High-Pressure Mass Spectrometry HPMS

High-pressure mass spectrometry (HPMS) has largely been applied to the determination of bonding energetics in adducts (also called clusters when several ligands are bonded to one cation) of alkali metal cations with simple molecules [105, 106]. 

As for all mass spectrometric techniques, Umitations arise in part from the possibility of generating gas-phase ions. Usually, molecules are vaporized and allowed to react with metal ions. In early studies, alkali metal cations were produced by thermionic emission from a filament coated with an appropriate melt of alkali metal oxide (or carbonate) with silica and alumina. Most recently, electrospray ionization (ESI), one of the atmospheric pressure ion sources mentioned previously, has been used to form gas-phase adducts directly, avoiding some problems associated with volatility, thermal stability, and so on [105], 

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The advent of techniques that enable the study of fast reactions in the gas phase, such as ion cyclotron resonance (ICR) spectrometry, Fourier-transform ion cyclotron resonance spectrometry (FT-ICR) and high pressure mass spectrometry (HPMS), allowed the measurement of the gas-phase proton affinities for strong bases84-86 as well as for... 

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... 

Gas-phase acid-base studies are usually performed by using one of the following techniques high-pressure mass spectrometry (HPMS), chemical ionization mass spectroscopy (CIMS) with mass-analysed ion kinetic energy spectroscopy/collision induced dissociation (MIKES/CID), flowing afterglow (FA) or ion cyclotron resonance (ICR) spectrometry. For a brief description of all methods, Reference 8 should be consulted. 

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. 

Many TCT values were determined by both the ion cyclotron resonance (ICR) and high-pressure mass spectrometry (HPMS) variants [3, 55-59]. In the ICR method AG values are calculated from measured ion ratios and known concentrations at a single temperature and AH obtained by making assumptions about AS. For example, in the above study of naphthoquinone and benzoquinone, the entropy changes are assumed to be the same [56]. In the HPMS method measurements are made as a function of temperature so that AH and AS can be obtained (AH = AG + T AS). Good agreement between the independently obtained values... 

Several methods exist that allow the determination of the standard enthalpies of formation of the ionic species. The reader is referred to two recent rigorous and detailed chapters by Lias and Bartmess and Ervin. The vast majority of the experimental data reported here are obtained by means of Fourier transform ion cyclotron resonance spectroscopy (FT ICR), high-pressure mass spectrometry (HPMS), selected ion flow tube (SIFT), and pulsed-field ionization (PFI) techniques, particularly pulsed-field ionization photoelectron photoion coincidence (PFI-PEPICO). All these experimental techniques have been examined quite recently, respectively, by Marshall, Kebarle, B6hme," ° Ng" and Baer. These chapters appear in a single (remarkable) issue of the International Journal of Mass Spectrometry. An excellent independent discussion of the thermochemical data of ions, with a careful survey of these and other experimental methods, is given in Ref. 37. 

The value of the PA of ammonia has been extensively discussed, and values ranging from 202 to 208 kcal mol have been proposed. In particular, since the originally accepted value of 204 kcalmol, a diffeent value of 208.3 kcalmol has been suggested on the basis of high-pressure mass spectrometry (HPMS) experiments. The same authors subsequently revised their arguments, returning to the accepted value of BA(NH3) = 204 kcal mol-. ... 

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