Electron and ion cyclotron resonance studies

Ion cyclotron resonance (i.c.r.) spectrometry is a specialised technique of growing importance. It enables the detailed study of ion-molecule reactions (and their mechanisms) in the gas phase, giving both absolute and relative thermochemical quantities and rate constants. 

The rate constant for the capture of low energy (thermal 1 eV) electrons by COClj has been measured by electron cyclotron resonance (e.c.r.) as 5 x 10 cm s [1817]. Thus, phosgene is a much more effective electron scavenger than dichlorine or chloroalkanes. The process of electron capture is believed to be [1817]  

At 30-70 eV, three primary ions are formed by electron impact [1071,1720]  

The relative abundance of these ions is shown in Fig. 9.13. The most abundant ion is 

The secondary negative ion, [COClj] , is fonned in two parallel processes, the former being eight times faster than the latter [1071]  

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The cation-TT interaction involves binding of a cation to Ji-electrons (e.g., benzene, acetylene). Early mass spectrometry and ion cyclotron resonance studies have established that alkali-metal cations (e.g., Na+) bind strongly to simple aromatics. Later, Deakyne, and Meolner showed that organic ions (e.g., alkylammoniums) display affinities for aromatics. Moreover, extensive work by Dougherty and CO workers has established models of the cation- n- interaction that have been successfully demonstrated in a variety of synthetic receptors. ... 

This is not significantly different from the value of-91 kcal mol" derived by Sharpe from the data given by Bills and Cotton. It indicates that the value of-71 kcal mol", derived from Altshuller s measurements, is seriously in error. Beauchamp, in his ion cyclotron resonance study for the electron affinity of UF, used this latter value. With sub-... 

The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes... 

Rate Measurement. We have used pulsed ion cyclotron resonance (ICR) spectroscopy to study these gas-phase, ion-molecule reactions. The method has been described elsewhere in considerable detail (5). Basically, ions are generated by pulsed electron impact and held in a magnetic-electric field trap for times up to about 1 sec, during which they can react with a... 

Chemical ionization is, as might be expected from its name, more chemically interesting and is closely allied to ion cyclotron resonance, which will be discussed in the next section. The principle of chemical ionization is simple. The molecule to be studied is injected into the ionizing region of the mass spectrometer in the presence of 0.5-1.5 mm Hg pressure of a gas, usually methane. Electron impact causes ionization of the methane, which is present in relatively large concentration. The ionization products of methane then react with the compound to be analyzed and convert it to ions. The gas mixture then exits into a low-pressure zone (10 4 mm) and the ions are analyzed according to mje in the usual way. 

Attempts have been made to observe and experimentally determine the structure of CH5+ in the gas phase and study it in the condensed state using IR spectroscopy,764 765 pulse electron-beam mass spectrometry,766 and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).767 However, an unambiguous structure determination was unsuccessful. Retardation of the degenerate rearrangement was achieved by trapping the ion in clusters with H2, CH4, Ar, or N2. 

The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9) ultrahigh mass resolution (>1,000,000 at m/z. < 200) with accurate mass measurement even 1n gas chromatography/mass spectrometry experiments sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers versatile Ion sources (electron impact (El), self-chemical ionization (self-Cl), laser desorption (LD), secondary ionization (e.g., Cs+-bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission) trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. 

Until this point, results appear to be rather divergent, possibly because these studies were performed on SMA dissolved in various solvents and solvation might modify the distribution of electrons in the Si-C-N unit. In order to circumvent this problem, gas phase studies and theoretical approaches were developed. In the gas phase (ion cyclotron resonance mass spectroscopy), trimethylsilylmethyldimethylamine was reported to be more basic than the analogous neopentyldimethylamine from proton affinity measurements based upon proton affinity of ammonia (201.0 kcal/mol) 227.1 and 225.8 kcal/mol respectively.32 Conversely, the ionization potential of MSMA indicates a basicity lower than that of its carbon analog and the authors emphasize the fact that this result is opposite... 

Radical cations of saturated hydrocarbons have strong electronic absorptions in the visible and near-infrared region of the spectrum. The strongly colored nature of alkane radical cations is in striking contrast to neutral alkanes that absorb electronically only in the vacuum UV. The electronic absorption of alkane radical cations has been studied in the solid phase by matrix isolation using y-irradiation [1-3] and in the gas phase by ion cyclotron resonance (ICR) photodissociation in either the steady-state or pulsed mode of operation [4]. Both methods have their specific merits and drawbacks. A major concern in matrix isolation spectroscopy is spectral purity (because of the possible presence of other absorbing species) and... 

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