Stable ions

Another approach to mass analysis is based on stable ion trajectories in quadnipole fields. The two most prominent members of this family of mass spectrometers are the quadnipole mass filter and the quadnipole ion trap. Quadnipole mass filters are one of the most connnon mass spectrometers, being extensively used as detectors in analytical instnunents, especially gas clnomatographs. The quadnipole ion trap (which also goes by the name quadnipole ion store, QUISTOR , Paul trap, or just ion trap) is fairly new to the physical chemistry laboratory. Its early development was due to its use as an inexpensive alternative to tandem magnetic sector and quadnipole filter instnunents for analytical analysis. It has, however, staned to be used more in die chemical physics and physical chemistry domains, and so it will be described in some detail in this section. 

Over a decade of research, we were able to show that practically all conceivable carbocations could be prepared under what became known as stable ion conditions using various very strong acid systems (see discussion of superacids) and low nucleophilicity solvents (SO2, SO2CIF, SO2F2, etc.). A variety of precursors could be used under appropriate conditions, as shown, for example, in the preparation of the methylcyclopentyl cation. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Under superacidic, low nucleophilicity so-called stable ion conditions, developing electron-deficient carbocations do not find reactive external nucleophiles to react with thus they stay persistent in solution stabilized by internal neighboring group interactions. 

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

Passage through the quadmpole assembly is described as stable motion, while those trajectories that lead ions to strike the poles is called unstable motion. From mathematical solutions to the equations of motion for the ions, based on Equation 25.1, two factors (a and q Equation 25.2) emerge as being important in defining regions of stable ion trajectory. 

For small values of a and q, the shaded area in Figure 25.4 indicates an area of stable ion motion it shows all values for a and q for which ions can be transmitted through the quadmpole assembly. 

Relationship between a and q. The shaded area indicates regions of stable ion motion through the quadrupolar field. 

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 alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

The 2-butyl cation can be observed under stable-ion conditions. The NMR spectrum corresponds to a symmetrical species, which implies either very rapid hydride shift or a symmetrical H-bridged structure. 

Hydride-bridged ions of this type are evidently quite stable in favorable cases and can be observed under stable-ion conditions. The Itydride-bridged cyclooctyl and cyclononyl cations can be observed at — 150°C but rearrange, even at that temperature, to methylcy-... 

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

The case for the generality of the o-complex mechanism is further strengthened by numerous studies showing that benzenium ions (an alternative name for the o-complex) can exist as stable entities under suitable conditions. Substituted benzenium ions can be observed by NMR techniques under stable-ion conditions. They are formed by protonation of the aromatic substrate ... 

The first step in the addition of an electrophile such as HBr to an alkyne involves protonation and subsequent formation of an intermediate vinyl cation. Where does propyne protonate Compare energies of 1-methylvinyl and 2-methylvinyl cations. Which is more stable Why Measure CC bond distance in the more stable cation. Does the cation incorporate a full triple bond (as in propyne) or a double bond (as in propene). Examine atomic charges and electrostatic potential maps to locate the positive charge in the two cations. Is the more stable ion the one in which the charge is better delocalized Use the charges together with information about the ions geometry to draw Lewis structures (or a series of Lewis structures) for 1-methylvinyl and 2-methylvinyl cations. 

In general, fragmentation in a mass spectrometer gives rise to the most stable ions. For unsaturated compounds resonance stablized ions may be possible. 

Conducting salts are usually sodium, potassium or ammonium salts, or the acid of a stable ion already present from other ingredients. Apart from energy conservation, the higher the conductivity of a bath the better the distribution of thickness on complex shapes. 

The structures shown for ions, such as in Equations 7 and 8, are entirely speculative since the mass spectrum gives only relative abundance of the various m/e-groups formed. Chemical reasoning, meta-stable-ion peaks, peak shifts in the deuterated analogs, and published mass spectra of similar compounds are the bases for the proposed modes of fragmentation and for the postulated structures.)... 

Experiment shows that a gaseous fluorine atom can acquire an electron to form a stable ion, F (g). We can discuss the energy of formation of this ion in the same way that we treated ionization energies. The first ionization energy of fluorine atom is the energy required to remove an electron from a neutral atom in the gas phase. We shall call this energy Ei. Then the heat of reaction can be written in terms of Ei. 

The most striking feature of the contrasts shown in Table 23-II is that the seventh-row elements display the multiplicity of oxidation states characteristic of transition elements rather than the drab chemistry of the +3 rare earth ions. Whereas Ce+3(aq) can be oxidized to Ce+4(aq) only with an extremely strong oxidizing agent, Th+Yaq) is the stable ion found in thorium salts and Th+3(aq) is unknown. In a similar... 

The separator is frequently a sintered glass frit, but it may also be any of a wide range of inert, porous materials such as celloton, vycor or porvic or an ion exchange membrane. A number of stable ion exchange membranes suitable for use in aqueous and non-aqueous solvents have become available in recent years. 

Several phenonium ions have been prepared as stable ions in solution where they can be studied by NMR, among them are 45, 46,and the unsubstituted 44. These were prepared by the method shown for 44 treatment of the corresponding P-arylethyl chloride with SbFs—SO2 at low temperatures. These conditions are even more extreme than the solvolysis in... 

Sufficient stability of the hydrocarbon ions, as the salt or in the solution, is an obvious prerequisite for these procedures, and, in practice, selecting or designing the stable ions and choosing a proper solvent are tasks of primary importance. As an ordinary stability index for the ions, thermodynamic scales referred to the water molecule, i.e. p CR+ and pKa values, are chosen for the carbocation and carbanion, respectively. 

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