Flowing afterglow technique

Gas-phase acidities and basicities for many organic compounds are now available, primarily due to the development within the past decades of three new experimental techniques pulsed high-pressure i.e. 0.1... 1300 Pa) mass spectrometry (HPMS) [22, 23, 118], the flowing afterglow (FA) technique with a fast-flowing gas like helium in the pressure range of ca. 10 . .. 10 Pa [119], and pulsed electron beam, trapped ion cell, ion cyclotron resonance (ICR) spectrometry, carried out at ca. 10 ... 10 Pa [24-26, 115]. 

Three new experimental techniques, developed within the past decades, now make it possible to study ionic reactions in the gas phase as well. These are pulsed ion-cyclotron-resonance (ICR) mass spectrometry, pulsed high-pressure mass spectrometry (HPMS), and the flowing afterglow (FA) technique [469-478 see also the references given in Section 4.2.2]. Although their approaches are quite independent, the results obtained for acid/base and other ionic reactions agree within an experimental error of 0.4... 1.3 kJ/mol (0.1... 0.3 kcal/mol) and are considered as reliable as those obtained in solution. 

Some of the important features of the flowing afterglow experimental technique are the following ... 

B. L. Moiseiwitsch How Opaque Is a Star M. J. Seaton Studies of Electron Attachment at Thermal Energies Using the Flowing Afterglow-Langmuir Technique, David Smith and Patrik Spanel... 

Reactions with neutral molecules have been investigated by the flowing afterglow (FA) technique (see [2, 3]) and earlier by ICR spectroscopy. The reaction usually starts with an initial nucleophilic attack of PHg on the neutral, followed by Intramolecular proton transfer and/or expulsion of a neutral fragment [4]. The table on p. 110 lists the rate constants k at 298 K (with an estimated error of 25% for the FA measurements [4]), efficiencies k/k oo (with kADo calculated by the average-dipole-orlentation theory of [5]), products (neutral products were not detected), and branching ratios, k and the branching ratio depend on the total pressure, when adducts are formed. Molecules, for which no reaction could be observed, are listed below the table. 

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)... 

Squires R R 1997 Atmospheric chemistry and the flowing afterglow technique J. Mass Spectrum. 32 1271-72... 

Since the flowing-afterglow method is quite well established, a brief review of the basic features of this technique should be sufficient. The experiments of Gougousi et al.46 on H3 recombination and those of Adams et al.18 and of Smith and Spanel24 used nearly the same experimental method, but there are significant differences in the data analysis and in the interpretation of the results. 

One may ask why some experiments, for instance those done by microwave-afterglow technique 15,16 and the experiments by Canosa et al., 21,22 gave no indications of an anomalous decay. In part, the answer may be that small variations of the deionization coefficients are not easily detected in the presence of ambipolar diffusion. They were detected in the work of Adams et al. and of Smith and Spanel only because the diffusion losses were unusually slow in their large flow tube. 

Tandem mass spectrometry H6,i46) both stationary 116> and flowing afterglow-methods 118,147) and drift tube techniques U6> have also been applied to some of the clustering reactions. Results for the gas-phase solvation of H+ by H2O and NH3 generally agree well with the values obtained by high pressure mass spectro-metric observations 148). 

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. 

At the higher pressures of other ion-molecule techniques, such as flowing afterglow or pulsed high-pressure mass spectrometry," both of which operate with a bath gas pressure of about 1 torr, collisions of such an excited intermediate with the bath gas occur on a nanosecond to microsecond time-scale, in competition with the unimolecular dissociation rate. For these techniques, ions that are the... 

Introduction 198 Experimental techniques 200 Ion cyclotron resonance spectrometry 201 Flowing afterglow 203 High pressure mass spectrometry 204 General features of gas-phase ion-molecule reactions 204 Gas-phase SN2 reactions involving negative ions 206 Thermochemical considerations 206 General aspects of gas-phase SN2 reactions 207 Stereochemistry 209... 

Three basic techniques, and variations thereof, have been used in recent years to study aspects of gas-phase ion-molecule reactions pertinent to organic systems they are ion cyclotron resonance spectrometry, flowing afterglow, and high pressure mass spectrometry. The essential feature of these techniques is that ions produced under vacuum are allowed to undergo from few to many collisions with neutrals before they are neutralized at the walls of the instrument. 

The data shown in Table 2 indicate that association of the ion in the gas phase lowers significantly the rate constant of the SN2 reaction. An even better example of this behaviour can be seen in the recent experiments of Bohme and Mackay (1981). The use of flowing afterglow techniques, in a sample rich in water vapour, allowed the measurement of the rate constant for reaction (30) as a function of successive degrees of hydration. These... 

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