Mass spectrometry bimolecular reactions

One rather unfortunate aspect of the M + hydrocarbon (and M + OX) reactions mentioned thus far is that the products of the reactions were not detected directly, but were instead inferred via the pressure and temperature dependencies of the measured rate constants for metal reactant consumption and by comparison to ab initio calculations. Exceptions are the reactions of Y, Zr + C2H4 and C3H6, for which the Weisshaar group employed the 157 nm photoionization/mass spectrometry technique to identify the products of the reaction as those resulting from bimolecular elimination of H2.45 47 95... 

Characterization of ion structures by bimolecular reactions, in which an ion is allowed to react with a neutral gas of known structure and the ionic products are analysed by mass spectrometry, depends on isomeric species having distinctive reactivities which reflect the functional group(s) that are present. This method is conceptually analogous to the use of structure-specific test reagents in classical solution chemistry. Sometimes a group may be transferred to a particular ion from the gas (methylene transfer is commonly encountered) on other occasions, hydrogen transfer is monitored. The latter is conveniently combined with isotopic labelling. 

The quasi-equilibrium theory (QET) of mass spectra is a theoretical approach to describe the unimolecular decompositions of ions and hence their mass spectra. [12-14,14] QET has been developed as an adaptation of Rice-Ramsperger-Marcus-Kassel (RRKM) theory to fit the conditions of mass spectrometry and it represents a landmark in the theory of mass spectra. [11] In the mass spectrometer almost all processes occur under high vacuum conditions, i.e., in the highly diluted gas phase, and one has to become aware of the differences to chemical reactions in the condensed phase as they are usually carried out in the laboratory. [15,16] Consequently, bimolecular reactions are rare and the chemistry in a mass spectrometer is rather the chemistry of isolated ions in the gas phase. Isolated ions are not in thermal equilibrium with their surroundings as assumed by RRKM theory. Instead, to be isolated in the gas phase means for an ion that it may only internally redistribute energy and that it may only undergo unimolecular reactions such as isomerization or dissociation. This is why the theory of unimolecular reactions plays an important role in mass spectrometry. 

In the present review, a new variation on an existing experimental method will be used to show how accurate unimolecular dissociation rate constants can be derived for thermal systems. For example, thermal bimolecular reactions are amenable to study by use of several, now well-known, techniques such as (Fourier transform) ion cyclotron resonance spectrometry (FTICR), flowing afterglow (FA), and high-pressure mass spectrometry (HPMS). In systems where a bimolecular reaction leads to products other than a simple association adduct, the bimolecular reaction can always be thought of as containing a unimolecular... 

To overcome this, instrumental techniques such as pulsed high-pressure mass spectrometry (PHPMS), the flowing afterglow (FA) and allied techniques like the selected-ion flow tube (SIFT), and ion cyclotron resonance (ICR) spectrometry and its modem variant, Fourier transform mass spectrometry (FTMS), have been developed. These extend either the reaction time (ICR) or the concentration of species (PHPMS, FA), so that bimolecular chemistry occurs. The difference in the effect of increasing the pressure versus increasing the time, in order to achieve bimolecular reactivity, results in some variation in the chemistry observed with the techniques, and these will be addressed in this review as needed. 

The field of ion-molecule reactions has profited immensely from the introduction of new techniques in mass spectrometry such as Fourier Transform Ion-Cyclotron Resonance Mass Spectrometry (FT-ICR) and Selected-Ion Flow Tube methods (SIFT). Several accounts of the progress in the field of ion-molecule reactions involving silicon-containing molecules have been published. For review articles the reader is referred elsewhere1,3,4,40-51 A detailed compilation of the kinetic data for bimolecular ion-molecule reactions of positive silicon ions has been published by Anicich52. 

In this review we focus both on major developments in new mass spectrometric techniques and on novel chemical applications of existing mass spectrometric techniques that have been reported since 1990. Emphasis is given to the application of these techniques to the study of bimolecular ion/molecule reactions, radiative association, and dissociative recombination of positive ions. Particular attention is given to the emerging field of interstellar metal-ion chemistry and recent studies of fullerene-ion chemistry and the influence of charge state on this, and related, chemistry. Mass spectrometric studies of the photochemistry of interstellar ions are briefly considered as is interstellar negative-ion chemistry. We conclude with a brief description of the use of mass spectrometry to examine interstellar material that has made the long journey to our solar system. 

The laboratory study of IS ion chemistry has its origins in measurements of bimolecular ion/molecule reaction kinetics. Experimental conditions that are found in most of the mass spectrometers employed in the measurement of bimolecular ion/molecule reactions are far from those found in the cold low-pressure environments of space. Nevertheless, because of the pressure independence of bimolecular reactions and the absence of significant activation energies in most bimolecular ion/molecule reactions, MS measurements performed here on earth do have relevance for the chemistry in space. The substantial database available in the early 1990s on the kinetics of bimolecular ion/molecule reactions important in IS chemistry [8-10] was obtained almost entirely using ion cyclotron resonance (ICR) and flow-tube (FT) mass spectrometry techniques. Both techniques are well established and continue to be used extensively for ion/ molecule reaction measurements generally. 

