Solid solutions of normal paraffins

The results of the X-ray diffraction and thermal X-ray diffraction studies of the paraffins C_nH_2n+2 are revised based on new principles asserting the existence of several types of rotator states of paraffins. Along with the known isomorphism factors, such as the parity (symmetry) of the starting components and the difference between the chain lengths of their molecules (Δn), there is a factor that is specific to paraffins: differences in the energy state of molecules entering solid solutions. St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 5, pp. 929–938. September–October, 1996. Translated by L. Smolina     

The determination of normal paraffins in petroleum products

A normal paraffin, outside the carbon number range of the sample, is quantitatively added to the sample as an internal standard, and 10–100 μl of the mixture are injected into a small absorber unit which contains 400 mg of activated molecular sieve. The temperature of the absorber unit is raised from ambient to 300° at about 15°/min while nitrogen is passed. This elutes the non-linear hydrocarbons but leaves the normal paraffins absorbed. The sieve is removed from the absorber unit and the paraffins are released by destroying the sieve structure with hydrofluoric acid. A pellet of potassium hydroxide is then added to neutralise the excess acid and the released normal paraffins are extracted in 0.3 ml of isooctane. This isooctane solution is examined on a programmed-temperature chromatograph; the chromatogram obtained consists of a solvent peak followed by well-resolved peaks representing the normal paraffins. These are easily measured and the concentrations of the normal paraffins in the original sample are calculated against the internal standard.     

Types of rotary crystal state of solid solutions of paraffins

Thermal deformations and polymorphous transformations of solid solutions of paraffins in C₁₇–C₁₉, C₁₉–C₂₁, C₂₁–C₂₃, C₂₂–C₂₄, and C₂₃–C₂₄ systems are investigated by thermal X-ray diffractometry using a temperature step of several tenths of a degree. It is examined how the length of a molecular chain of a homolog (n) and the difference in length (Δn) between the chains as well as the molecular composition of a solid solution affect these transitions, and the data are compared with those for the individual homologs of paraffins. St. Petersburg University. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 3, pp. 380–394, May–June, 1998. This work was supported by RFFR grant No. 97-05-65534 and ISSEP grants No. 156p and a97-2633.     

Fluorine chemistry statistics: numbers of organofluorine compounds and publications associated with fluorine chemistry

Data are presented which demonstrate the considerable growth in the numbers of new organofluorine compounds produced annually, and also the numbers of papers and patents published concerned with fluorine chemistry. An overview of some information sources of relevance to fluorine chemists is also presented.     

Fullerene-like chemistry at the interior carbon atoms of an alkene-centered C26H12 geodesic polyarene

The title compound (1) undergoes 1,2-addition reactions of both electrophilic and nucleophilic reagents preferentially at the "interior" carbon atoms of the central 6:6-bond to give fullerene-type adducts 2, 3, 4, and 5. Such fullerene-like chemistry is unprecedented for a topologically 2-dimensional polycyclic aromatic hydrocarbon and qualifies this geodesic polyarene as a "bridge" between the old flat world of polycyclic aromatic hydrocarbons (PAHs) and the new round world of fullerenes. The relief of pyramidalization strain, as in the addition reactions of fullerenes, presumably contributes to the atypical mode of reactivity seen in 1. Molecular orbital calculations, however, reveal features of the nonalternant pi system in 1 that may also play an important role. Thus, the fullerene-like chemistry of 1 may be driven by two or more factors, the relative importances of which are difficult to discern.     

