Ion-molecular reactions

Most nitrile oxides are unstable, some of them are explosive. This fact hinders the study of their physical properties. Nevertheless, there are a number of publications concerning not only stable but also unstable nitrile oxides. In particular, mass spectral data for nitrile oxides among other unstable compounds containing an N+-X bond are summarized in a review (9). In such studies, the molecular ions must be generated using indirect procedures, including dissociative electron ionization, online flash-vacuum pyrolysis mass spectrometry, or ion-molecular reactions. Their characterization is mainly based on collisional activation and ion-molecular reactions. 

Electron ionization is a perfect method for the analysis of labeled molecules as in this case ion-molecular reactions are suppressed. It is better to use for the calculations the most intense spectral peaks with the highest m/z values. Molecular ion is the best choice. However, if notable [M + H]+ or [M — H]+ peaks are present in the spectrum of the unlabeled compound the correct calculation will be problematic. To eliminate [M + H]+ peaks it is helpful to record a spectrum with the minimum quantity of sample. To consider interference with [M — H]+ ions one should know from what position the hydrogen atom is lost and whether deuterium could be in this position. 

Moens et al.124 demonstrated that ion-molecule reactions in a dynamic reaction cell (DRC) of ICP-QMS allow the 87Rb+/87Sr+ isobaric interference to be eliminated. A special ion-molecular reaction in a collision cell was studied in order to avoid this isobaric interference of atomic ions... 

The physicochemical stage includes the chemical processes in electron excitation states, as well as the chemical transformations of the active intermediates under nonequilibrium conditions. These are the predissociation and the ion-molecular reactions that take about 1013 s the recombination of positive ions with thermalized electrons (1CT12-10 10s) and the electron-solvation reactions (10 12-10-1° s). Thus, the physicochemical stage lasts from 1CT13 to 10-I0s. 

The alkane radical-cations generated in electron-pulse irradiated n-dodecane show an absorption band in the visible with its maximum at 800 nm (Fig. 15) [93], The position of the absorption maximum changed from 600 nm to 900 nm depending on the carbon number of the alkane. It was noted that the lifetime of alkane-radical cations was shorter than that of the solvated electrons observed in the near infrared region. These phenomena were interpreted in terms of the following ion-molecular reaction. 

Thermal effects caused by the heating of the sample to volatilize it into the mass spectrometer have been studied by several groups of workers. Anomalous M+ 14 ions in the mass spectrum of the alkaloid, voacamine, were shown by deuterium labelling to be due to intermolecu-lar methyl transfer from a carbomethoxy group to, probably, a basic nitrogen atom (Thomas and Biemann, 1965). The intensity of the M +14 peak varied with vapourization temperature, but not sample pressure which tends to rule out an ion-molecular reaction. 

Ion-molecular reactions are used to resolve isobaric interferences, as discussed, in ICP-MS with a collision/reaction cell or by utilizing ion traps. The mass spectra of Sr, Y and Zr (Fig. 6.10a) without O2 admitted into the collision cell and (Fig. 6.10b) with 10 Pa Oj are different. By introducing oxygen, selective formation of YO and ZrO, but not SrO, is observed. This behaviour of different oxide formation is relevant for an interference free determination of Sr. Ultrahigh mass resolving power ICP mass spectrometry (at m/Am 260 000) selectively removes unwanted ions prior to transfer to the FTICR analyzer cell by gas-phase chemical reactions, e.g., for separation of Ca from " Ar+ obtained with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer equipped with a 3 tesla superconducting magnet. ... 

Many kinds of organic molecules liave been detected in interstellar clouds. The formation mechanisms of these molecules have been one of the most significant issues in cosmochemistiy. Recent tlieoretical and experimental studies suggest that tlie important mechanisms are ion-molecular reactions in tlie gas phase [59], grain surface reaction in dusts [60], gas-pliase pyrolysis [61], and photochemical reaction in icy grain mantles [62]. The ion-molecular reaction results from a collision of ions and molecules in a gas phase. Because of tlie lack... 

