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Topics in Ion Photodissociation

The ICR spectrometer has come to be recognized as one of the best ways of looking at the photochemistry of gas-phase ions, and by the same token, ion photochemistry is an important, lively and (to take a biased view) exceptionally interesting area of ICR research. There are at least five ICR laboratories following such studies, all of them represented at the 1981 ICR Conference at Mainz, and pursuing a variety of aspects of ICR photochemistry. Here, the aim will not be to survey this field, but rather to note some of the different ideas and perspectives that have been current in our laboratory recently. (Recent reviews [1–41 are more comprehensive).
Robert C. Dunbar

Ion Structures and Relaxation of Vibrationally Excited Ions as Studied by Photodissociation

The rapid development of experiments on the photodissociation of ions in beams or trapped in an ICR cell has been described in several review papers [1]. Therefore, in this paper we will only give a survey of some experimental results recently obtained in this laboratory.
P. N. T. van Velzen, W. J. van der Hart

Infrared Photochemistry of Gas Phase Ions

Experiments involving molecules which are truly isolated for time periods .exceeding several milliseconds require special techniques for particle storage at low pressure. In the case of charged species, crossed electric and magnetic fields can be used to restrain particle motion for time periods exceeding several hours, establishing conditions in the laboratory which exist in nature only in the interstellar medium. The phenomenon of ion cyclotron resonance provides a sensitive and selective means to detect charged particles stored in a magnetic field. At pressure below 10 torr stored ions are forced to maintain equilibrium with their environment by the absorption and emission of infrared radiation rather than in collisions with other molecules. Infrared lasers offer the possibility of upsetting this equilibrium by exposing molecules to an enormous photon flux at specific wavelengths. What fraction of the ion population will absorb infrared radiation at a specific wavelength? Can more than one photon be absorbed?
L. R. Thorne, C. A. Wright, J. L. Beauchamp

Study of Atomic Metal Ions Generated by Laser Ionization

The development of a combined pulsed laser source-ion cyclotron resonance spectrometer in our laboratory has proven to be a convenient and powerful method of generating metal ions and for studying their subsequent chemistry in the gas phase. Based on this technique we have embarked on a wide range of studies on metal ions [1–7]. In particular our main emphasis this past year has been on the applications of metal ions as selective chemical ionization reagents and our progress in this area will dominate this discussion. In addition we report the first results combining laser ionization with a Nicolet FTMS prototype spectrometer recently put into operation and discuss some of the advantages of this approach.
R. C. Burnier, G. D. Byrd, T. J. Carlin, M. B. Wiese, R. B. Codym, B. S. Freiser

Gas-Phase Atomic Metal Cations. Ligand Binding Energies, Oxidation Chemistry and Catalysis

Ion cyclotron resonance (ICR) spectroscopy with a pulsed laser volatilization/ ionization source of atomic metal cations has recently been applied to studies of the reactions of gas-phase metal ions with neutral molecules 11,2]. Initial results from this laboratory have included a variety of mechanistic studies [1–4] as well as measurements of gas-phase ligand binding energies for a number of different metal cations [5–9]. These mechanistic and thermodynamic studies yield data which is useful in developing models for understanding molecular interactions.
Manfred M. Kappes, Ralph H. Staley

Transition Metal Ions in the Gas Phase

For several years we have been studying the chemistry of atomic transition metal ions with simple organic molecules. This research was stimulated by our interest in examining the consequences of oxidation and reduction of transition metals in their gas phase ion molecule chemistry. This is an area of chemistry, redox chemistry, which has received little attention from those of us interested in gas phase chemical dynamics. An initial discovery in our investigations was that atomic transition metal ions are quite subject to oxidative addition; a concerted process in which XY adds to M so that the metal is inserted into the XY bond to form XMY. As Pearson notes [1], it has only been in the last 15 years that oxidative addition has been recognized as an important elementary reaction. In fact our results appear to be the first direct observation of oxidative addition to transition metals in the gas phase. We have been particularly interested in oxidative addition processes that involve formation of metal carbon bonds. We have obtained evidence that such processes occur in alkyl halides and alcohols [2–4], in halobenzenes [5] and in alkanes [6,7].
Douglas P. Ridge