Surface chemistry of CN bond formation from carbon and nitrogen atoms on Pt(111)

The mechanism of CN bond formation from CH3 and NH3 fragments adsorbed on Pt(111) was investigated with reflection absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The surface chemistry of carbon-nitrogen coupling is of fundamental importance to catalytic processes such as the industrial-scale synthesis of HCN from CH4 and NH3 over Pt. Since neither CH4 nor NH3 thermally dissociate on Pt(111) under ultrahigh vacuum (UHV) conditions, the relevant surface intermediates were generated through the thermal decomposition of CH3I and the electron-induced dissociation of NH3. The presence of surface CN is detected with TPD through HCN desorption as well as with RAIRS through the appearance of the vibrational features characteristic of the aminocarbyne (CNH2) species, which is formed upon hydrogenation of surface CN at 300 K. The RAIRS results show that HCN desorption at approximately 500 K is kinetically limited by the formation of the CN bond at this temperature. High coverages of Cads suppress CN formation, but the results are not influenced by the coadsorbed I atoms. Cyanide formation is also observed from the reaction of adsorbed N atoms and carbon produced from the dissociation of ethylene.     

Reaction of carbon monoxide with ozone and oxygen atoms

The reaction between ozone and carbon monoxide was reinvestigated in the range of 80–160°C. The previously reported rate law ?d[O₃]/dt = k_a[O₃][CO] + k_b[O₃]² was confirmed and simulated using a mechanism based on an impurity-initiated chain reaction. When the CO was sufficiently purified, k_b tended to zero and k_a reduced to the value expected for the thermal decomposition of O₃. Subsequent reactions of O atoms with CO produced chemiluminescence which was used to measure k₃ for as 10^?14.0±0.3 exp[?(1630 ± 325)/T] cm³ molecule^?1 s^?1. The implications of this are discussed.     

Antenna chemistry with metallic single-walled carbon nanotubes

We show that, when subjected to microwave fields, surfactant-stabilized single-walled carbon nanotubes (SWNTs) develop polarization potentials at their extremities that readily drive electrochemical reactions. In the presence of transition metal salts with high oxidation potential (e.g., FeCl3), SWNTs drive reductive condensation to metallic nanoparticles with essentially diffusion-limited kinetics in a laboratory microwave reactor. Using HAuCl4, metallic particles and sheaths deposit regioselectively at the SWNT tips, yielding novel SWNT-metal composite nanostructures. This process is shown to activate exclusively metallic SWNTs; a degree of diameter selectivity is observed using acceptors with different oxidation potentials. The reaction mechanism is shown to involve Fowler-Nordheim field emission in solution, where electric fields concentrate at the SWNT tips (attaining approximately 10(9) V/m) due to the SWNT high aspect ratio (approximately 1000) and gradient compression in the insulating surfactant monolayer. Nanotube antenna chemistry is remarkably simple and should be useful in SWNT separation and fractionation processes, while the unusual nanostructures produced could impact nanomedicine, energy harvesting, and synthetic applications.     

Structure of silver clusters with magic numbers of atoms by data of molecular dynamics

The behavior of silver clusters (cubic octahedron habit) with magic numbers of atoms N = 13, 55, 146, 309, 561, 923, 1415, and 2057 in the 0–1300 K temperature range is studied for the embeded atom model by the molecular dynamics method. The structural method for the analysis of the dynamics of local configurations of atoms based on the construction of angular characteristics of simplexes of the Delone partition of a cluster is proposed. Structural transitions of clusters with a cubic octahedron habit to the stable clusters with an icosahedron habit are revealed. Motions of atoms in clusters with an icosahedron habit are transformed into the stationary vibration mode. Middle positions of atoms in clusters tend to form shells with a regular structure. At N = 561, there are 15 such shells. The cluster with N = 561 at 650 K is characterized by a reduced density close to that of silver melt.     

Global warming potential predictions for hydrofluoroethers with two carbon atoms

Global warming potentials are predicted using computational chemistry and thermodynamics approaches for four hydrofluoroethers where no data have previously been available. We also compare results with the same methodology for six other species. We combine predictions of radiative forcing values from density functional theory computations at the B3LYP/6-31g* level of theory with previous experimentally determined or newly estimated hydroxyl radical-hydrogen abstraction rate constants to obtain these global warming potentials. We find that many of the HFEs studied have lower global warming potentials than the hydrofluorocarbons and chlorofluorocarbons they may soon replace, although other environmental and technical issues may need to be addressed first. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

On the possibility of chain folding in solutions of normal paraffins and polyethylene

A simple statistical thermodynamic argument is presented in which the probability of intramolecular chain folding of normal paraffins and polyethylene in solution is examined. This possibility is based on the existence of low-energy intramolecular conformations which are stable enough to overcome the tendency of a chain molecule to assume a random arrangement in solution. Some rough estimates of the magnitudes of energetic interaction in straight-chain hydrocarbons are made to demonstrate the plausibility of this hypothesis. The experimental support for this model arises from NMR spectra of normal paraffins in aromatic hydrocarbon solvents.     

