding hydrogen nuclei inside the groove, while shielding those outside, indicated by Δδ of 1.47u2009ppm for geminal hydrogens bonded to C‐3,21. These findings should be valuable in future NMR studies of diamondoids/nanodiamonds of increasing size. Copyright