Elucidation of the Transfer Mechanism in Ion-Molecule Reactions by ICR

Ion cyclotron double-resonance (ICDR) techniques have been widely used to determine the pathways of ion-molecule reactions [1]. Methods using isotopi-cally labelled reactants, mainly hydrogen isotopes, have also been previously applied to studies of reaction mechanisms [2]. These methods are not always straight-forward. In this work the mechanisms of some positive and negative ion-molecule reactions were studied employing ICDR techniques in combination with isotope labelling.
Itzhak Dotan, Ze’ev Karpas, Fritz S. Klein

Equilibrium Studies of Electron Transfer Reactions

The Ion Cyclotron Resonance (ICR) technique has been utilized extensively in recent years to measure equilibrium constants for ion-molecule reactions. In our laboratory at the University of California at Irvine, most of our effort has been focused on bimolecular proton transfer reactions. Equilibrium constants for positive ion-molecule reactions such as
provide a quantitative measure of the gas-phase basicity of the base B relative to water. Several hundred reactions of this type have been investigated for a wide variety of alcohols, amines, pyridines, esters and ethers [1,2]. Recently Michael Locke in our laboratory has measured the gas-phase basicity of several amino acids, purines, pyrimidines and nucleosides using a direct insertion probe to admit these low volatility compounds to the ICR analyzer cell [3,4].
Robert. T. McIver, Elaine K. Fukuda

ICR Study of Negative Ions Produced by Electron Impact in Water Vapor

Negative Ions produced by electron impact in water vapor have been studied extensively. The primary ions are the H- and O- formed by the dissociative attachment processes:
Masao Inoue

Reactions with Alkoxide Anions

The development of negative chemical ionization (NCI) as a technique in the field of analytical mass spectrometry has stimulated interest in detailed studies of processes involving negative ions such as O-·, OH-, alkoxide and halide anions [1]. In a recent publication, we have demonstrated the potential use of the reactant ion 0-· to probe the structure of aliphatic alcohols [2]; it was found that the product distribution in these systems allows for analysis of structural isomers. On the other hand, reaction of OH- with alcohols resulted in formation of the alkoxide anion, thus allowing for the determination of molecular weights; only minor formation of the corresponding enolate anion was observed. It could be that the latter product ion might be formed via pyrolytic dehydrogenation of the alcohol molecule to the corresponding aldehyde or ketone that would in turn transfer a proton to a strong base such as OH to form the enolate anion. Preliminary results [2] indicated that under our conditions (very diluted alcohol.N2O or alcohol/H2O mixtures), pyrolysis plays a minor role and that the formation of enolate occurs in a collisionary induced loss of H2 from the alkoxide anion, reaction 1, in which M stands for a bath gas molecule.
Geneviève Boand, Raymond Houriet, Tino Gäumann

A Fourier Transform Ion Cyclotron Resonance Study of Negative Ion-Molecule Reactions of Phenyl Acetate, Phenyl Trifluoroacetate and Acetanilide

Recently we have shown by 18O-labelling [1] that in the gas-phase reaction of OH - with methyl phenyl ether (C6H5OCH3) 15% of the phenoxide anions (C6H5O-) are formed via an ipso substitution reaction (1a), whereas the remaining 85% are formed via an SN2 displacement [2] reaction 1b.
J. C. Kleingeld, N. M. M. Nibbering

Site of Protonation in Gaseous Five-Membered Ring Systems C4H4X(X=NH,O,S,CH2)

The study of acid-base properties of isolated systems have resulted in a fast growing output of data obtained in high pressure mass spectrometers and in low pressure ion cyclotron resonance spectrometers (ICR), (see Ref. 1 for recent reviews). These studies provide the basis for evaluating intrinsic substituent effects and Taft et al. [2] have recently stressed the predominance of polarization effects over polar (inductive) effects in stabilizing charged species. Subsequent comparison of gas phase and condensed phase data have the potentiality of disclosing solvent effects operating in the condensed phase [3]. While the understanding of the basic properties of monofunctional compounds is a relatively unambiguous matter, the situation becomes evidently more complex in multifunctional systems. Recent examples of the latter are provided by the ketene molecule in which it was found that the carbonyl group is less basic than the methylene group by 18 ± 8 kcal/mol [4], and also by enamine systems where the N-site was found to be less basic than the p-C-site by approximately 9 kcal/mol [5].
Raymond Houriet, Helmut Schwarz

