Mass spectrometry high pressure

The Use of High Pressure Mass Spectrometry to Determine the Energy Dependence of Ion-Molecule Reaction Rates... 

High Pressure Mass Spectrometry with a m.e.v. Proton Beam... 

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... 

Table 10. Thermodynamic data for individual clustering steps in derived by high pressure mass spectrometry gas-phase solvation of H+, ...

Williamson, D.H. Knighton, W.B. Grimsrud, E.P. Effect of Buffer Gas Alterations on the Thermal Electron Attachment and Detachment Reactions of Azu-lene by Pulsed High Pressure Mass Spectrometry. Int. J. Mass Spectrom. 2000, 795/796,481-489. 

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... 

These data thus show that high-pressure mass spectrometry can, in addition to its many other impressive capabilities, be a powerful tool for the determination of lifetimes of transient intermediates on the 10 s timescale. 

Pressure dependence analysis. Kofel and McMahon pointed out that if the apparent bimolecular association rate constant is measured as a function of pressure, k and can be obtained from the slope and intercept of the pressure plot, provided that k and k are independently known k is often taken equal to the Langevin or ADO orbiting rate constant k (the strong collision assmnption), and kf is either taken equal to k or is measured independently by high-pressure mass spectrometry. 

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. 

Understanding the behavior of organic bases in solution requires some knowledge of their gas phase (intrinsic) basicities (proton affinities (PA)). These can be determined by ICR methods or by variable-temperature pulsed high-pressure mass spectrometry. Both methods afford basicities (termed thermodynamic vs. kinetic basicity), which have been compared in (91JOC179). 

From the temperature dependence of the equilibrium constant for proton exchange between some deuterated and undeuterated primary and secondary amines, monitored by high-pressure mass spectrometry, the reaction enthalpy, or difference in proton affinity, could be measured.101 Protonation of the deuterated amine is favored by 0.2kcalmol-1, varying with structure by 0.1 kcal mol-1 but with no obvious pattern. However, the equilibrium, at least for CH3CD2NHCH3, appears to be entropy driven, not enthalpy. 

ICR techniques or high-pressure mass spectrometry have been used to determine the equilibrium constants for proton transfer between two bases (equation 23). The determined relative free energies of protonation are accurate ( 0.2 kcalmol-1 in most instances). 

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