Charge transfer, and ion—molecule reactions

Luminescence in Charge Transfer and Ion-Molecule Reactions Charge Transfer in Atomic Systems... 

Charge-Transfer and Ion-Molecule Reactions. Both positive and negative ion-molecule reactions have recently been studied by a variety of experimental methods, and consistent values for many rate constants... 

The major problem in method (a) is that in ion-molecule interchange, considerable momentum in the direction of travel of the incident ion is imparted to both final products. Hence, in a perpendicular type apparatus only transfer of low weight particles can be observed at all and only at very low velocities of the incident ions (1, 9, 10, 11, 12, 13, 19, 20, 23, 27). Cross-sections cannot be measured. The value of these investigations is that some ion-molecule reactions—e.g., proton transfer and hydride ion transfer—can be identified. The energetics and the competition between charge exchange and ion-molecule reactions can be discussed, and by using partially deuterated compounds, one can obtain a detailed picture of the reaction. 

Ion-molecule reactions can be investigated in a double mass spectrometer in two ways (a) In the collision between the incident ion and the gas molecule, transfer of part of one of these structures can take place. The pressure in the collision chamber must be low (b) The pressure in the collision chamber is increased. The slow incident ions ionize the gas molecules by charge exchange. Then ion-molecule reactions take place between the ionized gas molecules or their fragment ions and other gas molecules. 

The most common reagent ions used in SIFT-MS are H30+, NO+, and 02 +, and their reactions with many different classes of volatile organics have been well documented.44 The NO+ reagent ion can react with the VOCs, depending on their chemistry, in one or two of several different ways - charge transfer (Equation (17)), hydride ion transfer (Equation (18)), hydroxide ion transfer (Equation (19)), alkoxide ion transfer, and ion-molecule association (Equation (20)). 

Chemical ionization (Cl) The formation of new ionized species when gaseous molecules interact with ions. This process may involve the transfer of an electron, proton, or other charged species between the reactants in an ion-molecule reaction. Cl refers to positive ions, and negative Cl is used for negative ions. 

The pulsed source method, despite several limitations, appears to be a very useful technique for studying ion-molecule reactions at thermal energies. Although the studies to have date been limited primarily to simple hydrogen transfer reactions, the technique should also prove useful for studying charge transfer and hydride ion transfer reactions at thermal energies. 

Mass spectrometric studies of the ionic species which arrive at the cathode of both glow and corona discharges yield useful information regarding ion-molecule reactions which occur within these systems. Glow discharges have been used to study endothermic reactions, and their usefulness and limitations have been demonstrated by studies of the dissociative charge transfer reactions Ar+ + N2 N+ + N + Ar N2+ + N2 N+ + N + N2 N2+ + 02 0+ + O + N2. Exo-... 

The choice of a particular type of gas discharge for quantitative studies of ion-molecule reactions is essential if useful information is to be obtained from ion abundance measurements. Generally, two types of systems have been used to study ion-molecule reactions. The pulsed afterglow technique has been used successfully by Fite et al. (3) and Sayers et al. (1) to obtain information on several exothermic reactions including simple charge transfer processes important in upper atmosphere chemistry. The use of a continuous d.c. discharge was initiated in our laboratories and has been successful in both exothermic and endothermic ion-molecule reactions which occur widely within these systems. 

A second consequence of the relatively high pressures used in these systems is the simultaneous occurrence of many side reactions of the ions with trace quantities of impurity gases. Two of the most serious side reactions are charge transfer processes and fast ion-molecule reactions which often interfere with the reaction under study. Indeed, in systems... 

The observed H+(NH3)n and H (NH3)n(PA) clusters are thought to be formed in a two-step reaction sequence taking place after ionization of the PA(NH3) cluster. The first step is a charge transfer (CT) reaction between the resonantly ionized PA+ and the NH3 molecules in the cluster. The second step is an intracluster ion-molecule reaction (ICIMR) of the charged ammonia cluster leading to the formation of an (n - 1) protonated cluster ion this has been previously established for NH3 clusters33 and is sufficiently exothermic for fragmentation of the cluster. 

There are basically two kinds of neutralization processes for the cation, reaction with the electron and with a negative ion. In each case, it may be assumed that neutralization will occur with the parent or fragment ion of lowest energy. It is believed that the various degradation processes for the cation-fragmentation, ion-molecule reaction, and so forth—are much faster than the neutralization process. In addition, one considers charge transfer, without decomposition, from the cation formally as a neutralization of that species. To effect that, of course, one... 

Binary ion-molecule reactions are indicated by thin arrows (c.t. indicates charge transfer), the radiative association reaction of C+ with H2 is indicated by the thick arrow and the dissociative recombination reactions are indicated by dashed arrows leading to the neutral molecules inside the compound brackets (e indicates free electrons). The molecules indicated in bold are known (observed) interstellar molecules. 

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