Gas-Phase Radical-Ion Cycloadditions : Experiment and Theory

The importance of cycloaddition pathways in ion-molecule reactions has been the subject of recent investigations [1]. This is in some measure because of the importance of cycloaddition reactions in the synthesis of organic neutrals [2]. Indeed, the selection rules which govern the reactivity as well as regio- and stereo-selectivity of organic neutrals in solution are well established [31. There are now enough data available to test applicability of some of these selection rules to the cycloaddition reactions of radical cations and neutrals. Some of these concepts have been incorporated into a simple theory, a Frontier Molecular Orbital approach modified to take into account the unique features of radical cations [4].
J. O. Lay, M. L. Gross

Ring Ions in the Ion Chemistry of Thiirane, Ethanediol-1,2, Ethanedithiol-1,2 and 2-Mercaptoethanol

The molecular ion of thiirane retains the ring structure of the neutral molecule during ionization with great probability [1,2]. The same is true for phosphirane [3]. The three-ring heterocycles C2H4X (X = N,O) containing the lighter elements of group V and VI react different. The oxirane molecular ion ring-opens before reaction with n-donor bases[4]. Aziridine has no ion chemistry characteristic for its structure at all [2]. The cyclopropane molecular ion [5,6] shows ion-molecule reactions indicating three equivalent carbon atoms. The tendency to form rings is quite pronounced in ion chemistry of the sulfur containing compounds 2-mercaptoethanol and ethane-dithiol-1,2. The same is found for the oxygen analogue ethanediol-1,2. Among the most abundant product ions in the ion chemistries are cyclic acetals or ketals and thioacetals or thioketals, respectively.
G. Baykut, K.-P. Wanczek, H. Hartmann

Kinetic Energy of Fragment Ions Produced in Charge Transfer Reactions of He+ and Ar++ with CO

Determination of the amount of kinetic energy (KE) released into the products of charge transfer reactions from rare gas ions at thermal energy is often sufficient to decide whether or not a reaction goes through an energy resonant mechanism. This is true at least for all non dissociative charge transfers and for dissociative ones involving diatomic molecules.
R. Derai, M. Mencik, G. Mauclaire, R. Marx

A Tandem ICR Study of the Reaction of N2 + with SO2

The title reaction is an example of a growing group of reactions which exhibit very strong dependence on ion translational energy and/or internal energy of the reactant species. An extensive investigation of the charge transfer reaction between N2 + and SO2 utilizing a flow drift tube has demonstrated that this reaction system has an extremely strong dependence on translational energy [I]. The rate constant reported by Dotan et al. has a miximum value of 7 × 10-10 cc’s per molecule second at thermal energy, declines sharply to a minimum at 0.5 eV and increases at higher energy. Figure 1 illustrates this kinetic energy dependence.
Jean H. Futrell, Robert G. Orth

Internal Energy Dependence of the Reaction of NH3 + with H2O; A Tandem ICR Study

The Tandem ICR (TICR) is a concept with a great deal of appeal. The concept originated with Smith and Futrell [1] at Utah who constructed the first instrument. The TICR constists of an ion source, a 180° Dempster magnetic mass selector and a differentially pumped ICR cell. All three are positioned between the pole camps of a 12″ electromagnet. The ions are formed in the source, either by direct electron impact or electron impact followed by chemical reaction. They are then mass analyzed and injected through an entrance slit into the ICR cell where their subsequent reactions are studied. The advantages over a conventional ICR are obvious: reaction complexity is greatly reduced with a single primary ion; reactions of product ionsmay be easily studied; ionsmay be formed and thermalized under high pressure source conditions and reacted under low pressure ICR conditions (ions such as Ar2 + may be studied this way) and lastly ions with different amounts of internal en.ergy may be formed in the source using charge transfer reactions permitting a determination of absolute reaction rate constants and branching rations and their dependence on internal reaction energy. In this paper we describe briefly the tandem instrument at UCSB and preliminary results of a study of a reaction as a function of ion internal energy.
P. R. Kemper, M. T. Bowers

