By Ion-Molecule Reactions

Reaction +H2 NH + H. The nearly thermoneutral hydrogen-atom abstraction reac- 

The rate constants of reaction (1) were measured at 300 K using different techniques, and values (in 10 ° cm -molecule s ) of ki =5.6 (flowing afterglow reaction tube) [18], 4.8di0.2 (ion cyclotron resonance) [19, 20], and 4.8 (SIFT = selected-ion flow tube) [21] were obtained (the values ki =6.4 [22] and 6.2 [23] presumably include other reaction channels see also [1]). 

The results obtained from SIFDT [24], guided Ion beam [27], Ion trap [28, 29], and CRESU [31] experiments are compiled in Table 4, p. 130. Fig. 3, p. 130, shows Arrhenius-type plots of In k versus KE J obtained from the SIFDT experiments at 300 K [24]. 

Reaction of with H2S. A minor product channel for the reaction of N Ions with H2S is the formation of NH +SH with a branching ratio of 3% compared to the formation of H2S + N, SH +NH, and S + NH2 with branching ratios of 56, 29, and 12%, respectively measurements with a selected-ion flow tube (SIFT) at 300 K gave a total rate constant of 1.9x 10 cm -molecule [1, 21]. 

Rate constants kj at T = 300 K (thermalized reactants) the AHj values measured at 300 H  

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In addition to the effects discussed above, two further possible sources of discrimination peculiar to ion-molecule reactions must be considered. First, although it is known that most primary ions are formed without kinetic energy, such may not be the case for ions produced by ion molecule reactions. Secondary ions formed in exothermic ion-molecule reactions could retain a considerable fraction of the exo-thermicity as kinetic energy and diffuse from the sampling region at a rate considerably greater than predicted from the ambient temperature. The limited evidence to date (40) indicates that the kinetic energy of the product ions is small, but this may not be true for all types of reactions. 

Origin of Ions in Hydrocarbon Flames. Many ions, both positive and negative, are observed in hydrocarbon flames studied by mass spectrometric methods (9, 14, 26). Most of these are produced by ion-molecule reactions following the formation of primary ions from neutral species. 

The filament operates in the same way as a filament in chemical ionization by generating reactive species from solvent molecules in the high-pressure region of the source. These ionize the analyte by ion-molecule reactions (see Section 3.2.2 above). The discharge electrode, which may also provide more stable conditions when the mobile phase contains a very high proportion of water, provides the electrons required to generate the reactive species by means of a continuous gas discharge. 

In APCI, droplets are generated by a combination of heat and a nebulizing gas. While the analytes are embedded in a droplet, and thus protected to some extent from the heat, many thermally labile materials are decomposed. In addition, ionization occurs mainly by ion-molecule reactions and yields predominantly singly charged ions. If, therefore, compounds do not undergo thermal degradation, a mass spectrometer with extended mass range would be required to detect any ions formed. 

Atmospheric-pressure chemical ionization (APCI) An ionization memod in which a liquid stream is passed through a heated capillary and a concentric flow of a nebulizing gas. Ions are formed by ion-molecule reactions between me analyte and species derived from me HPLC mobile phase. 

We use laser photofragment spectroscopy to study the vibrational and electronic spectroscopy of ions. Our photofragment spectrometer is shown schematically in Eig. 2. Ions are formed by laser ablation of a metal rod, followed by ion molecule reactions, cool in a supersonic expansion and are accelerated into a dual TOE mass spectrometer. When they reach the reflectron, the mass-selected ions of interest are irradiated using one or more lasers operating in the infrared (IR), visible, or UV. Ions that absorb light can photodissociate, producing fragment ions that are mass analyzed and detected. Each of these steps will be discussed in more detail below, with particular emphasis on the ions of interest. 

GC-IMR-MS is based on gentle ionisation of gas molecules by ion-molecule reactions (IMR) [242]. Such reactions between reaction ions and sample gases produce a significantly smaller excess of energy than does electron impact ionisation. Thus, IMR provides a... 

