Simple bond cleavage

Duffield and coworkers65 studied the El- induced mass spectra of five arene- (215-219) and four alkane sulfonylthioureas (220-223) and observed two rearrangement processes, namely loss of S02 from 215-219 and the elimination of ArS02 and RS02 with the thione sulfur atom from 215-223. The other fragmentations involved simple bond cleavages with and without hydrogen transfer (equation 48). The loss of H2S was evident for all the compounds studied except 221 and 222. It was, however, found to be a thermal and not an ionization process. 

Figure 5.13. The variation of the potential energy along the reaction coordinate for the simple bond cleavage (left) and rearrangement (right).

Note The validity of Stevenson s rule requires no reverse activation energy barrier to exist for the fragmentation pathway. This requirement is usually fulfilled for simple bond cleavages, but not in case of rearrangement fragmentations. 

Example The El mass spectmm of butanal mainly shows carbenium fragment ions due to simple bond cleavage that can be easily recognized from their odd-numbered m/z values. However, the base peak is represented by a [M-28] ion, m/z 44, obviously resulting from rearrangement. A closer look reveals that the charge-migration product, m/z 28, is also present in the spectrum. 

Whatever the mechanism of CO loss might be, in none of the above cases CO loss proceeds by a simple bond cleavage. Instead, multistep rearrangements are necessary to cut the CO moiety out of the precursor ion. 

Table 1. TRPD Results Interpreted as Simple Bond Cleavages Yielding Direct Thermochemical Thresholds...

Phase space theory (PST) has been widely used for estimation of rates and energy partitioning for ion dissociations. It can be considered within the framework of transition-state theory as the limiting case of a loose transition state, in which all product degrees of freedom are statistically fully accessible at the transition state. As such, it is expected to give an upper limit for dissociation rates and to be best suited to barrierless dissociations involving reaction coordinates with simple bond cleavage character. 

Besides fragmentation that occurs by simple bond cleavage, ions can be formed through molecular rearrangement. These rearrangements can be simple, as in the... 

Simple bond cleavages can proceed with the shared pair ultimately residing on either of the previously joined elements. In either case one atom has a sextet... 

Primary hydrogen isotope effects on simple bond cleavage... 

In a recently published book [1280] on mass spectrometry, the fragmentation of organic molecules under electron impact, electron capture, and other ionization methods is described not in the traditional way - by classes - but rather by isomerization and fragmentation types with simple bond cleavage, hydrogen- and skeletal rearrangements with systematic thermodynamic approach and separately for positive and negative ions. Our review on mass spectrometry of nitroazoles has been reported in 1998 [1281],... 

Bulk controlled-potential electrolysis experiments have shown that most of the (O) ester anion radicals decay via a simple bond cleavage mechanism to form car-boxylate anions in very high yield, while the (S) ester radicals decay via a very complicated mechanism often involving aromatic substitution reactions [442], Using CV, many of the compounds are shown to display chemically (and electrochemi-cally) reversible behavior at slow scan rates, in the sense that the ratios... 

Secondary-ion mass spectrometry (SIMS) of a thin layer of nucleic acid bases deposited on a silver foil under bombardment with Ar ions at 3 kV gives intense pseudomolecular ions [M H] but practically no simple bond cleavage fragments. Another new technique is that of (pulsed) laser induced desorption (LD). When applied to nucleotide bases such as cytosine or adenine (266 nm, quadruplet neodymium laser or 347 nm, ruby laser) the technique has good detection limits, particularly for ions with a short lifetime (up to 100 nsec). The technique makes use of a time-of-flight instrument and is utilized in both modes, positive (PI) and negative ions (NI). Both bases exhibit an intense [BH]" ion. These results are similar to those obtained by Cf plasma desorption (PD). 

Fig. lb). For rearrangement reactions to compete with simple bond cleavages they must have low E... 

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