Precision Determination of Cyclotron Frequencies of Free Electrons and Ions

Within the last two decades the electrodynamical storage of electrons and ions developed into an experimental method of great versatility. That this method is now being used in so many different fields of physics and chemistry results primarily from the long storage times which nowadays can be achieved. Under ultrahigh vacuum conditions and in sufficiently strong electromagnetic fields the particles can easily be trapped for hours or even days. This really long storage time offers the possibility of studying reactions of very slow rate to the chemist and of precision measurement of photon-ion interactions to the physicist. The accuracy of photon-ion interaction measurement is finally limited by Heisenberg’s uncertainty relation. Therefore long interaction times correspond to narrow line widths. An excellent example is the determination of the hyperfine structure of stored Barium ions [1] . The transition frequency is about 10 GHz, the absolute line width achieved in this experiment was a few Hz only. Therefore the fractional line width is of the order of one part in 1010 opening the introduction of this method as a future frequency or time standard. In an analogous fashion electrons were trapped to measure the anomalous part of their magnetic moment to one part in 108 , now the best known elementary particle poperty at all [2]. Last not least atomic masses have been measured to high accuracy.
G. Gräff

Toward a Frequency Scanning Marginal Oscillator

The advantages of a frequency scannable detector in ICR spectrometry have long been obvious. The ability to operate at constant magnetic field allows uniform trapping efficiency in trapped ion experiments and, in drift cell work, eliminates differential effects due to changing ion density, drift times and extents of reaction. Experiments where one ion is continually ejected are possible as well. That a great need exists for this type of detector is obvious from the tremendous interest in Bridge Circuit Detectors (BCD) which exists at present. A BCD suitable for ICR work was first presented by Wobschall [1] . McIver has recently developed a solid state version, [2,3] and other workers have followed [4] . Throughout this development, the possibilities of a frequency scannable Marginal Oscillator (MO) have been ignored. Recent work in our laboratory and others [1, 4, 5] indicate, however, that the sensitivity of the MO surpasses that of the BCD by a significant factor. While not universally applicable, the scanning MO appears to be the detector of choice in many experiments. We present here a summary of the requirements a scanning MO must fulfill, the basic approaches we have taken to satisfy them, and finally a short derivation of the relative sensitivities of Bridge Circuit and Marginal Oscillator detectors. A complete description of the scanning MO will be submitted for publication elsewhere.
Paul R. Kemper, Michael T. Bowers

An FTICR Spectrometer — Design Philosophy and Practical Realisation

Many papers have appeared in the literature describing the theoretical background to Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR) and presenting block diagrams for such spectrometers [1–6]. This paper will endeavour to draw attention to some of the practical difficulties which must be solved by constructors, and to describe in detail how the most serious problems have been solved in the building of the Amsterdam FTICR instrument.
J. H. J. Dawson

A Microcomputer-Based Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Detection System

Since its development less than ten years ago, the technique of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry [1] has been applied in a number of laboratories and has shown great promise for advances in several fields of analytical and physical chemistry. Developments in FTICR theory [2–5] enhancements of the technique based in part on theoretical concepts [6–9] and some limited applications [9–13] have appeared. All of the recently published work has involved FTICR spectrometers utilizing relatively expensive minicomputers developed originally for use in Fourier transform nuclear magnetic resonance spectrometry.
R. J. Doyle, T. J. Buckley, R. Eyler

FT ICR Spectrometry with a Superconducting Magnet

Since ICR spectrometry [1] was introduced in 1965 [2] great progress has been made in instrumentation. Among the most important new methods were: 1. Introduction of pulsed spectrometry with a trapped — ion analyzer cell [3] and 2. Fourier transform ICR spectrometry [4]. The early instruments utilized a magnetic field scan to obtain a mass spectrum. This has several disadvantages: The scan is slow, trapping efficiency and sensitivity are changing with magnetic field strength. The strength of the magnetic field is not well defined and its homogeneity is not high. Therefore a frequency scan at constant magnetic field is preferable. Once operating at constant magnetic field the use of superconducting high field magnets yields substantial improvements:
Very high resolution,
Large mass range
Very long trapping times
Capability of simple mass scale calibration stable for extended periods of time utilizing the great field stability of the superconducting magnet.
M. Allemann, Hp. Kellerhals, K. P. Wanczek

Analytical Fourier Transform Mass Spectrometry

This is a survey concerning the current state of Fourier Transform mass spectrometry (FT-MS) for chemical analysis. Four general analytical applications are considered: (1) High resolution measurements, (2) Precise and accurate mass measurement, (3) Chemical ionization spectra, and (4) Gas chromatography/mass spectrometry. Most work to date has been devoted to feasibility studies with the objective of assessing the technique’s capabilities. Accordingly, this chapter should be considered a progress report, rather than a definitive description of long-established analytical FT-MS practises.
Michael L. Gross, Charles L. Wilkins