A second successful prediction is that many so-called metastable species (i.e. isomers) are abundant even if they are quite reactive in the laboratory.66 Perhaps the simplest interstellar molecule in this class is HNC, but large numbers of others can be seen in Table 1. It is assumed that most metastable species are formed in dissociative recombination reactions along with their stable counterparts at approximately equal rates, and that both are destroyed by ion-molecule reactions so that the laboratory reactivity, which is normally determined by reactions with neutral species, is irrelevant. Both HCN and HNC, for example, are thought to derive from the dissociative recombination reaction involving a linear precursor ion ... 

Figure 7. Schematic diagram of a flowing-afterglow electron-ion experiment. The diameter of flow tubes is typically 5 to 10 cm and the length is 1 to 2 meters. The carrier gas (helium) enters through the discharge and flows with a velocity of 50 to 100 m/s towards the downstream end of the tube where it exits into a fast pump. Recombination occurs mainly in the region 10 to 20 cm downstream from the movable reagent inlet, at which the ions under study are produced by ion-molecule reactions. The Langmuir probe measures the variation of the electron density in that region. A differentially pumped mass spectrometer is used to determine which ion species are present in the plasma.

We are being somewhat disingenuous here. If performed and interpreted correctly and with the appropriate ancillary phase-change enthalpy information, the enthalpy of formation of an arbitrary species by ion-molecule reaction chemistry and by combustion calorimetry must be the same. That the ionization potential of quinuclidine is higher than l,4-diazabicyclo[2.2.2]octane simply says that there is a stabilizing effect in the radical cation of the latter not found in the former. This information does not say that there is a stabilizing effect in the neutral molecular form of the latter not found in the former. After all, we trust the reader is not bothered by the fact that the ionization potential order of the cyclohexenes increases in the order 1,3-diene < 1,4-diene < 1-ene < 1,3,5-triene (benzene). 

Compound-specific isotope analysis (CSIA) by GC-IRMS became possible in 1978 due to work of Mathews and Hayes [634], based on earlier low-precision work of Sano et al. [635]. The key innovation was the development of a catalytic combustion furnace based on Pt with CuO as oxygen source, placed between the GC exit and the mass spectrometer. The high pressure of helium (99.999% purity or better) ensures that all gas flows are viscous. After being dried in special traps avoiding formation of HC02 (i. e., interferes with 13C02) by ion-molecule reactions in the ion source, the C02 is transmitted to a device that regulates pressure and flow and then into the ion source [604]. 

Definition A distonic ion is a positive radical ion, which would formally arise by ionization of a zwitterion or a diradical, by isomerization or fragmentation of a classical molecular ion, or by ion-molecule reactions. Consequently, distonic ions have charge and radical at separate atoms in a conventional valence bond description. [42,43]... 

C. K. Gase-Phase Halo Alkylation of C6o-Fullerene by Ion-Molecule Reaction Under Chemical Ionization. J. Am. Soc. Mass Spectrom. 1993,4, 894-897. 

The heat of formation of [MolCOlg] has been determined as -960 + 12 kJ mol by measuring its heat of decomposition. The Mossbauer parameters for the 100 keV transition of in [W(CO)g] and some tungsten(vi) complexes have been measured and discussed in terms of known bonding and structure. Secondary ions [M (CO) ] (M = Mo, m = I or 2 M = W, m = 1—4 n = 0—14) formed by ion-molecule reactions have been observed in the mass spectra of the hexacarbonyls. A mixt u re of [Cr(CO) ] and [MolCO) ] vapours affords [CrMo(CO) ] ( = 5—7). [MofCOl ] and [WICO) ] catalyse the condensation of isocyanates with aldehydes to give imines in high yields. ... 

Two mechanisms have been proposed to account for the deuterium enrichment (1) for organic molecules, high D/H ratios can be explained by ion molecule reactions that occur in interstellar space and (2) for the phyllosilicates the enrichment can be produced via isotope exchange between water and hydrogen (Robert et al. 2000). 

