Flowing afterglow instruments

Proton abstraction from fluorobenzene yields the o-fluorophenyl anion (139) which, under CA conditions in a flowing afterglow instrument, gives the 2,3-dehydrophenyl anion (140) via HF loss317. The expected products from this reaction would be F and benzyne, since benzyne is less acidic than HF (the AH°.dCld values are 372 and 366 kcal mol-1, respectively) and 140 should readily undergo proton transfer from HF. The possibility of a concerted loss of HF was tested by analyzing the reaction of the D-labeled anion 141. The nearly equal amount of HF and DF eliminated from this precursor imply the involvement of the species 142. 

Flow tube studies of ion-moleeule reaetions date baek to the early 1960s, when the flowing afterglow was adapted to study ion kineties [85]. This represented a major advanee sinee the flowing afterglow is a thennal deviee under most situations and previous instruments were not. Smee that time, many iterations of the ion-moleeule flow tube have been developed and it is an extremely flexible method for studying ion-moleeule reaetions [86, 87, 88, 89, 90, 91 and 92]. 

Alternatively, the translational energy threshold for endothermic proton transfer from MH+ to R can be measured using a flowing afterglow triple quadrupole instrument.127 These data define the proton affinity of M, relative to that of R. Thus, the PA of cyclopropenylidene was found to exceed that of ammonia by 23.3 1.8 kcal/mol (Table 6).128 In order to obtain absolute proton affinities, the enthalpies of formation of both the base and the conjugate acid must be known from other measurements (Eq. 9). Numerous reference compounds with known absolute PA are available.124... 

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. 

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. 

Flowing afterglow (FA) was developed in the early 1960s primarily to collect data on atmospheric ion chemistry (Ferguson et al 1969). The instrumentation (Fig. 2) consists basically of a plasma created in a long tube (usually 1 m long) which is carried by a fast flowing gas like helium (usually around... 

Fig. 3 Schematic drawing of a typical flowing afterglow (FA) instrument...

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. 

This chapter deals with instrumentation and experimental methods to study gas-phase ion-molecule reactions, especially in relation to ion attachment with alkali metal ions. The obvious tools for this are mass spectrometers and tandem mass spectrometers. These are discussed in Sects. 4.3 and 4.4. Alternatively, gas-phase reactions may be studied in a variety of reaction chambers, either in static mode or in flow systems. Examples of the static reaction chambers are high-pressure MS devices (see Sect. 4.7). Examples of flow devices are flowing afterglow and drift-tube systems (see Sects. 4.5 and 4.6, respectively). However, next to these tools, there are a number of other tools that may be helpful. 

Figure 1.5 Schematic of a SiFT instrument. In contrast to a flowing afterglow experiment the reagent ion is mass-selected using a quadrupole mass filter from the many possible ions produced in the discharge source and in this particular figure the ions selected are shown to be HsO. Proton transfer creates product ions YFI. ...

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