Reactions, ion-molecule

Ion-molecule reactions involve a positive ion and a neutral molecule, frequently the parent molecule. Historically, there has been a dichotomy in the interpretation of the radiation-chemical yields in hydrocarbon gases. Early work by Lind (1961) and by Mund (1956) indicated the involvement of ion clustering, exemplified in the radiation-induced polymerization of acetylene as follows  

C2H2+ + nC2H2—(C2H2)+n + 2 (clustering), e + (C2H2)+n + 1 - (polymer product), 

Ion-molecule reactions are ubiquitous. A simple example in hydrogen is given by 

In the case of hydrocarbons, following Ausloos and Lias (1967) we can look at the various ion-molecule reactions in terms of transfer of H and H2 from the molecule to the ion, transfer of H, H2, and H+ from the ion to the molecule, and condensation reactions. Some examples are now given. 

A simple model for the reaction of ions with neutrals that do not have a permanent dipole moment is known as the Langevin model [17]. The potential for ion-induced dipole interactions is given by equation (52), from which we see that n — 4, and C — az e /2, so the reactive cross-section, (7 = nbi 

Note that /cl is independent of the kinetic energy of the reactants. For ion-molecule reactions without an energy barrier, and in the absence of a permanent dipole moment, the Langevin model gives reasonably good estimates of rate coefficients. For example, the rate coefficient for the reaction of Ar ions with H2 is predicted to be 1.5 x 10 cm mole-and the experimental value is reported to be 

If the neutral has a permanent dipole moment, the potential for the interaction of an ion with the dipole will be given by equation (53). The ion-dipole rate coefficient, km, averaged over cos 9, is given by equation (68). Su and Bowers [20] have tabulated values of (cosO) 

During the ion-dipole interaction, polarization of the neutral must also occur. The reaction rate coefficient may be considered to be composed of contributions from both the ion-dipole and the ion-induced dipole interactions. An estimate of the total rate coefficient may be made as the sum of the Langevin and the ion-dipole rate coefficients. 

At an early stage of investigations it has been assumed that ion clusters, i.e. ions surrounded by neutral molecules and recombining with oppositely charged ions to form an appreciable amount of chemically-active species, played an important role in radiation-chemical reactions and were responsible for the high yields. The 

The ion-molecule reactions in inorganic gases have been known for a relatively long time. For instance, the ion-molecule reaction 

Many ion-molecule reactions have been studied [255, 417]. The above reactions involve transfer of a proton or of a hydrogen atom. Collisions of ions with molecules can also include more complicated rearrangements. 

On the other hand, another type of reactions, involving electron transfer is known, the so-called charge exchange, i.e. oxidation-reduction reactions. 

A distinction is made between symmetric and non-symmetric charge exchange processes. The first involve ions and neutral particles of the same gas, e.g. 

Finally, a low temperature chemical reactor can be obtained using collimated uniform supersonic flows generated by the isentropic expansion of a buffer gas through an axisymmetric convergent-divergent Laval nozzle. Uniform supersonic flows were initially developed in a continuous flow version for the study of ion-molecule reactions. Later, the methodology was adapted to the study of reactions between neutral species in a continuous flow version and also in a pulsed flow version.This technique known as CRESU will be described in detail in Section 2.3. 

Brief Overview of Techniques Used to Date 2.2.2.1. Ion-Molecule Reactions 

In the 1980s and early 1990s there was a formidable development of techniques allowing the study of ion-molecule reactions down to temperatures close to 10K. The state of the art was reviewed by M.A. Smith 1994 and a more up-to-date review of the methods presently used in this field, excepting supersonic flow reactors, is given in the chapter by Gerlich. 

The second group of methods relies on supersonic expansion as the source of cooling and the use of supersonic flows as flow reactors. In the early 1980s the group of Rowe, at the rarefied wind tunnel facility, SR3, in the laboratoire d Aerothermique du CNRS in Meudon (France) developed the CRESU technique an ion-molecule reactor that made use of the uniform supersonic flows generated by a Laval nozzle. The first measurements of rate coefficients down to 20 K were reported in 1984 and soon results for temperatures as low as 8 K were obtained.Note that in the CRESU method the temperature is a true temperature, i.e. LTE prevails in the flow. The up-to-date version of this apparatus and the basis of the technique will be described in some detail in the following paragraphs. 

