Ion clustering

Laser-desorption mass spectrometry (LDMS) or matrix-assisted laser desorption ionization (MALDI) coupled to a time-of-flight analyzer produces protonated or deprotonated molecular ion clusters for peptides and proteins up to masses of several thousand. 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

FIG. 9 Example of a two-ion cluster the cluster contains sodium and chloride ions surrounded by water molecules to form a two ion cluster. The bottom cluster shows the sodium ion more clearly, after some water molecules in front of it have been removed [25]. 

Harden T.C. Imeson, Detection and Identification of Trace Quantities of Organic Vapors in the Atmosphere by Ion Cluster Mass Spectrometry and the Ionization Detector System ,... 

From the earliest days of radiation chemistry it has been known that acetylene polymerizes to a cuprene-like ( alprene ) solid (5, 6,25,28). The characteristics of the polymerization—e.g., lack of effect of temperature, doso rate, and pressure on polymer yield and negligible effect of radical scavengers—led Lind (24) to postulate an ion cluster mechanism. 

The presence of ions in an otherwise organic matrix is not thermodynamically stable. As a result these materials undergo slight phase separation in which the ions cluster together in aggregates. These ionic clusters are quite... 

In addition to the formation of these ions of direct analytical utility, APCI leads to the formation of ion clusters involving solvent molecules. Since these tend to make interpretation more difficult, they need to be removed and this may be accomplished either by the use of a curtain gas or by cone-voltage fragmentation (see Section 4.7.4 above) which is also applicable to APCI. 

Cluster Metsd ion Cluster fragment Ground state Ref. 

Table I. The electrostatic energies of the central silicon site and one nearest-neighbor oxygen site for a 977 point-ion cluster, and the Madelung energies for silicon and oxygen in alpha-quartz...

Calculated Electrostatic Energy of the Site(eV) Ewald Summation 977 Pt. Ion Cluster... 

Table II. Differences in Mulliken population analyses for an Si50i5Hi2 molecule and four combinations of point-ion cluster, using a 3-21G basis set...

Figure 2. Negative FAB mass spectra of maitotoxin. The numbers denote the mass number at the centroid of each peak. A A survey scan at a low resolution (R=300). B. Resolution enhanced spectrum (R=3000) for ion clusters at around m/z 3300. C. Resolution enhanced spectrum (R=3000) for ion clusters at around miz 3400.

Fic. ],—Schcinatic R< preseiU8lion of (he Appearainx of Typical, MoIccular-ion Cluster, ((ul Near mass SOO, Ih) near mass 1300, anJ tc) near mass 3000.]... 

Fig. 6.—One of the Molecular-ion Clusters Obtained from a Sample of Deuteropermethy-lated. Cyclic /3(l- 2)-Glucans (see Section VI,S). [All high-mass samples give unresolved clusters of this type if the mass spectrometer is operated at low resolution. The peak is 6 mass units wide at half height. The mass is assigned by using the mass marker, which gives marks every 4 mass units, as shown. The center of the peak corresponds to the chemical molecular weight of an [M + NKJ species.]...

A possible explanation comes from X-ray analyses of the sulfonic acids [45]. All X-rayed crown ether crystals contained water and the sulfonic acid moiety was dissociated. Therefore in crystals of [45], macrocyclic ben-zenesulfonate anions and hydronium ions (sometimes hydrated) are present. The ions are bound to each other by hydrogen bonds. The size of the included water-hydronium ion cluster (varying by the number of solvating water molecules) depends on the ring size. In the 15-membered ring, HsO" was found, whereas in a 21-membered ring HsO and in the 27-membered ring were present. This means the sulfonic acid functions in [45] are... 

Just as each element has a characteristic molar mass, so does every chemical compound. Chemical compounds are composed of atoms bound together into molecules or ions clustered together in electrically neutral aggregates. In either case a chemical formula describes the atomic composition of a compound. 

The ion clusters (Si2H+, Si.sH, ,...) are also present, and larger clusters (positive and neutral) can be formed through reactions with silane molecules. Negative ions have been detected [321], which are thought responsible for the powders in the discharge [322]. 

Caldwell J, Dang LX, Kollman PA (1990) Implementation of nonadditive intermolecular potentials by use of molecular-dynamics - development of a water water potential and water ion cluster interactions. J Am Chem Soc 112(25) 9144—9147... 

Figure 1. Fast-flow reaction apparatus. Ions or ion clusters are introduced into the flow tube from various sources and reactions proceed after they encounter the reactants added through a ring injector located at a selected position in the flow tube. The disappearance of the reactant ions and formation of products is monitored with the quadrupole mass spectrometer/electron multiplier shown. Taken with permission from ref. 19.

Figure 6. Possible intensity profiles for two mechanisms for the formation of ion clusters (a) Ionization through AID mechanism, (b) Ionization through ADI mechanism. The signal would persist for long times due to the lifetime of the NH4 in the cluster, and its ensuing ionization, (c) Ionization through both AID and ADI mechanisms. Taken with permission from ref. 65.

The operative mechanisms have been deduced by considering various peaks identified in the daughter spectrum employing the reflectron technique. One corresponds to a mass loss of 32 amu from a parent ion cluster that enters the drift region as H+(CH3OH) via ... 

Table 2 gives a summary of results70,71 which provide the solvent number r at which simple solvent loss and charge reduction become competitive. Charge reduction becomes dominant for solvent numbers lower than r. Ion clusters for... 

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