Thermochemical Information from Ion-Molecule Rate Constants

In the study of ion-molecule reactions, there has long been practice of inferring exothermicity from the fact that a reaction is observed to occur, or, on occasion, endothermicity from the non-occurrence of a particular reaction. This “bracketing” technique has, for example, been used to establish relative proton affinities by ascertaining whether or not the reaction:
$$M{H^ + } + B \rightleftarrows B{H^ + } + M$$
occurs preferentially from left to right or from the right to the left. Since the advent of the measurement of equilibrium constants for a bimolecular ion-molecule reactions in 1973 [1], most quantitative thermochemical information about ionmolecule reactions is derived from such measurements. Nevertheless, there are situations in which equilibrium constant determinations can not be made; this happens, for example, when one of the neutral bases is a free radical, or when a fast competing process precludes the establishement of an equilibrium.
Sharon G. Lias

Ion-Molecule Association Reactions

The overall reaction for the general ion-molecule association process is
$${A^ + } + B\xrightarrow{{{k_2}}}A{B^ + }$$
where k2 is the second-order rate constant for formation of the association product AB. In this paper we will deal with association reactions which proceed through the formation of a long-lived excited intermediate complex according to the mechanism where kf, kb, ks, kr and kd represent the rate constants for formation of the excited complex, back-dissociation of the complex, stabilization of the complex via collisions with bath gas M, stabilization of the complex via radiative emission of a photon, and dissociation of the complex into some fragment channel other than the reactant channel. Note that the incorporation of more than one such channel into mechanism 2 will not be discussed here, but is straight-forward. For each of the specific systems which will be discussed here either kr or kd or both will be zero.
Lewis Bass, Michael T. Bowers

Theory for Pulsed and Rapid Scan Ion Cyclotron Resonance Signals

Several recent developments have stimulated renewed interest in use of the cyclotron resonance principle for high performance mass spectrometry. A number of laboratories have constructed Fourier transform mass spectrometers (FT-MS) which are capable of ultrahigh mass resolution and rapid scanning [1–4]. These instruments function by storing ions in a one region ion cyclotron resonance (ICR) cell and detecting them by exciting their cyclotron motion in a homogeneous magnetic field [5,6]. The cyclotron resonance principle has also been used in conjunction with ion trapping techniques to study the energetics and dynamics of gaseous ion-molecule reactions [7–10] and to study laser photodetachment and laser photodissoci-ation of ions [11–13]. The most advanced new instruments utilize high-field superconducting magnets to store the ions efficiently in the analyzer cell for long periods of time (up to several minutes).
Richard L. Hunter, Robert T. McIver

Signals, Noise, Sensitivity and Resolution in Ion Cyclotron Resonance Spectroscopy

This chapter discusses ion cyclotron resonance (ICR) signal generation, noise generation in ICR spectrometers, and ICR mass resolution. The quotient of the ICR signal strength and the ICR noise level is the ICR sensitivity. Many important aspects of the above topics have been separately discussed in the literature. This chapter is an attempt to discuss the topics in a coherent manner and to describe additional aspects of the topics which have heretofor been ignored. In the following sections, the origin of ICR signals is discussed in terms of a signal model which accounts for the properties of the signal. The time dependence of the ICR signal is then shown to be related to the ICR resolution. Many factors which affect the time dependence of the ICR signal are discussed and the relative importance of the factors is assessed. The origin of electronic noise in ICR spectrometers is also discussed.
Melvin B. Comisarow

Theoretical Tools for the Description of Ion Motion in ICR Spectrometry

In ICR spectrometry the motion of ions under the influence of magnetic and electric field is considered. This motion can be described by different methods which may be principally divided into two groups. One group includes descriptions based on classical particle mechanics, the other group utilizes field theoretical methods, where especially the quantum theoretical description has to be mentioned which will be discussed in more detail later on. In the following, we want to consider examplarily a single ion with thermal energy. Additional statistical and thermodynamical effects may be taken into account subsequently.
Dieter Schuch, Kyu-Myung Chung, Hermann Hartmann


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