Other Ion-Molecule Reactions

N +NH3, NH +NH3. Formation of N2H was observed following electron beam ionization of gaseous ammonia (at pressures up to 0.6 Torr) in the ion source of a mass spectrometer. The cross sections 0.89 A and 1.1 were reported for the NH + NH3- NsH + H + H2 and N + NH3- N2H + H2 reactions, respectively [1]. In the N +NH3 system, the N2H product channel (branching fraction 0.1) was found to be dominated by the charge transfer channel [2 to 4]. 

N3 +H2. The rate constant k 2 x 10 cm molecule s for the reaction N3 + H2 - N2H + NH was determined using a selected-ion flow tube technique. Only branching into the N2H product channel was observed [2]. 

T-HjCHD, Dj). An ion-beam target-gas study showed that the N4 +D2 N2D + N2 + D channel dominates other available reaction channels in addition to N2D, only a comparatively small amount ( 5%) of N2 from collision-induced dissociation was detected [5]. Drift tube studies showed significantly more (13%) [2] or only N4H (100%) [6] to be formed in N4+H2 reactive collisions. The difference in product distributions is attributed [5] to the single [5] and multiple collision [2, 6] conditions that were present. Thermal rate constants at 300 K for the N2H and N2D product channels in the reactions of N4 with H2 

The energy dependence of the cross sections for the reactions of thermal N4 ions with H2, HD, and D2 was determined for center-of-mass collision energies from thermal up to 5 eV using the ion-beam target-gas technique. The total cross section for the reactions with HD (N4 + HD- N2H (N2D ) +D(H)) is equal to the cross sections for the reactions with H2 and D2. An isotope effect was observed in the reaction with HD at energies above 0.1 eV, formation of N2D over N2H is favored [5]. 

Typical for reactions with a barrier, the cross sections for the N2H and N2D product ions rise with increasing collision energy up to 3 eV. Above 3 eV the cross sections decrease, possibly due to dissociation of the product ion. For the reactions with H2, D2, and HD, an activation energy of 0.09 0.03 eV at 0 K was obtained from the reaction thresholds which were determined in ion-beam target-gas experiments [5]. From Arrhenius plots of temperature-dependent [7] and collision-energy-dependent [9] drift tube data, the following activation energies were obtained H2, 0.15 0.01 [7], 0.16 0.01 D2, 0.17 0.01 eV [9]. 

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Many other ion-molecule reactions involving highly unsaturated hydrocarbon ions and neutral olefins or the equivalent strained cycloalkanes have been studied by mass spectrometry98. For example, we may mention here the addition of ionized cyclopropane and cyclobutane to benzene radical cations giving the respective n-alkylbenzene ions but also isomeric cyclodiene ions such as ionized 8,9-dihydroindane and 9,10-dihydrotetralin, respectively. Extensive studies have been performed on the dimerization product of charged and neutral styrene4. 

Other ion-molecule reactions with methane will occur in the plasma, such as... 

During the 1970s the ECD became firmly established as the most sensitive gas chromatographic detector for some compounds. The kinetic model was described in terms of a numerical solution of the differential equations. This was assisted by the development of the constant current mode of measuring the response and the development of Ni-63 sources for the detector. The purification of the carrier gas and the further development of capillary columns improved the operation of the ECD. In addition, chemical reactions were used to make derivatives with a greater sensitivity in the ECD. Other ion molecule reactions were used to improve the sensitivity of... 

Section 2 of this paper describes the apparatus we use at Orsay to study ion-molecule collision processes. The following four sections discuss the results we have obtained for reactions (1)—(4). These are a representative sample of the type of systems we can study at present. Section 7 discusses future prospects for using synchrotron radiation to study other ion-molecule reactions. 

Apart from the proton transfer reactions discussed in Section II, phosphorus species undergo a range of other ion-molecule reactions in the gas phase. The types of instruments which have been used to study ion-molecule reactions of phosphorus species include ion cyclotron resonance (ICR) mass spectrometers and the related FT-ICR instruments, flowing afterglow (FA) instruments and their related selected-ion flow tubes (SIFT) and also more conventional instruments This section is divided into four topics (A) positive ion-molecule reactions (B) negative ion-molecule reactions (C) neutralization-reionization reactions and (D) phosphorus-carbon bond formation reactions. 

The ion HCO is formed and destroyed by proton transfer reactions in the inner region of the envelope and formed by other ion-molecule reactions at r 10 cm and has a column density of 3x10 cm 2. Glassgold et al. (1987) have presented an extensive discussion of the HCO abundance and its sensitivity to parameters such as mass-loss rate and the cosmic-ray ionisation rate. In particular, their calculated antenna temperature for the J = 1 - 0 line is consistent with the upper limit obtained by Lucas et al. 

