Fragmentation rates

B/ Fragmentation rate g/s Ib/s (AL/Ll Critical agglomerate deformation strain ... 

SNMS sensitivity depends on the efficiency of the ionization process. SNs are post-ionized (to SN" ) either hy electron impact (El) with electrons from a hroad electron (e-)heam or a high-frequency (HF-) plasma (i.e. an e-gas), or, most efficiently, hy photons from a laser. In particular, the photoionization process enables adjustment of the fragmentation rate of sputtered molecules by varying the laser intensity, pulse width, and/or wavelength. 

Advection is important in fragmentation processes, and an initially homogeneous system may evolve spatial variations due to spatially dependent fragmentation rates. For example, Fig. 36 shows the spatial distribution of eroded clusters in the journal bearing flow operating under good mixing... 

Table 3.3. Fragmentation Rate Constants and Standard Potentials for Aryl Chlorides Anion Radicals in DMF...

FIGURE 3.19. Correlation between the fragmentation rate constant (in s-1) and the standard potential and the fragmentation standard free energy (in V vs. SCE) (top) and activationdriving force relationship (free enthalpies in eV) (bottom) for aryl chloride anion radicals. Data from Table 33. Adapted from Figure 3 of reference 30, with permission from the American Chemical Society. 

With systems such as those represented in Figure 3.22a, one therefore expects that the log of the fragmentation rate constant varies with the driving force with a slope of 1/0.059 eV, at 25°C, within a large portion of the accessible domain of driving forces. 

The electron-transfer mechanism for electrophilic aromatic nitration as presented in Scheme 19 is consistent with the CIDNP observation in related systems, in which the life-time of the radical pair [cf. (87)] is of particular concern (Kaptein, 1975 Clemens et al., 1984, 1985 Keumi et al., 1988 Morkovnik, 1988 Olah et al., 1989 Johnston et al., 1991 Ridd, 1991 Rudakov and Lobachev, 1991). As such, other types of experimental evidence for aromatic cation radicals as intermediates in electrophilic aromatic nitration are to be found only when there is significant competition from rate processes on the timescale of r<10 los. For example, the characteristic C-C bond scission of labile cation radicals is observed only during the electrophilic nitration of aromatic donors such as the dianthracenes and bicumene analogues which produce ArH+- with fragmentation rates of kf> 1010s-1 (Kim et al., 1992a,b). 

Example Secondary kinetic isotope effects on the a-cleavage of tertiary amine molecular ions occurred after deuterium labeling both adjacent to and remote from the bond cleaved (Chap. 6.2.5). They reduced the fragmentation rate relative to the nonlabeled chain by factors of 1.08-1.30 per D in case of metastable ion decompositions (Fig. 2.18), but the isotope effect vanished for ion source processes. [78] With the aid of field ionization kinetic measurements the reversal of these kinetic isotope effects for short-lived ions (lO -lO" s) could be demonstrated, i.e., then the deuterated species decomposed slightly faster than their nonlabeled isoto-pomers (Fig. 2.17). [66,76]... 

Figure 21. Fragmentation rates of Na3 C state vibrational bands versus excitation energy [22]. Insert Highly resolved TPI (lower) and depletion spectrum (upper) of the Na3 C state...

Figure 26. Fragmentation rates /t of the observed mother molecules (type I) recorded at excitation energies of (a) 1.47 eV and (b) 2.94 and 2.00 eV [23],...

Mathivanan, Johnston and Wayner examined the effect of substituents on the rates of cleavage of a-phenoxyacetophenone radical anions (equation 6)43. In this study, the radical anions were generated by trapping solvated electrons produced by laser-induced photoionization of 4,4 -dimethoxystilbene in CH3CN and DMF. Their results show that the fragmentation rate is enhanced when Y is electron withdrawing, but retarded when X is electron withdrawing. 

The distinctive structure of CP (III), coupled with an absence of any published oxidation rate constants for it, requires that kox be estimated from additivity of fragment rate constants for the major structural units 3,5,6-trichloropyridinyl (TCP), diethoxyphospho-rothioate (DEP), and the phosphorothioate =P=S bond. 

The factors affecting the relative reactivity of aryl halides in SrnI reactions have been analysed and compared645. Competition experiments of pairs of substrates, in photo-stimulated reactions with pinacolone enolate ion in liquid ammonia, reveal a spread of reactivity exceeding three powers of ten. The ease of formation of the radical anion of the substrate appears to dominate the overall reactivity. The rate of dehalogenation of the radical anion may become important when its stability exceeds a certain threshold. When the fragmentation rate of the radical anion intermediate is fairly slow, the overall reactivity diminishes. 

The extensive determination of fragmentation rates of aryl halide radical anions, due to Saveant and coworkers15a by electrochemical methods, indicates that they range from values of 10-2s-1 for nitro-substituted phenyl halides up to 1010 s-1 for />-cyanophenyl halides. These values are in agreement with measurements by pulse radiolysis42. The fragmentation rates for unsubstituted phenyl halides are too high to be measured even by electrochemical techniques. Besides, 1-bromo- and 1-iodoanthraquinone radical anions have been shown to dissociate from their photoexcited state (Section V. D). 

The differences in the bond-fragmentation rates of the radical-anion intermediates of these reactions have been studied. The frangibility of the C—Z bond in the radical-anion intermediate was found to increase in the order Sphotostimulated reactions of 1-Adi with Se2"or Te2"in liquid ammonia are also known122. 

The different behaviour of these substrates indicates that in the radical anion (43) , the fragmentation rate to give 45 is faster than the ET to 42 for X = Br and I, while for X = Cl the intermolecular ET to give 43a (equation 49) is faster than the fragmentation reaction. 

The overall lower yield of phenols obtained with 4-chlorobenzonitrile vs 4-bromobenzophenone was ascribed to the faster fragmentation rate of the former radical anion (9.3 x 108 s 1 vs 590 s1 for 4-bromobenzophenone). This difficulty was overcome by redox... 

When 1-adamantyl phenyl tellurium was treated with sodium in liquid ammonia, the Te-adamantyl bond is preferentially cleaved, although the Te-phenyl bond is also attacked. The ratio of the fragmentation rates is approximately 132. 

The limited available data suggest that solvent has relatively small effect on the fragmentation rate, although the change in solvent may affect the relative magnitudes of entropy and enthalpy of activation [102], For those reasons the... 

---