Alkali atoms stability

At high alkali coverages (near monolayer coverage), when the adsorbed alkali overlayer shows a metal-like character, alkali-methoxy species are formed. As shown by TPD experiments in the system K/Ru(001) these alkali-methoxy species are more stable than the methoxy species on clean Ru(001) and adsorbed methanol on 0.1K/Ru(001). On metal surfaces inactive for methanol decomposition, e.g. Cu(lll), these alkali-methoxy species are formed even at low alkali coverages, due to the weaker interaction of the alkali atoms with the metal surface. The formation of these species stabilizes the methoxy species on the metal surface and enhances the activity of the metal surface for methanol decomposition. 

A motif found in the majority of alkali metal stabilized carbanion crystal structures is a nearly planar four-membered ring (13) with two metal atoms (M ) and two anions (A ), i.e. dimer. This simple pattern is rarely observed unadorned as in (13), yet almost every alkali metal and alkaline earth carbanion aggregate can be built up from this basic unit The simplest possible embellishment to (13) is addition of two substituents (S) which produces a planar aggregate (14). Typically the substituents (S) in (14) are solvent molecules with heteroatoms that serve to donate a lone pair of electrons to the metal (M). Only slightly more complex than (14) is the four coordinate metal dimer (15). Often the substiments (S) in (15) are joined by a linear chain. The most common of these chains are tetramethylethylenediamine (TMEDA) or dimethoxyethane (DME) so that the spirocyclic structure (16) ensues. Alternatively the donors (S) in (16) have been observed as halide anions (X ) when the metal (M ) is a divalent cation, e.g. (17) or (18). Obviously, the chelate rings found in (16) are entropically favorable relative to monodentate donors (S) in (14), (15), (17) or (18) (Scheme 2). 

When such clusters are formed in beams, essentially by many slow collisions of alkali atoms within a nozzle, it is found experimentally (by analysing the mass distribution using a time of flight spectrometer) that there are discontinuities in the intensity distribution of the peaks from which abundances can be computed for each cluster size. These discontinuities correspond to enhanced stability of metallic clusters around specific sizes (8, 20, 40, 58, etc). They are the same for all the different alkali metals, and are therefore referred to as magic numbers see fig. 12.5. They also turn out to be the same (at least, for the first few magic numbers) as those observed in nuclear physics. This similarity has led to an explanation based on the shell model and to the suggestion that the jellium model can be used to account for the properties of metallic clusters [683]. 

Matrix isolation has been shown to be a very useful technique to stabilize C02 generated by reduction of CO2 with alkali metal atoms during their codeposition at low temperature in a neat or inert matrix [82-86], An early matrix isolation spectroscopic (IR) study on alkali atoms and CO2, codeposited at 14 K in solid Ar, was conducted by Jacox and Milligan [82], Further studies showed a rich reactivity, except for sodium which was the least reactive among the alkali metals (Li, Na, K, Cs) investigated. 

Validity of shell models in Na V and Cs V clusters has been addressed by Reveles et al. [59]. Relative stability of these clusters at different sizes is measured by alkali addition energy the energy gain in adding successive alkali atoms. 

When hydroxypteridines are considered, it must be borne in mind that these compounds exist principally in the pteridinone forms, containing thermodynamically stable amide functions, and consequently have low reactivity. Their stability towards acid and alkali correlates well with the number of electron-donating groups which apparently redress the deficit of ir-electrons located at the ring nitrogen atoms. Quantitative correlations can be seen in the decomposition studies of various pteridinones (Table 7). These results are consistent with the number of the oxy functions and their site at the pteridine nucleus. The... 

In coating fullerenes with alkali metals, the stability of the cluster seemed to be determined primarily by the electronic configuration. The units C qM and C7oMg, where M is any alkali metal, proved to be exceptionally stable cluster building blocks. Coating a fullerene with more than 7 alkali metal atoms led to an even-odd alternation in the mass spectra, inter-... 

In Chapter 5 we identified metals by their high electrical conductivity. Now we can explain why they conduct electric current so well. It is because there are some electrons present in the crystal lattice that are extremely mobile. These conduction electrons move throughout the metallic crystal without specific attachment to particular atoms. The alkali elements form metals because of the ease of freeing one electron per atom to provide a reservoir of conduction electrons. The ease of freeing these conduction electrons derives from the stability of the residual, inert gas-like atoms. 

---