As well as the PH3 ion, the ions PH, PH and P, caused by fragmentation are observed. Furthermore the signal for the phosphonium ion, formed according to Eq. (5), is seen. Many other, heavier ions are the products of ion molecule reactions in PH3. These have the general formulae P2H (n = 0 - 5), PjH (n = 0 - 2), and Pj. Analogous ions were also formed by ion molecule reactions in ammonia. The reactions listed in Table 3 were identified with the help of the ion cyclotron double resonance technique. 

Reiner, T., M. Hanke, and F. Arnold, Atmospheric Peroxy Radical Measurements by Ion Molecule Reaction-Mass Spectrometry A Novel Analytical Method Using Amplifying Chemical Conversion to Sulfuric Acid, J. Geophys. Res., 102, 1311-1326 (1997). 

Tiedemann and Riveros (1974) first discussed the gas-phase reactions which are equivalent to an esterification reaction. An icr study of alcohols and acetic acid revealed that the formation of protonated acetic acid by ion-molecule reactions of fragment ions with acetic acid is followed by the rapid reaction (72). 

Herbst, E. (2003) Isotopic fractionation by ion-molecule reactions. Space Science Reviews, 106, 293-304. 

Reaction 4 is favored by the strong electron affinity of nitroethylene (30). The carbanion may be formed by ion-molecule reaction between the anion radicals and the nitroethlene molecules (reaction 5), to which the latter add successively, and polymerization proceeds by anionic propagation (reaction 6)... 

Doubly charged ions of 1-heptene were observed in its field ionization mass spectrum by Tecon, Stahl and Gaumann267. By using 13C-labeled compounds they were able to prove that these ions are formed in an absorption process on the emitter surface, partly by ion-molecule reactions. 

Amides. Primary amides exhibit behaviour similar to the corresponding acid and methyl esters substituted amides resemble the higher alkyl esters. There is a common tendency to form M + 1 ions by ion-molecule reactions. Primary amides generally give a strong peak at m/z 44 ... 

If, on the other hand, the excitation energy of the metastable is lower than the dissociation energy of the species AB, the latter can only be decomposed either by dissociative de-excitation of short-lived electronically excited states, or by ion-molecule reactions, or by electron impact. Much higher plasma energy values are necessary to produce a high rate of decomposition of the species AB in such systems28,44 (e.g. C/02,TiN/Cl2). 

The selected primary ions can also be characterized by ion molecule reactions with the FTMS-2000 dual cell and/or surface induced reactions (48). Reactions specific for particular functionalities can be used to count the number of these in a primary ion such as the number of hydrogen atoms that can be exchanged with a deuterium-containing species (59). Further, their position may well be distinguished by the reaction s effect on fragment ions in MS-II spectra. 

An ion of nominal m/z 442 was produced from cis-dichloro-trans-dihydroxo-bis-2-propanamine/platinum (IV) (CHIP) by ion/molecule reactions (IMR) of the major electron impact (El) fragment ion with the neutral molecule. As shown in Figure 3, this ion dissociated when irradiated by the cw laser for ca. 500 ms to produce two daughter photofragment ions, m/z 366 and m/z 311, by loss of various ligands (Equation 1). 

Shown in Figure 4 is the photodissociation of protonated N,N -bis(4,6-dimethoxysalicylidene)-4-trifluoromethyl-o-phenylenediimin-ato cobalt (II) (CoSALOPH), which is also produced by ion/molecule reactions of electron impact fragments with the neutral molecule. This ion dissociated by loss of a methyl group (Equation 2). 

A heated pneumatic nebuliser is used to produce the aerosol in APCI and the ions are produced by ion-molecule reactions initiated by corona discharges in the ion source region. White et al. (1998) found atmospheric pressure ionisation MS and LC-ICP-MS to be complementary techniques. 

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