Fission energetics

In the previous sections, we showed that consideration of triaxial (ellipsoidal) shapes in the framework of the SCM leads to overall substantial systematic improvement in the agreement between theory and experimental observations pertaining to the major and the fine structure of the size-evolutionary patterns associated with the energetics of fragmentation processes (monomer/dimer dissociation energies and fission energetics) and ionization. 

Note that in contrast with normal paraffins (Equation 10), beta fission at the branch point is exothermic, and the reaction is energetically allowed. A similar exothermic reaction can be written using C2H5 + as the reactant ion, and two other reactions equivalent to Reaction 12 can be written—namely, those with the charge in the initially formed C2i+ ion on the other two branches of the molecule. One of these other reactions will also produce a Ci4 + ion, but the other will produce a Ci5 + ion. 

In effect we postulate that the olefin ion is formed by a 1-3 hydride ion shift accompanied by a beta homolytic bond fission. The fact that olefin ions are formed only at branch points (except methyl branch points) could be explained on an energetic basis if it were not for the contrary fact that the over-all energetics are highly unfavorable. Thus in Reaction 20 we see that a disubstituted olefin ion is formed, and this will be true for any branch other than a methyl branch. Thus ... 

Homolytic fission of an R3C—X bond is, in the gas phase, always less energy-demanding than heterolytic fission. This energetic advantage is, however, often reversed in polar solvents, because of the energy then developed—in heterolytic fission—from solvation of the developing ions. 

Lewis et al.106 calculated four possible decomposition pathways of the ot-HMX polymorph N-N02 bond dissociation, HONO elimination, C-N bond scission, and concerted ring fission. Based on energetics, it was determined that N-N02 dissociation was the initial mechanism of decomposition in the gas phase, whereas they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The more recent study of Chakraborty et al.42 using density functional theory (DFT), reported detailed decomposition pathways of p-HMX, which is the stable polymorph at room temperature. It was concluded that consecutive HONO elimination (4 HONO) and subsequent decomposition into HCN, OH, and NO are the most energetically favorable pathways in the gas phase. The results also showed that the formation of CH20 and N20 could occur preferably from secondary decomposition of methylenenitramine. 

The observed survival probability depends not only on the survival probability of an ion-pair formed with an initial separation, r0, but also on the distribution of initial separations, w(r0). As will be discussed in Sects. 3 and 4, the rate of loss of energy of ions after heterolytic bond fission (or, alternatively, the range of electrons formed by ionisation of a molecule) depends sensitively on the energetics of the solvent molecules. These separation distances can range up to 10 nm or more and are specifically discussed in Sect. 3. The survival probability of a collection of isolated ion-pairs, P(t), formed at a time t0 — 0, and with a distribution of initial distances w(r0) is... 

The relatively small bond specific energy for heavy nuclei makes the process of the fission of these nuclei into fragments energetically favourable. The most energetically favourable and thus the most widespread process is decay with a-particle emission X — 1VX + jHe... 

Each observed decay sequence involved the observation of three energetic a-particle decays followed by a spontaneous fission. 

A problem with all mass spectroscopy of large molecules is how to get them into the vapor phase so that they can be ionized and their fragmentation patterns determined. Simple heating may cause excessive degradation and formation of ions not corresponding to the desired substance. Two useful methods that involve only intense short-term local heating of the sample appear to have promise in this connection. One method uses a burst from a powerful infrared laser to volatilize part of the sample, and the other uses bombardment by heavy and energetic particles from fission of californium-252 nuclei to raise the local temperature of the sample to about 10,000°. The latter technique both volatilizes and ionizes the sample molecules. 

The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9) ultrahigh mass resolution (>1,000,000 at m/z. < 200) with accurate mass measurement even 1n gas chromatography/mass spectrometry experiments sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers versatile Ion sources (electron impact (El), self-chemical ionization (self-Cl), laser desorption (LD), secondary ionization (e.g., Cs+-bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission) trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. 

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