Composite metastable peak

Evers, E.A.I.M. Noest, A.J. Akkerman, O.S. Deconvolution of Composite Metastable Peaks a New Method for the Determination of Metastable Transitions Occurring in the First Field Free Region. Org. Mass Spectrom. 1977, 72, 419-420. 

Astonishingly, the study of the mechanism of formaldehyde loss from anisole revealed two different pathways for this process, one involving a four- and one a five-membered cyclic transition state (Fig. 6.37). [129] The four-membered transition state conserves aromaticity in the ionic product, which therefore has the lower heat of formation. Prompted by the observation of a composite metastable peak, this rather unusual behavior could be uncovered by deconvolution of two different values of kinetic energy release with the help of metastable peak shape analysis (Chap. 2.8). 

Fig. 6.37. Energetics of formaldehyde loss from anisole. The inset shows the composite metastable peak due to two different amounts of kinetic energy release. Adapted from Ref. [129] with permission. American Chemical Society, 1973.

It was also observed that loss of CO from the ion [CF2BrCO]+ is associated with a composite metastable peak. The authors propose for this ion the two reacting structures 106 and 107, the former associated with the narrow component of the peak characterized by a low KER (Tbroad component with a large KER value (Ty= 1056 me V). 

Loss of water from the molecular ions of cyclopentanol occurs by two pathways as evidenced by the composite metastable peak [415, 430], the major pathway being a 1, 3 (and/or 1, 4) elimination from the a-cleaved molecular ion. Deuterium substitution at the 3 or 4 positions on the ring has been shown to result in an isotope effect of 2—2.5 for this mode of decomposition. 

Energy releases for the different components of a composite metastable peak. 

Composite metastable peaks are also known and these can arise from the following situations ... 

The basic and most commonly used method for identifying the amorphous structure is X-ray diffraction (XRD). The diffraction patterns consist of broad bands instead of discrete peaks because of the lack of long-range atomic order of the amorphous state. This method is used to follow the changes in the amorphous structure that occur during pretreatment (37, 38), heat treatment (21, 23, 39-44), and reaction (22, 45-49). XRD permits the identification of different intermediate metastable crystalline phases (2,39, 40,50-52). Also, changes in the surface chemical composition induced by catalytic transformation are detected by XRD (46,53,54). Finally, X-ray line broadening is used to determine the mean crystallite size (21, 53). 

Ce02 crystallizes in the fluorite (fee) structure with octahedral coordination of the cations. This structure is maintained upon reduction to at least CeOu and sometimes lower. Metastable phases of varying compositions, all with defective fluorite structures, have been observed, suggesting that the diffusion of oxygen vacancies into the bulk is rapid at reduction temperatures this is a reason why the surface and bulk reduction peaks in the TPR spectrum overlap. The (defective) fluorite structure is retained even to high degrees of reduction in CeOx/ZrOx. ... 

The spectra of the unstable solid allotropes of sulfur are not known well. Sg, St, Sg, Si2, and other species (8, 9, 10) form yellow to white solids. Their solution spectra are discussed later. Quenched liquid sulfur or vapor produces yellow polymeric sulfur. Its spectrum shows a shoulder at 360 nm, as shown later. If liquid sulfur or sulfur vapor is quenched rapidly to 77 °K or below, deep colored films are formed. The spectrum of red sulfur produced by quenching a boiling liquid film is shown in Figure 2. The color is a result of three spectral features the absorption edge of polymeric sulfur, an absorption peak at 400 nm which is caused by the lowest allowed electronic transition of Sa, and an absorption peak at 550 nm which is a result of S4. The spectrum of trapped vapor consists of several broad peaks it changes with the composition of the vapor source and depends on speed of deposition and many other factors. The deep color of all trapped metastable species fades at — 90°C to deep yellow. Above this temperature, the spectrum shows only polymeric sulfur and Sg. 

The formation enthalpy AHf of amorphous alloys is less negative than that of crystalline materials of similar composition, which means that the former alloys are metastable. As a function of temperature and time the amorphous alloys will therefore transform into the stable crystalline phases. This transformation can conveniently be studied by means of diffraction methods. As will be discussed later on, no sharp diffraction lines occur in the diffraction diagrams of amorphous alloys. The transformation into the crystalline state is generally accompanied by the occurrence of sharp diffraction peaks. In some cases the stable crystalline phases are not reached directly. First one or more metastable crystalline phases may be formed which transform into the stable end products at a later stage of the crystalUzation process. 

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