Deisen ° using a shock tube with time-of-flight mass spectrometry detection has investigated the kinetics of the F2-N2F4, system in the range 1100-1600 °K. From an analysis of the variation of NFj with time at different temperatures a bimolecular rate coefficient of 4.8 x lO exp (—14,400/RT) l.mole . sec was obtained for the reaction... 

The reaction of OH with COS is again thought to proceed via an OH-COS addition complex, but in the absence of a rate-accelerating effect of 02, the main fate of the complex is reversion to the original reactants. The actual forward reaction yielding new products is a minor pathway showing bimolecular behavior. By means of mass spectrometry, Leu and Smith (1981) identified HS as one of the products, so that the reaction takes the course... 

L.W. Sieck, S.K. Searles, and P. Ausloos, High-pressure photoionization mass spectrometry. I. Unimolecular and bimolecular reactions of C4Hg from cyclobutane,... 

Several additional products formed by the Diels-Alder condensations of butadiene and styrene were also characterised by mass spectrometry (MS) and by matching the pyrogram retention times of the samples with retention times obtained for samples spiked with the same dimers. Vapour phase dimerisation of butadiene is known to be a clear-cut bimolecular reaction. Formation of the dimers can take place in the pyrocell as well as in the gas chromatograph (GC) transfer line. Equation 2.4 shows the dimerisation of two molecules of butadiene to form a molecule of 4-vinyl-l-cyclohexene ... 

Middaugh, J.E. (2014) The Study of Bimolecular Radical Reactions Using a Novel Time-resolved Photoionization Time-of-flight Mass Spectrometry and Laser Absorption Spectrometry Apparatus. PhD Thesis, Massachusetts Institute of Technology. 

Radiative Association Reactions The study of radiative association reactions, (Eq. 2.2), has been of considerable interest [6-8] in chemical kinetics, planetary and interstellar chemistry, flames, and a variety of other areas. The kinetic study makes it possible to model the formation of complex molecular species in the interstellar science. At the very low molecular number densities in interstellar environments, the probability of formation of the products of association reactions by collisional stabilization is very low. Therefore, the radiative association process becomes an extremely important one for the production of the complex molecular species observed by astronomical physicist. The methodology is either flowing afterglow (FA) or Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. For the study of the apparent bimolecular rate constant for formation of association products as a function of pressme of a third body (N), the pressure should be set up to be sufficiently high in order to release the energy in the associated complex. Under the high pressure conditions collisional stabilization has competed with and usually dominated over radiative associatioiL As a result, the radiative association rate was then extrapolated from the intercept of a plot of apparent rate constant versus pressure of a third body, N. 

The termination mode of poly(alkyl methacrylate) radicals has also been the subject of much research.[9] Model compound studies of the bimolecular reactions of l-methoxycarbonyH-methylethyl radicals and the higher esters ethyl and butyl have resulted in for MMA, 0.72 for EMA, and 1.17 for nBMA.[9,10]. A k /k =4.37 was obtained by means of Matrix-assisted laser desorption/ionisation-time of flight mass spectrometry (MALDI-TOF MS) to the end-group analysis of low MW PMMA.[11] The use of fluorinated derivatives of BPO in combination with F NMR analysis of PMMA also indicated that the termination occurred mainly due to the disproportionation in this system.[12]... 

So far we have mostly considered first-order reactions of radical ions and ions in the gas phase. Here we will consider bimolecular reactions between ions and neutral molecules. Such reactions can be studied by Cl mass spectrometry or ion cyclotron resonance (ICR) spectrometry (see Section 7.1). 

Cl differs from what we have encountered in mass spectrometry so far because bimolecular processes are used to generate analyte ions. The occurrence of bimol-ecular reactions requires a sufficiently large number of ion-molecule collisions during the dwell time of the reactants in the ion source. This is achieved by significantly increasing the partial pressure of the reagent gas. Assuming reasonable... 

Bimolecular ion-molecule reactions such as those in Reactions 2.2-2.4 are often very fast, by which we mean that they can often occur on every collision, which makes them useful for efficient ionization of target molecules in mass spectrometry. Furthermore, the energy available to deposit into the product cation is typically low, and may be well under 1 eV if an appropriate Cl reagent is chosen. This low excess energy means that CI-MS is considered to be a soft ionization technique since it tends to leave parent ions intact. However, it is also wise to bear in mind that some ion-molecule reactions can occur via concerted mechanisms which lead to fragmentation even with much smaller excess energies, so soft ionization is not necessarily fragment-free. 

The study of bimolecular gas reaction rate coefficients has been one of the primary subjects of kinetics investigations over the last 20 years. Largely as a result of improved reaction systems (static flash photolysis systems, flow reactors, and shock tubes) and sensitive detection methods for atoms and free radicals (atomic and molecular resonance spectrometry, electron paramagnetic resonance and mass spectrometry, laser-induced fluorescence, and laser magnetic resonance), improvements in both the quality and the quantity of kinetic data have been made. Summarizing accounts of our present knowledge of the rate coefficients for reactions important in combustion chemistry are given in Chapters 5 and 6. 

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