Helium compound formation

G. Ottaviani and M. Costato have investigated the process of compound formation at the platinum-silicon interface. Platinum films, 115, 205, 268 and 357 nm thick, were sputtered on Si(l 11) single crystals under hard vacuum. The interaction of the Pt and Si phases during isothermal annealing was followed using Rutherford backscattering spectroscopy of helium ions. 

Besides thermolysis, the photochemical decomposition of solid trihalo-methylmercury compounds RHgCCli, CF3HgOCOCF3 and Hg(OCOCF3)2 has been studied (Scheme 2). The irradiation of samples placed in an evacuated quartz tube, which was connected to a helium cryostat, was carried out at -50 to +10°C. Thus, a desorption into the gas phase of the primary products of the photolysis occurred, and consequent low-temperature matrix stabilization of them was made. As a result, the formation of only the radicals CCI3 (1 3 898 cm" ) and CF3 (vi 1084, V2 702, P2+ V4 1205, r>3 1249 cm ) or of products of their secondary reactions was observed (Mal tsev et al., 1974, 1975, 1977b). 

Compound-specific isotope analysis (CSIA) by GC-IRMS became possible in 1978 due to work of Mathews and Hayes [634], based on earlier low-precision work of Sano et al. [635]. The key innovation was the development of a catalytic combustion furnace based on Pt with CuO as oxygen source, placed between the GC exit and the mass spectrometer. The high pressure of helium (99.999% purity or better) ensures that all gas flows are viscous. After being dried in special traps avoiding formation of HC02 (i. e., interferes with 13C02) by ion-molecule reactions in the ion source, the C02 is transmitted to a device that regulates pressure and flow and then into the ion source [604]. 

Here, then, for the first time in the history of Chemistry, we have the undoubted formation of one chemical element from another, for, leaving out of the question the nature of the emanation, there can be no doubt that radium is a chemical element. This is a point which must be insisted upon, for it has been suggested that radium may be a compound of helium with some unknown element or, perhaps, a compound ofhdium with lead, since it has been shown that lead is probably one of the end products of the decomposition of radium. The following considerations, however, show this view to be altogether untenable (i.) All attempts to prepare compounds of helium with other... 

Compounds of the three heavier noble gases, krypton (Kr), xenon PCe), and radon (Rn), have been made, but the formation of stable compounds of the hghter noble gases, helium (He), neon (Ne), and argon (Ar), has been more difficult. Recently a positive ion has been formed by combining hydrogen with hehum (HeH ). 

IR spectrometric data reveal that the thermolysis of octa(methylsilsesquioxane) is a heterogeneous reaction involving the formation of a non-volatile solid phase similar to SiOj In a helium atmosphere at 150-300 °C (CHjSiO j)g almost completely evaporates without marked decomposition. When heat i in the air, however, this compound begins to decompose slowly at 270 °C, and at 450 °C an exo-effect attributed to the oxidation by air oxygen is observed When the temperature is raised to 500 °C, 66% of (CH3SiOi,5)g are oxidized to SiOj. 

In contrast to a straightforward and predictable decomposition pattern of photolysis with >400 nm light, irradiation of nitrosamides under nitrogen or helium with a Pyrex filter (>280 nm) is complicated by the formation of oxidized products derived from substrate and solvent, as shown in Table I, such as nitrates XXXIII-XXXV and nitro compound XXXVI, at the expense of the yields of C-nitroso compounds (19,20). Subsequently, it is established that secondary photoreactions occur in which the C-nitroso dimer XIX ( max 280-300 nm) is photolysed to give nitrate XXXIII and N-hexylacetamide in a 1 3 ratio (21). The stoichiometry indicates the disproportionation of C-nitroso monomer XVIII to the redox products. The reaction is believed to occur by a primary photodissociation of XVIII to the C-radical and nitric oxide followed by addition of two nitric oxides on XVIII and rearrangement-decomposition as shown below in analogy... 

Photodecomposition of the benzenesulfonamide derivative 183 in aqueous solution was investigated by irradiation from a 254 nm low pressure lamp under a helium atmosphere. Homolytic fission of the S-N bond gives arylsulfonyl and arylsulfonylamino radicals. Gyclization from the latter radical via C-N bond formation and hydrogen atom abstraction afforded the /3-sultam 47 in 7% yield. Other photodissociation processes resulted in the formation of compounds 184-186 (Scheme 59) <2002JPH109>. 

Until now, no compound of helium has been discovered, and as radon has intense a-radioactivity, information about its chemistry is very limited. Compounds of the other rare gas elements, Ne, Ar, Kr, and Xe, have been reported. For example, experimental investigation of CUO(Ng) (Ng = Ar, Kr, Xe n = 1, 2, 3, 4) complexes in solid neon have provided evidence of their formation. The computed structures of CUO(Ne)4 (Ar)M (n = 0, 1, 2, 3, 4) complexes are illustrated in Fig.17.5.1. 

Two main nuclear methods for the formation of the radioactive fluorine have been developed. The first involves a reaction of neon with deuterons, 20Ne(d, a) 18F, using elemental fluorine as a carrier. The second method uses irradiation of natural water with 30-MeV helium-3, [160 (3He,p)18F] or irradiation of lsO-enriched water with 16-MeV protons resulting in [lsO(p,n)18F]. This is a no carrier method and results in an anionic 18F fluoride. We will review here some of the synthetic ways based on these two major methods for introducing 18F isotope into organic compounds. Many of these compounds have special affinity to certain organs or receptors in the living body. 

Reagents Helium was purified by passage through charcoal, molecular sieve and an Oxisorb trap. Tenax adsorbant was obtained from Alltech Assoc. Inc. Chemical compounds for comparisons of mass spectra and GC retention times were obtained from commercial sources (Aldrich). Sec-butyl and isopropyl formates were synthesized according to Dlralcky (7). 

I he previous chapters showed how the laws of conservation of mass and con--1- servation of atomic identity, together with the concept of the mole, determine quantitative mass relationships in chemical reactions. That discussion assumed prior knowledge of the chemical formulas of the reactants and products in each equation. The far more open-ended questions of which compounds are found in nature (or which can be made in the laboratory) and what types of reactions they undergo now arise. Why are some elements and compounds violently reactive and others inert Why are there compounds with chemical formulas H2O and NaCl, but never H3O or NaCli Why are helium and the other noble gases monatomic, but molecules of hydrogen and chlorine diatomic All of these questions can be answered by examining the formation of chemical bonds between atoms. 

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