Hydrides magnetic structure

Boron s electron deficiency does not permit conventional two-electron bonds. Boron can form multicenter bonds. Thus the boron hydrides have structures quite unlike hydrocarbons. The 11B nucleus, which has a spin of 3/2, which has been employed in boron nuclear magnetic resonance spectroscopy. 

Yvon, K. and Fisher, R, Crystal structure and magnetic structures of ternary metal hydride a comprehensive review, in Hydrogen in Intermetallic Compounds I Electronic, Thermodynamic, and Crystallographic Properties, Preparation, L. Schlapbach, Editor. 1988, Springer Verlag Berlin, p. 88-138. 

The incompletely filled 4f-shell confers a magnetic moment to the lanthanide ions and it is the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism via polarization of the conduction electrons which is responsible for the occurrence of long-range ordered magnetic structures in the lanthanide metals. Details can be foimd in numerous reviews, among others in vol. 1, chs. 3, 6 and 7 (1978) of this Handbook series and a recent one by Jensen and Mackintosh (1991). The addition of hydrogen, both in solution or resulting in hydride formation, leads to a decrease of the conduction electron density e (cf. sect. 5)... 

In the 1960s, after Kennedy and Thomas [25] had established the isomerisation polymerisation of 3-methylbutene-l, this became a popular subject. From Krentsel s group in the USSR and Aso s in Japan there came several claims to have obtained polymers of unconventional structure from various substituted styrenes by CP. They all had in common that an alleged hydride ion shift in the carbenium ion produced a propagating ion different from that which would result from the cationation of the C C of the monomer and therefore a polymer of unconventional structure the full references are in our papers. The monomers concerned are the 2-methyl-, 2-isopropyl-, 4-methyl-, 4-isopropyl-styrenes. The alleged evidence consisted of IR and proton magnetic resonance (PMR) spectra, and the hypothetical reaction scheme which the spectra were claimed to support can be exemplified thus ... 

If the electron-cloud radius yrms were exactly equal to the structural radius r, Wasastjerna s criterion would be obviously true. But in fact, for ions r ce. 2 yrms (Table 3). Hence the criterion needs justification. It is obviously most probable for isoelectronic ions (cp. Eauling (/)), but the electron-cloud radii should refer to the ions in the crystals, not to the free ions. For, with a gross difference between crystal and free-ion electron-cloud radii for the hydride ion, there may be significant differences for others 40). For the crystals the electron-cloud radii could be obtained either from polarizeability or from magnetic susceptibility. The theory of polarizeability is less certain and there is a considerable correction to infinite wavelength. We therefore adopt the magnetic evidence. But this must be corrected for the inner shell contribution (Table 3). 

Boron compounds contain two isotopes B10 and B11 of natural abundances 19% and 81%, respectively. Although both these isotopes possess magnetic moments, the Bn nucleus is better suited to the high resolution experiment because of its (1) greater natural abundance, (2) smaller quadrupole moment, and (3) larger nuclear moment. Because of the broad range of structures possible in boron compounds, particularly the hydrides, there has been considerable NMR work done in this field to confirm previously proposed structures and in a few cases to first establish geometry of a compound. The B11 spectra of tetraborane and a tetraborane derivative arc considered below. 

Apparent NMR equivalence of nuclei can also arise by a quantum mechanical intramolecular tunneling process. In principle, this process may be differentiated from intermolecular exchange processes because although the exchanging nuclei are rendered equivalent insofar as the NMR experiment is concerned, spin-spin splitting by other magnetic nuclei is not washed out. This type of intramolecular exchange is manifested in several boron hydride derivatives. It was first proposed by Ogg and Ray (98) to explain the NMR spectra of aluminum borohydride, whose structure is... 

The properties of some rare-earth binary alloys with platinum group metals are also important in view of the role they can play in the chain of preparing ternary hydrides. Many of the alloys of the series R-M, where R is a rare earth element and M is a Group VIIIB metal, have been investigated structurally and magnetically. The alloys with iridium all have cubic structures, whereas those... 

Metallic hydrides are usually nonstoichiometric compounds, as expected from their relatively low heats of formation and the mobility of hydrogen. They are ordinarily described, chemically, in terms of any of three models in which hydrogen is considered a small interstitial atom, a proton, or a hydride anion. These models are discussed critically with particular reference to the group V metal hydrides. The interstitial atom model is shown to be useful crystallographically, the protonic model is questioned, and the hydridic model is shown to be the most useful at present. The effect of hydrogen content on the lattice parameter of VHn and the structural and magnetic properties of several hydrides are discussed in terms of these models. 

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