Belevskii, V. N., Shchapin, I. Y. Rearrangement and ion-molecular reactions of C5H8+.-related radical cations as studied by EPR spectroscopy in the solid and liquid phase. Acta Chem. Scand. 1997, 51, 1085-1091. 

This is a simple and versatile method for investigating many ion-molecular reactions caused by metal ions. 

In order to detect radical ions, mass spectrometry, electron spectroscopy, and ESR are used in combination with optical bleaching. For example, according to mass spectrometric data, ethylene forms a dimer particle, C4H8, as a result of a primary ion-molecular reaction. This particle reacting with monomeric ethylene produces growing chains up to C14 with a radical-ion active centre ... 

Ion-Molecular Reactions, Ion-Molecular Polarization Collisions, and the Langevin Rate Coefficient... 

Some ion-molecular reactions were already discussed earlier. Thus, the positive ion conversion A+ -I- B -I- M AB+ + M was considered in Section 2.2.2 as a preliminary stage of the dissociative electron-ion recombination. Ion-molecular reactions not only make a contribution in the balance of charged particles but also provide plasma-chemical processes by themselves. Ion-cluster growth in dusty SiH4 plasmas and ion-molecular chain reactions of SO2 oxidation in air during exhaust gas cleaning are good relevant examples, which will be discussed later in the book. 

The typical valne of the rate coefficient for ion-molecular reactions is 10 cm /s, which is 10 times higher than the gas-kinetic value for neutral particles ko 10 cm /s. The preceding relations describe the interaction of a charge with an induced dipole. If an ion interacts with a molecnle having a permanent dipole moment /xd, the Langevin capture cross section becomes larger (Sn Bowers, 1973,1975) ... 

The whole ionization process in air, as a result, can be significantly focused on the formation of water ions (H2O+) even though the molar fraction of water is low. The generated water ions can then react with neutral water molecules via the quite fast ion-molecular reaction ... 

Most of the exothermic ion—molecular reactions have no activation energy (Talrose, 1952). Quantum-mechanical repulsion between molecules, which provides the activation barrier even in the exothermic reactions of nentrals, can be suppressed by the charge-dipole attraction in the case of ion-molecular reactions. Thus, rate coefficients of the reactions are very high and often correspond to the Langevin relations (2 8)-(2-50), sometimes partially hmitedby quantum-mechanical factors (Su Bowers, 1975 Virin et al., 1978). The efiect obviously can be apphed to both positive and negative ions. 

A significant contribution to NO synthesis in non-thermal air plasma can be provided by ion-molecular reactions, especially those involving positive atomic oxygen ions ... 

This reaction proceeds fast without any energy barrier but requires the formation of an expensive ion, 0+. An additional NO molecule can be produced in this case through exothermic reaction (6-3). NO synthesis can be also provided by ion-molecular reactions of molecular ions ... 

Guertin, Christe, and Pavlath (1966) considered another mechanism of NF4ArF6 synthesis in plasma through a set of ion-molecular reactions. The mechanism starts with the formation of positive ions (NF and Fj through ionization, and the formation of negative ions (F ) through dissociative electron attachment to NF3, AsFs, or F2 molecules. The initial positive ions lead to the formation of an ion radical NF4 ... 

NO Synthesis in Non-Thermal Plasma Provided by Positive Ions and Electronically Excited Molecules. Estimate the energy cost of NO molecule formation in air at non-thermal plasma conditions produced by each of the three following processes (1) dissociation of molecular nitrogen through its electronic excitation by direct electron impact (2) electron-ion recombination of positive molecular nitrogen ion and (3) ion-molecular reactions of... 

Neglecting ion-ion recombination, the particle size growth in the sequence of ion-molecular reactions (8-129)-(8-132) can be described based on (8-137) by the following equation ... 

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