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At the limit of extremely low particle densities, for example under the conditions prevalent in interstellar space, ion-molecule reactions become important (see chapter A3.51. At very high pressures gas-phase kinetics approach the limit of condensed phase kinetics where elementary reactions are less clearly defined due to the large number of particles involved (see chapter A3.6). 

The decrease in reactivity with increasing temperature is due to the fact that many low-energy ion-molecule reactions proceed tln-ough a double-well potential with the following mechanism [82] ... 

Although this reaction is exothemiic, the reaction has a small rate constant. This is one of the most studied ion-molecule reactions, and dependences on many parameters have been measured [148]. 

Franklin J L (ed) 1979 Ion-Molecule Reactions, Part I, Kinetics and Dynamics (Stroudsburg, PA Dowden, Hutchinson and Ross)... 

Troe J 1992 Statisticai aspects of ion-moiecuie reactions State-Selected and State-to-State Ion-Molecule Reaction Dynamics Theory ed M Baer M and C-Y Ng (New York Wiiey)... 

Ikezoe Y, Matsuoka S, Takebe M and Viggiano A A 1987 Gas Phase Ion-Molecule Reaction Rate Constants Through 1986 (Tokyo Maruzen)... 

Smith M A 1994 Ion-molecule reaction dynamics at very low temperatures Unimolecular and Bimolecular Ion-Molecule Reaction Dynamics ed C-Y Ng, T Baer and I Powis (New York Wiley)... 

Ferguson E E 1992 A personal history of the early development of the flowing afterglow technique for ion molecule reactions studies J. Am. Soc. Mass Spectrom. 3 479-86... 

Adams N G and Smith D 1988 Flowing afterglow and SIFT Techniques for the Study of Ion-Molecule Reactions ed J M Farrar and W FI Saunders Jr (New York Wiley)... 

Flierl P M ef a/1996 Flowing afterglow apparatus for the study of ion-molecule reactions at high temperatures Rev. Scl. Instrum. 67 2142-8... 

McFarland M, Albritton D L, Fehsenfeld F C, Ferguson E E and Schmeltekopf A L 1973 Flow-drift technique for ion mobility and ion-molecule reaction rate constant measurements. I. Apparatus and mobility measurements J. Chem. Phys. 59 6610-19... 

Ferguson E E 1979 ion-moiecuie reactions in the atmosphere K/nef/cs of Ion-Molecule Reactions ed P Ausioos (New York Pienum)... 

A comprehensive look at the effect of state selection on ion-molecule reactions from both experimental and theoretical viewpoints. 

Collision-induced dissociation mass spectrum of tire proton-bound dimer of isopropanol [(CH2)2CHOH]2H. The mJz 121 ions were first isolated in the trap, followed by resonant excitation of their trajectories to produce CID. Fragment ions include water loss mJz 103), loss of isopropanol mJz 61) and loss of 42 anui mJz 79). (b) Ion-molecule reactions in an ion trap. In this example the mJz 103 ion was first isolated and then resonantly excited in the trap. Endothennic reaction with water inside the trap produces the proton-bound cluster at mJz 121, while CID produces the fragment with mJz 61. 

The same procedure as outlined above can be used to study ion-molecule reactions [15, 34]- Mass-selected ions will react with neutral species inside the trap. The presence of the damping gas means that stable (tliemiodynamic and... 

Grover R, Decouzon M, Maria P-C and Gal J-F 1996 Reliability of Fourier transform-ion cyclotron resonance determinations of rate constants for ion/molecule reactions Eur. Mass Spectrom. 2 213-23... 

Mass spectrometric investigations of the ionosphere show an abundance of molecular ions such as NO and watercluster ions [4T ]. This is an indication of the result of ion-molecule reactions which change the chemical state of the ions in this plasma ... 

V. Sidis, in Siaie-Selected and State-io-Siate Ion-Molecule Reaction Dynamics, M. Baer andC. Y, Ng., eds., John Wiley Sons, Inc, New York, 1992, Vol. II, Chap, 2. 

However, in both FI and FD, there are other neutral molecules on or close to the surface of the emitter and, in this region, ion/molecule reactions between an initial ion and a neutral (M(H)) can produce protonated molecular ions ([M + H]+), as seen in Equation 5.2. 

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