Neutral fragments

For every positively charged fragment ion, at least one neutral fragment is also formed. These fragments are unaffected by the electric and magnetic fields of the mass spectrometer and therefore cannot be studied in the normal ways their structures and properties can only be ascertained by examining the ions with which they 

Fragmentation reactions are often promoted by the formation of certain stable neutral fragments or molecules. The most common of these stable species are shown in Table 2.5. They are discussed in greater detail in the next section. 

---

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.

Tabulate the probable groups that (a) give rise to the prominent charged ion peaks and (b) list the neutral fragments. 

The presence of Cl, Br, S, and Si can be deduced from the unusual isotopic abundance patterns of these elements. These elements can be traced through the positively charged fragments until the pattern disappears or changes due to the loss of one of these atoms to a neutral fragment. 

Characteristic Low-Mass Neutral Fragments from the Molecular Ion... 

Constant neutral loss (or fixed neutral fragment) scans. The linked scan at constant B[1 -(E/Eg)] /E gives a spectrum of all product (daughter) ions that have been formed by loss of a preselected neutral fragment from any precursor (parent) ions. 

Stable ion. An ion that is not sufficiently excited to dissociate into a daughter ion and associated neutral fragments, or to react further in the time frame of the mass spectrometric analysis under stated experimental conditions. 

The behaviour under electron impact of IV- and C-trimethylsilylpyrazoles (mono-, di-and tri-substituted) has been studied by Birkofer et al. (740MS 8)347). Loss of a methyl radical followed by loss of HCN is the most common fragmentation feature of these compounds. When more than one trimethylsilyl group is present, a neutral fragment CaHgSi is expelled. Mass spectrometry of pyrazolium salts has been studied by Larsen etal. (8i OMS377, 830MS52). 

The mass spectra of alcohols often completely lack a peak corresponding to the parent ion. This is due to extremely rapid loss of neutral fragments following initial ionization. For example, the mass spectrum of 2-methyl-2-butanol lacks a parent peak and contains strong peaks at M-15 (loss of CH3O and M-18 (loss of H2O). 

Examine fragment ions to determine the mass of the neutral fragments that were lost from the molecular ion, even though these high-mass peaks may be of low abundances. Compare the neutral loss from the molecular ion with the neutral losses tabulated in Part III to see if these losses agree with the suspected structural type. 

Neutral Loss Only a limited number of neutral fragments of low mass which are eliminated in decompositions of molecular ions. Examples are H, H2, CH3 and OH. Therefore, the presence of a major ion below the molecular ion at an improbable interval (eg, loss of 4 to 14, 21 to 25 amu) will indicate that the latter is not the molecular ion Postulation of Molecular Structures The. postulation of the structure of an unknown molecule is based on several major kinds of general structural information available in the mass spectmm. McLafferty (Ref 63) suggests the following systematic approach ... 

Comprehensive correlation tables of various ion series, characteristic fragment ion and neutral fragments from different classes of compounds are available in various sources (Refs 10, 11 63). A number of general rules in understanding fragmentation mechanisms and for prediction of prominent peaks in a spectrum are summarized in Refs 10 63... 

To integrate one must know ts, which of course is a function of tp, and the form of this function depends upon the mechanism assumed for Reaction P. At this point we restrict Reaction P to a hydrogen transfer reaction in which the transferred species may be either a proton, hydrogen atom, or hydride ion and for which the masses of the primary ion, the molecule, the secondary ion, and the neutral fragment are identical and large compared with the transferred hydrogen. Three situations must be considered where the type of collision is defined by the relationship between uP and vQ, the velocities of the primary and secondary ions ... 

Here, a primary ion P+ formed by the radiation field reacts with a gas molecule M to give an intermediate complex [PM +] which can either dissociate to a secondary species S + and a neutral fragment N or react with another molecule to produce another complex [PM2 + ]. The latter then dissociates into a tertiary ion T+ or propagates the chain by forming a third intermediate [PM3 + ]. A quaternary ion Q+ may result from dissociation of [PM3 + ], or the chain may continue through reaction of [PM3 + ]. Wexler and Jesse (38), on the other hand, have suggested a model which states that reactive intermediate complexes are not involved in the propagation, but rather the polymerization proceeds by chains of simple consecutive and competitive ion-molecule reactions,... 

The evaporative loss of neutral fragments proceeds following ionization, in a manner analogous to the processes discussed above. 

The adiabatic ionization potential (1A) of a molecule, as shown in Figure 4.1, equals the energy difference between the lowest vibrational level of the ground electronic state of the positive ion and that of the molecule. In practice, few cases would correspond to adiabatic ionization except those determined spectroscopically or obtained in a threshold process. Near threshold, there is a real difference between the photoabsorption and photoionization cross sections, meaning that much of the photoabsorption does not lead to ionization, but instead results in dissociation into neutral fragments. 

In CP2MXY complexes (M Ti, Zr, Hf), the ligands X and Y together donate a total of four electrons to the neutral fragment CP2M. When Y is a non-ff-donor, as CH3 in complex III, all ir bonding is provided by X(0R) this leads to maximum contraction of the Zr-0 distance. It was noted previously (3.) that the Ti-Cl distance in... 

---