Pulsed ion cyclotron resonance

Hehre and co-workers (DeFrees et al., 1977, 1979a) have published both experimental and theoretical evidence in support of negative ion (anionic) hyperconjugation. These workers determined the free energies for the gas-phase hydron3 transfer equilibria (31), (32) and (33) by pulsed ion cyclotron resonance spectroscopy (Wolf et al., 1976). These equilibria, which involve the gas-phase formation of a methylamino, a methoxy and a thiomethoxy anion, all lie to the right, i.e. the formation of the isotopically light anion is favoured. These results were rationalized in terms of the MO... 

A variety of studies on nucleophilic displacement reactions have been carried out in the gas phase, utilizing pulsed ion cyclotron resonance (ICR) spectroscopy. Many of these reactions occur with conveniently measurable efficiencies... 

Rate Measurement. We have used pulsed ion cyclotron resonance (ICR) spectroscopy to study these gas-phase, ion-molecule reactions. The method has been described elsewhere in considerable detail (5). Basically, ions are generated by pulsed electron impact and held in a magnetic-electric field trap for times up to about 1 sec, during which they can react with a... 

The gas-phase heats of formation obtained from pulsed ion cyclotron resonance (ICR) spectroscopy showed that the tertiary 1-cyclopropyl-1-methylethyl cation (20) is more stable than the 1-phenyl-1-methylethyl cation by 0.8 kcalmol 1, while the secondary 1-cyclo-propylethyl cation (18) is less stable than the 1 -phenylethyl cation by 4.8 kcal moT125. Thus a substantial reversal of the stabilization of the phenyl over cyclopropyl groups is observed. The results were also rationalized by STO-3G calculations for the isodesmic reaction involving proton transfer (equation 71). 

Another exciting possibility for high sensitivity molecular surface mass spectrometry is the use of laser-excited ion desorption in a pulsed ion cyclotron resonance experiment using Fourier transform techniques. In an ideal situation, this scheme could include all those attributes which are desirable for solid-surface molecular characterization ... 

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. 

Even slower dissociation rates can be measured by storing ions in an ion trap such as a pulsed ion cyclotron resonance (ICR) cavity (So and Dunbar, 1988) or a quadru-pole ion trap (March et al., 1992), both of which can trap the ions up to several seconds. In the ICR, ions are trapped by a combination of DC electric and magnetic fields, while in the quadrupole trap, they are stored by a combination of RF and DC electric fields. Analysis of either the depleted parent ions or the newly formed product ions is carried out by pulsed extraction of mass selected ions. Thus, the timing with respect to the photodissociation pulse is achieved by delayed ion extraction. The long time limit in this experiment is determined not by the trapping time of the instrument, but by the IR fluorescence rate of ions which is typically 10 to 10 sec > (Dunbar, 1990 Dunbar et al., 1987). 

Three main types of experimental techniques have been used high pressure mass spectrometry (HPMS), flowing afterglow (FA or FA-SIFT) studies, and pulsed ion cyclotron resonance (ICR) spectrometry. See Chapter 7 for further discussion and leading references. 

The proton affinity Ap = 8.09 0.09 was determined from a pulsed ion cyclotron resonance spectrum [5]. Semiempirically calculated Ap values were also published [13, 17]. 

Acid-base equilibria have played a predominant role in structural theories of chemistry [1-8]. Yet, with the exception of limited studies in the gas phase with Lewis acids, such as B(CH3)3 [7, 8], the interpretation of the observations has been clouded by the uncertain role of the solvent. Recent advances in high pressure mass spectrometry [9,10] and pulsed ion cyclotron resonance spectroscopy [11-16] have made possible the study of proton-transfer equilibria in the gas phase with a precision which allows determination of even small effects of changes in molecular structure. 

The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes... 

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. 

Note The acronyms used here are OSPED (optical spectroscopy in a pulsed electrical discharge), FAMS (flowing afterglow mass spectrometry), SIFT (selected ion flow tube), TRAPI (time-resolved atmospheric pressure ionization mass spectrometry), PHPMS (pulsed high-pressure ionization mass spectrometry), ICRMS (ion cyclotron resonance mass spectrometry), and ADO (averaged dipole orientation collision rate theory). 

Requires a mass analyzer that is compatible with pulsed ionization techniques Time of flight Fourier-transformed ion-cyclotron resonance... 

Attempts have been made to observe and experimentally determine the structure of CH5+ in the gas phase and study it in the condensed state using IR spectroscopy,764 765 pulse electron-beam mass spectrometry,766 and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).767 However, an unambiguous structure determination was unsuccessful. Retardation of the degenerate rearrangement was achieved by trapping the ion in clusters with H2, CH4, Ar, or N2. 

Selected topics in Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry instrumentation are discussed in depth, and numerous analytical application examples are given. In particular, optimization ofthe single-cell FTMS design and some of its analytical applications, like pulsed-valve Cl and CID, static SIMS, and ion clustering reactions are described. Magnet requirements and the software used in advanced FTICR mass spectrometers are considered. Implementation and advantages of an external differentially-pumped ion source for LD, GC/MS, liquid SIMS, FAB and LC/MS are discussed in detail, and an attempt is made to anticipate future developments in FTMS instrumentation. 

Output from both gated continuous wave and pulsed carbon dioxide lasers has been used to desorb ions from surfaces and then to photodissociate them in a Fourier transform ion cyclotron resonance mass spectrometer. Pulsed C02 laser irradiation was most successful in laser desorption experiments, while a gated continuous wave laser was used for a majority of the successful infrared multiphoton dissociation studies. Fragmentation of ions with m/z values in the range of 400-1500 daltons was induced by infrared multiphoton dissociation. Such photodissociation was successfully coupled with laser desorption for several different classes of compounds. Either two sequential pulses from a pulsed carbon dioxide laser (one for desorption and one for dissociation), or one desorption pulse followed by gated continuous wave irradiation to bring about dissociation was used. 

Concerning the first field of application, the kinetics and equilibrium constants for several halide transfer reactions (equation 1) were measured in a pulsed electron high pressure mass spectrometer (HPMS)4 or in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR)5. From measurements of equilibrium constants performed at different temperatures, experimental values were obtained for the thermochemical quantities AG°, AH° and AS° for the reaction of equation 1. The heat of formation (AH°) of any carbocation of interest, R+, was then calculated from the AH0 of reaction and the AH° values of the other species (RC1, R Cl and R +) involved. 

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]. 

Fig. 6 Schematic of a FTICR MS instrument. This type of MS consists of an ion cyclotron resonance (ICR) analyzer cell that is situated in the homogeneous region of a large magnet. The ions introduced into the ICR analyzer are constrained (trapped) by the magnetic field to move in circular orbits with a specific frequency that corresponds to a specific mass-to-charge ratio (m/z). Mass analysis occurs when radiofrequency (rf) potential is applied (pulsed) to the ICR analyzer so that all ions are accelerated to a larger orbit radius. After the pulse is turned off, the transient image current is acquired and a Fourier transform separates the individual cyclotron frequencies. Repeating this pulsing process to accumulate several transients is used to improve the signal-to-noise ratio. (Courtesy of Bruker Daltonics, Billerica, MA.)...

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