Hi) Gas phase charge transfer means that ion formation occurs after volatilization or desorption of neutral species from the surface into the gas phase through ionization via proton/electron transfer or other ion-molecule reactions at atmospheric pressure. Indeed, the assumption of ion-molecule reactions, eventually purely in the gas phase above the sample, has led to the development of an DAPCI source (Chap. 13.2), in the first place to prove this mechanism of ion formation [7]. The solvent pH can be used to positively affect the vapor pressure of the analyte, e.g., the vapor pressure of volatile plant alkaloids is increased by addition of abase. 

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. 

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. 

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. 

Ton-molecule reactions are of great interest and importance in all areas of kinetics where ions are involved in the chemistry of the system. Astrophysics, aeronomy, plasmas, and radiation chemistry are examples of such systems in which ion chemistry plays a dominant role. Mass spectrometry provides the technique of choice for studying ion-neutral reactions, and the phenomena of ion-molecule reactions are of great intrinsic interest to mass spectrometry. However, equal emphasis is deservedly placed on measuring reaction rates for application to other systems. Furthermore, the energy dependence of ion-molecule reaction rates is of fundamental importance in assessing the validity of current theories of ion-molecule reaction rates. Both the practical problem of deducing rate parameters valid for other systems and the desire to provide input to theoretical studies of ion-molecule reactions have served as stimuli for the present work. 

Pulsed source techniques have been used to study thermal energy ion-molecule reactions. For most of the proton and H atom transfer reactions studied k thermal) /k 10.5 volts /cm.) is approximately unity in apparent agreement with predictions from the simple ion-induced dipole model. However, the rate constants calculated on this basis are considerably higher than the experimental rate constants indicating reaction channels other than the atom transfer process. Thus, in some cases at least, the relationship of k thermal) to k 10.5 volts/cm.) may be determined by the variation of the relative importance of the atom transfer process with ion energy rather than by the interaction potential between the ion and the neutral. For most of the condensation ion-molecule reactions studied k thermal) is considerably greater than k 10.5 volts/cm.). 

Here, a primary ion P+ formed by the radiation field reacts with a gas molecule M to give an intermediate complex [PM +] which can either dissociate to a secondary species S + and a neutral fragment N or react with another molecule to produce another complex [PM2 + ]. The latter then dissociates into a tertiary ion T+ or propagates the chain by forming a third intermediate [PM3 + ]. A quaternary ion Q+ may result from dissociation of [PM3 + ], or the chain may continue through reaction of [PM3 + ]. Wexler and Jesse (38), on the other hand, have suggested a model which states that reactive intermediate complexes are not involved in the propagation, but rather the polymerization proceeds by chains of simple consecutive and competitive ion-molecule reactions,... 

Acetylene Ion. No evidence for the contribution of ion-molecule reactions originating with acetylene ion to product formation has been obtained to date. By analogy with the two preceding sections, we may assume that the third-order complex should dissociate at pressures below about 50 torr. Unfortunately, the nature of the dissociation products would make this process almost unrecognizable. The additional formation of hydrogen and hydrogen atoms would be hidden in the sizable excess of the production of these species in other primary acts while the methyl radical formation would probably be minor compared with that resulting from ethylene ion reactions. The fate of the acetylene ion remains an unanswered question in ethylene radiolysis. 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

Most information concerning ion-molecule reactions in flames has been obtained from mass spectrometric measurements, but some inferences have been drawn from results of other types of experiments... 

Reaction 2 has been invoked because C3H3 + is apparently formed in a primary ionization step since the ion appears early in the flame front, its concentration maximizes in rich flames (this is true of no other positive ion observed), and it is present in the flame front in large concentrations (9). However, not all the experimental evidence is consistent with this mechanism for producing C3H3+ it might also be produced through an ion molecule reaction, which will be considered below. 

Consideration of work by Buchel nikova (4) on the dissociative attachment of electrons to HC1 leads to the conclusion that k4 is given approximately by k4 10-10 e 20 mtRT cm.3 molecule-1 sec.-1 Since K 4 2 X 10-3 el8 miRT at 2000°K., 4 10 13 cm.3 molecule-1 sec.-1 This is considerably smaller than rate constants for other exothermal ion-molecule reactions, which probably reflects the importance of participation of molecular vibrational energy in such reactions. Remember, however, that the uncertainty in 4 is probably at least an order of magnitude. 

Cl is an efficient, and relatively mild, method of ionization which takes place at a relatively high pressure, when compared to other methods of ionization used in mass spectrometry. The kinetics of the ion-molecule reactions involved would suggest that ultimate sensitivity should be obtained when ionization takes place at atmospheric pressure. It is not possible, however, to use the conventional source of electrons, a heated metallic filament, to effect the initial ionization of a reagent gas at such pressures, and an alternative, such as Ni, a emitter, or a corona discharge, must be employed. The corona discharge is used in commercially available APCI systems as it gives greater sensitivity and is less hazardous than the alternative. 

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