Molecule fragments

Understanding how molecules fragment upon electron impact permits a mass spec trum to be analyzed m sufficient detail to deduce the structure of an unknown compound Thousands of compounds of known structure have been examined by mass spectrome try and the fragmentation patterns that characterize different classes are well docu mented As various groups are covered m subsequent chapters aspects of their fragmentation behavior under conditions of electron impact will be descnbed... 

As small molecule fragments resulting from the initiator may plasticize the polymer and lower performance, approaches have been developed to avoid this. A dihydroxyamine can be used to form a polyester [52]. This accelerator gave a modest increase in the strength of unsaturated polyester resins. A polymerizable tertiary amine has been prepared by the reaction of A-methylaniline with glycidyl methacrylate [53] (Scheme 8). 

The MALDI-TOF mass spectra of the Cso-fullerene-annelated [3]- and [4]rotanes 127 and 128 also demonstrated that these molecules fragment at the spirocyclopropane units with successive loss of the fullerene moieties. Unfortunately, however, the peaks for cyclo-Ci and cyclo-C2o carbon clusters were not observed [38]. 

Following a description of femtosecond lasers, the remainder of this chapter concentrates on the nuclear dynamics of molecules exposed to ultrafast laser radiation rather than electronic effects, in order to try to understand how molecules fragment and collide on a femtosecond time scale. Of special interest in molecular physics are the critical, intermediate stages of the overall time evolution, where the rapidly changing forces within ephemeral molecular configurations govern the flow of energy and matter. 

In a unimolecular reaction, a molecule fragments into two pieces or rearranges to a different isomer, hi either case, a chemical bond breaks. For example, in the fragmentation of bromine molecules, breaking a ffbond gives a pair of bromine atoms Bf2 2 Br Recall that this unimolecular process is the first step of the reaction between molecular hydrogen and molecular bromine to give HBr. 

CI-MS and EI-MS address the same compound class. Cl is used mainly when the molecule fragments so completely in El mode that no M+" ions are observed or when the problem is only knowledge of the molecular weight of the sample component. In fact, El and Cl are usually both carried out on the same sample, as the two ionisation methods produce complementary information of value for the determination of structure and MW of a compound. The detection limits of Cl tend to be better than El, as the latter technique divides the ion current between molecular and fragment ions. A few ng of sample may be detected. 

Molecular interferences can be completely eliminated by exploiting the fact that multiply charged molecules fragment with 100 percent probability, because of the internal coulomb forces, when several electrons are removed [1]. 

Free radicals are the atom groups or molecule fragments having unpaired electrons. Most of them are unstable with high reactivity. Interacting between themselves and with other molecules they produce new compounds that continue chemical reactions based on chain principle - similar to neutrons in chain nuclear reactions. In many cases such processes are the main reason of pathologic condition of living systems [1]. 

A characteristic property of thiirane 1,1-dioxides is the ease with which such molecules fragment into sulfur dioxide and the related olefin on standing for several hours at room temperature. The title compound is no exception however, the rate of decomposition may be reduced substantially by storage under an inert atmosphere in a freezing compartment (ca. —5°). Under such conditions the product may be kept for many months. 

FIG. 5.6 The potassium chlorate/sulfur system. Sulfur endo-therms are seen near 105° and 119°C, as expected. A violent exothermic reaction is observed below 150 C. The ignition temperature is approximately 200 degrees below the melting point of the oxidi2er (KCIO3 m.p. = 356°C). Ignition occurs near the temperature at which S 3 molecules fragment into smaller units. 

Refining Catalyst Most catalysts used in the refining industry are solid heterogeneous catalysts. The chemical reactions they enhance would not proceed at all, or would do so quite slowly, in the absence of the catalyst. Reactions of this nature are believed to occur due to a dramatic disruption of the existing chemical bonds of an absorbed molecule. Molecules or molecule fragments may enter into reactions much different from those which occur in uncatalyzed reactions. 

The standard against which all peak intensities in a given mass spectrogram are measured is the most intense peak, called the base peak, which is arbitrarily assigned the value 100. If few parent molecules fragment—not a typical situation—the parent cation will furnish the base peak. 

Figure 15.3. (a) Orbital interaction diagram for the formation of a bond between molecule fragments a and b. (b) Orbitals for Dauben-Salem-Turro analysis of the rupture of the a—b bond left-hand side, orbitals of the bond with other occupied and unoccupied MOs of the reactant (molecule a—b) right-hand side, configurations which arise from the orbitals which made up the bond (other orbitals of fragments a and b not shown). 

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