New insights into the interaction of hydrogen atoms with boron-substituted carbon

Boron substitution in carbon materials has been comprehensively investigated using the density functional theory method. It was found that there is a correlation between the stability of the graphene sheet, the distribution of pi electrons, the electrostatic potential, and the capability for hydrogen-atom adsorption. Boron substitution destabilizes the graphene structure, increases the density of the electron wave around the substitutional boron atoms, and lowers the electrostatic potential, thus improving the hydrogen adsorption energy on carbon. However, this improvement is only ca. 10-20% instead of a factor of 4 or 5. Our calculations also show that two substitutional boron atoms provide consistent and reliable results, but one substitutional boron results in contradictory conclusions. This is a warning to other computational chemists who work on boron substitution that the conclusion from one substitutional boron might not be reliable.     

The crystal chemistry of four thorium sulfates

Four thorium sulfate compounds have been synthesized and characterized. [Th(SO₄)₂(H₂O)₇]·2H₂O (ThS1) crystallizes in space group P2₁/m, a=7.2488(4), b=12.1798(7), c=8.0625(5) Å, β=98.245(1)^o; Na₁₀[Th₂(SO₄)₉(H₂O)₂]·3H₂O (ThS2), Pna2₁, a=17.842(2), b=6.9317(8), c=27.550(3) Å; Na₂[Th₂(SO₄)₅(H₂O)₃]·H₂O (ThS3), C2/c, a=16.639(2), b=9.081(1), c=25.078(3) Å, β= 95.322(2)^o; [Th₄(SO₄)₇(OH)₂(H₂O)₆]·2H₂O (ThS4), Pnma, a=18.2127(9), b=11.1669(5), c=14.4705(7) Å. In all cases the Th cations are coordinated by nine O atoms corresponding to SO₄ tetrahedra, OH groups, and H₂O groups. The structural unit of ThS1 is an isolated cluster consisting of a single Th polyhedron with two monodentate SO₄ tetrahedra and seven H₂O groups. A double-wide Th sulfate chain is the basis of ThS2. The structures of ThS3 and ThS4 are frameworks of Th polyhedra and sulfate tetrahedra, and each contains channels that extend through the framework. One of the Th cations in ThS3 is coordinated by a bidentate SO₄ tetrahedron, and ThS4 is unusual in the presence of a pair of Th cations that share a polyhedral face.     

Recombination of carbon monoxide and oxygen atoms

The recombination of carbon monoxide and oxygen atoms was studied in reflected shock waves in H₂:O₂:CO:Ar = 0.1:1:24:75 with 1300 < T₅ 2200 K and 2 < P₅ < 4 atm. Reaction progress was monitored by observations of the carbon monoxide flame spectrum near 435 nm and carbon dioxide thermal emission near 4.2 μm. Data analysis was accomplished with the aid of computer modeling using a 27-reaction mechanism. Computer modeling experiments also showed that these measurements were sensitive primarily to the rate of the reaction CO + O + M = CO₂ + M and only slightly sensitive to the rates of other reactions. The best fit to the data was achieved with a rate constant for this reaction of 7.7 × 10^?35 exp[19 kJ/RT] cm⁶ s for the temperature range of these experiments. Correlation of this result and previous data covering the temperature range 250 < T < 11,000 K confirms that this recombination reaction is governed by a nonadiabatic curve crossing with an activation barrier of about 20 kJ and subsequent deactivation of a singlet CO₂ molecule.