Importance of metastability

The importance of metastable phases which persist at ambient pressure and temperature need not be emphasized. Control of the eutectoidal decomposition of the metastable BCC iron-carbon phase, austenite, below 996 K is essential to the steel... 

Mass spectrometric investigation of gas phase clusters has come to play a significant role in cluster research because it is presently the only technique that allows size-dependent analysis of cluster beams. We anticipate that, in the near future, CMS studies utilizing tandem mass spectrometric techniques will be invaluable in confirming the importance of metastable decompositions and in determining the factors responsible for the enhanced intensities of various ions in the CMS. Such an instrument is currently under construction in our own labs, and we should soon have the ability to perform collision-induced dissociation studies on large vdW clusters. 

H. Bohni, L. Stockert, The importance of metastable pitting corrosion in the case of high-alloy steek, Werkst. Korros. 40 (1989) 63-71. 

Ulrich, J. and Strege, C. 2002. Some aspects of the importance of metastable zone width and nucleation in industrial crystallizers. J. Cryst. Growth 237-239 2130-2135. 

Classic examples are the spontaneous emission of light or spontaneous radioactive decay. In chemistry, an important class of monomolecular reactions is the predissociation of metastable (excited) species. An example is the fonnation of oxygen atoms in the upper atmosphere by predissociation of electronically excited O2 molecules [12, 13 and 14] ... 

The probability matrix plays an important role in many processes in chemical physics. For chemical reactions, the probability of reaction is often limited by tunnelling tlnough a barrier, or by the fonnation of metastable states (resonances) in an intennediate well. Equivalently, the conductivity of a molecular wire is related to the probability of transmission of conduction electrons tlttough the junction region between the wire and the electrodes to which the wire is attached. 

The importance of linked scanning of metastable ions or of ions formed by induced decomposition is discussed in this chapter and in Chapter 34. Briefly, linked scanning provides information on which ions give which others in a normal mass spectrum. With this sort of information, it becomes possible to examine a complex mixture of substances without prior separation of its components. It is possible to look highly specifically for trace components in mixtures under circumstances in which other techniques could not succeed. Finally, it is possible to gain information on the molecular structures of unknown compounds, as in peptide and protein sequencing (see Chapter 40). 

Wilhelm Ostwald was the first to recognise this state of affairs clearly. Indeed, he went further, and made an important distinction. In the second edition of his Lehrbuch der Allgenieinen Chemie, published in 1893, he introduced the concept of metastability, which he himself named. The simplest situation is just instability, which Ostwald likened to an inverted pyramid standing on its point. Once it begins to topple, it becomes ever more unstable until it has fallen on one of its sides, the new condition of stability. If, now, the tip is shaved off the pyramid, leaving a small flat... 

Verwey and Hamaker (10) have modified the Morse curve to take into account the approach of two charged bodies, as shown in Figure 4. Here, as one moves outward toward increasing distance of separation, an electrostatic repulsion is encountered because the charges are similar. A secondary minimum is then encountered as a result of the concentration of counter ions around each charged particle. The shallowness of the secondary minimum shows that the deflocculated system is metastable. The importance of the Verwey and Hamaker concept lies in its ability to show graphically the correlation between the secondary minimum and the metastable position. A, Figure 1. 

In such cases, the Nogami method can be applied to the early points curve (Fig. 6) and the solubility, S, of the polymorph can be assessed. One of the important aspects of metastable polymorphs in pharmacy is exactly their higher solubility, since the dissolution rate will also be higher [Eq. (7)]. Hence the bioavailability will be increased where this is dissolution rate limited [21]. 

An important example of the application of this method is seen for the case of ammonia. Referring to Figure 13, the measured average kinetic energy release of metastable (NH3)nH+ (n = 4-17) is seen to display a maximum value of... 

The importance of framework density and molar volume is evident also for large pore, mesoporous silica [33] and for AIPO4 polymorphs [34], Data for the latter are included in Figure 7.19. For mesoporous silica a transition from a regime where cages and pores affects the energetics to one in which the large pores act as inert diluent is reported. A further increase in pore diameter does not appear to increase the enthalpy of the compound [33], The similarity in enthalpy of many different structures shows that the synthesis of metastable microporous framework... 

Pseudo-a-cleavage competes relatively poorly with a-cleavage in fast dissociations in conventional 70-eV El spectra. However, it becomes more important at lower internal energies and often dominates the fragmentation of metastable ionized amines. 

The most important example of alkene elimination is found for ionized 1-pentylamine, for which propene loss is the dominant fragmentation of metastable ions. However, this channel is not of general importance metastable ionized 1-butylamine eliminates a much smaller proportion (15%) of ethylene and higher homologues of metastable ionized n-pentylamine rarely expel alkenes to any appreciable extent44. Nevertheless, [M — C H2 ]+ ions do appear in LELT spectra of long-chain primary alkylamines (e.g. at m/z 31, 45, 59, 73, 87 and 101 for 1-octylamine, Figure 2)113. 

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