Neutral Fragment Stability

The loss of a stable molecule is always a favorable fragmentation. Examples are water (H2O, 18 Daltons), ethylene (C2H4, 28 Daltons), carbon dioxide (CO2, 44 Daltons) and acetic acid (C2H4O2,60 Daltons). These and other stable neutral fragments will be seen in many of the spectra that follow in this book. 

Cleavage at branched positions is the favored process of hydrocarbons and in this case leads to three of the most intense peaks in the spectrum, miz 43, 57, and 113. These secondary carbonium ions are primary cleavage products of the molecular ion. The fourth intense peak in this spectrum, at mk 71, is a typical example of a secondary cleavage ion. Elimination of a stable alkene molecule provides the driving force for this fragmentation. Note that no structures are drawn since it is likely that considerable rearrangement occurs in these hydrocarbon ions. 

Isolated double bonds in a hydrocarbon provide a driving force for the preferential cleavage of a carbon-carbon bond beta to the unsaturation in only certain cases. Normally, rapid rearrangement of hydrogens within the molecular ions of olefins occurs faster than allylic cleavage thus, there is actually a mixture of molecular 

Although in most cases the structures of ions are only speculative, and drawn merely to explain significant fragmentation reactions, the unusual intensity of the C7H7+ ion in many spectra has aroused the interest of several workers. 

The remainder of this book presents both types of ions without distinction since either representation leads to the same functional gronp in the molecular structure for which we will be searching. 

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Figure Bl.7.7. Summary of the other collision based experiments possible with magnetic sector instruments (a) collision-mduced dissociation ionization (CIDI) records the CID mass spectrum of the neutral fragments accompanying imimolecular dissociation (b) charge stripping (CS) of the incident ion beam can be observed (c) charge reversal (CR) requires the ESA polarity to be opposite that of the magnet (d) neutiiralization-reionization (NR) probes the stability of transient neutrals fonned when ions are neutralized by collisions in the first collision cell. Neutrals surviving to be collisionally reionized in the second cell are recorded as recovery ions in the NR mass spectrum.

The most accepted among the qualitative theories of mass spectral fragmentation are the conception of charge and unpaired electron localization and the estimation of ions and neutral particles stability. Despite their qualitative character these approaches are quite useful to work with mass spectra. Both theories use the principle of the minimal structural changes at each stage of fragmentation, while the structure of the molecular ion is considered to be the same as that of the initial molecule. Certain isomerization processes of M+ before the fragmentation are usually a matter of special study. 

The fragmentation of 2-phenyl-1,3,4-thiadiazolin-5-one and -5-thione begins with the breaking of the C—N bond followed by the ejection of the S=C=Y molecule (Y = O or S). The major part of AH°R is associated with the stabilization of the neutral fragment (810MS29). 

This cleavage is named after Fred W. McLafferty (United States 1923-). Shown is the McLafferty rearrangement for 9, where ethene (ethylene, with a mass of 28) is lost and ion 10 is formed. This ion is resonance stabilized, as shown, which accounts for the facility of the McLafferty rearrangement. Note that loss of a neutral fragment from the molecular ion generally leads to an even-mass ion such as 10, whereas loss of a radical cation daughter ion usually leads to an odd-mass ion such as 11 or 12. 

Postulate a structure by assembling the various mass fragments/neutral losses. Do the observed fragment ions make sense in terms of fragment/ neutral loss stability considerations Does the structure make sense in... 

In addition to fragmentation by the McLafferty rearrangement, aldehydes and ketones also undergo cleavage of the bond between the carbonyl group and the a carbon, a so-called a cleavage. Alpha cleavage yields a neutral radical and a resonance-stabilized acyl cation. 

It turns out that in solutions of c < 0.1 gL 1 thermosensitive homopolymers, such as PNIPAM, PVCL, and PVME, themselves, form stable colloids in water at elevated temperature in the absence of additives or chemical modification [141-147]. The colloids remain stable upon prolonged heat treatment, without detectable aggregation or precipitation. Also, core-shell particles consisting of PNIPAM and a hydrophobic block are stable not only below but also above the LCST up to 50 °C, when the PNIPAM block is expected to be insoluble [185]. Factors that determine the colloidal stability as defined in Sect. 1.1 do not explain, it seems, their stability. In this review we have compiled a fist of all the reported instances where the formation of stable particles was detected in aqueous solutions of neutral thermosensitive neutral polymers at elevated temperature. We present studies of homopolymers, as well as their copolymers consisting of thermosensitive fragments and ei-... 

It is known that in the vast majority of cases the activation energy E,. of the reverse reaction is very small or even negligible. Using Hammond s postulate [3], it is possible to assume that in the case of endothermic fragmentation the transition state will be much closer to the products than to the initial particle (Fig. 5.14). Thus, the stability of the products influences significantly the efficiency of fragmentation. It is important to consider stability of both products a neutral and a daughter ion. 

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