J. L. Beauchamp

A Versatile Trapped Ion Cell for Ion Cyclotron Resonance Spectroscopy

Experimental methods have been developed which permit operation of the standard ICR cell in a trapped ion mode. Appropriate configurations of applied electrostatic fields permit trapping of ions in the source region of the ICR cell. Detection is effected after a suitable delay by drifting the ions from the source through the analyzer region where their power absorption is monitored with the usual marginal oscillator‐detector. The minor modifications required do not inhibit normal operation of the cell, thus allowing for the full range of conventional ICR experiments with the additional capability of examining variation of ion abundance with time. The latter mode of operation greatly simplifies elucidation of reaction kinetics. The ion molecule reactions of methyl chloride have been investigated using this new technique. Accepted values of reaction rate constants are reproduced, demonstrating the accuracy of the method.

Gas-phase ion chemistry of fluoromethanes by ion cyclotron resonance spectroscopy. New techniques for the determination of carbonium ion stabilities. [Substituent effects]

The gas-phase ion chemistry of the fluoromethanes CH/sub 4-n/F/sub n/(n = 1-4) has been investigated using the techniques of ion cyclotron resonance spectroscopy. The kinetics of reactions involving parent and fragment ions have been determined over a range of pressure and electron energies using trapped ion techniques complemented by the more usual method of examining the variation of ion abundance with pressure. Fluoride-transfer reactions between substituted carbonium ions are a dominant feature of the observed ion chemistry. A detailed examination of these processes provides information relating to carbonium ion stabilities. Several criteria, including hydride affinities of carbonium ions (R/sup +/-H/sup -/ heterolytic bond dissociation energies) and adiabatic ionization potentials of the corresponding free radicals, indicate the order of decreasing stability of fluoromethyl cations to be CHF/sub 2//sup +/ is greater than CH/sub 2/F/sup +/ is greater than CF/sub 3//sup +/ is greater than CH/sub 3//sup +/. A second important feature of the observed ion chemistry concerns halonium ion formation. While (CH/sub 3/)/sub 2/F/sup +/, (CH/sub 2/F)/sub 2/F/sup +/, and (CHF/sub 2/)/sub 2/F/sup +/ are readily formed in nucleophilic displacement reactions involving the protonated parent and corresponding neutral of CH/sub 3/F, CH/sub 2/F/sub 2/, and CHF/sub 3/, respectively, the species (CF/submorexa0» 3/)/sub 2/F/sup +/ is not observed. The binding energies of fluoromethyl cations to fluoromethanes decrease with increasing fluorine substitution in the neutral. The basicities (proton affinities) of the fluoromethanes also decrease with increasing fluorine substitution.«xa0less

Determination of Ion Transit Times in an Ion Cyclotron Resonance Spectrometer

A method for directly determining ion transit times in an ion cyclotron resonance spectrometer has been developed which employs a simple combination of pulsed ion formation and time dependent trapping conditions. The variation of transit times with the adjustable parameters associated with the experiment (magnetic field strength, drift voltage, trapping voltage, pressure, and ion kinetic energy) is examined and found to be in quantitative agreement with the predictions of electrodynamic theory. The accurate determination of ion transit times facilitates the calculation of ion-molecule reaction rate constants of increased credibility. In addition, a technique for recording single resonance spectra by trapping voltage modulation is presented.

Sequential two photon dissociation of cyanobenzene cation

Abstract In continuation of our studies in the new area of multiple photon laser dissociation of ions, we report evidence for the sequential two photon dissociation of cyanobenzene radical cation, C6H5CN+, to produce predominantly C6H+4 and HCN. The complete excitation function for this process, as well as for a single photon process occurring at higher energies, are compared to the photoelectron spectrum of cyanobenzene to elucidate the nature of the transitions involved. An exact kinetic expression is derived and used to obtain information about the absorption spectra of C6H5CN+ in its ground vibronic state and with internal energy between 2.0 and 2.8 eV. Finally, data from our earlier study of benzene radical cation is reanalyzed and discussed.

Bimolecular infrared radiative association reactions. Attachment of Li+ to carbonyl compounds in the gas phase

Abstract Direct clustering of Li + with (C 2 H 5 ) 2 CO, CH 3 COC 2 H 5 , (CD 3 ) 2 CO, CH 3 CHO and H 2 CO is examined in the gas phase by ion cyclotron resonance spectroscopy. Initial interaction of Li + with the carbonyl compound leads to formation of a vibrationally excited adduct which can be stabilized by energy loss in collisional or infrared radiative processes. At pressures where the time between collisions exceeds 100 ms the dominant stabilization mechanism is assumed to be infrared emission; calculations of radiative rates are presented. Calculated rates for Li + attachment at low pressures are found to be in good agreement with experiment. Implications of bimolecular infrared radiative association reactions for molecular synthesis in interstellar space are discussed.

Reactions of U+ and UO+ with O2, CO, CO2, COS, CS2 and D2O

Abstract An ion-beam apparatus has been employed to investigate the reactions of uranium ions and uranium monoxide ions with O 2 , CO, CO 2 , COS, CS 2 and D 2 O. Reaction cross sections as a function of ion kinetic energy are determined and compared to simple models for exothermic and endothermic reactions. With two exceptions, all exothermic reactions exhibit large cross sections which decrease with increasing kinetic energy. Although expected to be exothermic, the reactions of UO + with CO 2 and COS to form UO + 2 exhibit substantial energy barriers. Uranium ions react with CO to yield both UO + and UC + in endothermic processes. The thresholds for these reactions agree well with literature thermochemistry.

Experimental and theoretical studies of the reaction Ba+(D2, D)BaD+: sequential impulse model for endothermic reactions

Abstract The sequential impulse model for direct reactions of Mahan, Ruska and Winn is extended to include endothermic reactions. The model is outlined and used to predict the variation in cross section with kinetic energy for heavy atom—light homonuclear diatom reactions. The results are found to agrees well with experimental data for the reaction Ba+(D2, D)BaD+. The bond dissociation energy of BaD+, 2.5 ± 0.1 eV, and the proton affinity of Ba, 250 ± 3 kcal/mol, are derived.

Trapped ion cyclotron resonance studies of symmetric electron- and proton-transfer reactions

Abstract Trapped ion cyclotron resonance techniques have been used to study symmetric electron-transfer processes involving N 2 , CO and CO 2 and symmetric proton-transfer processes involving H 2 , CH 4 and NH 3 . In most cases these reactions have been found to be fast and can be shown to be a very effective means of momentum transfer and energy relaxation. Comparisons of experimental and theoretical rate constants have been used to deduce models for electron- and proton-transfer reactions.

A novel trapped-ion mass spectrometer for the study of ion-molecule reactions☆

Abstract A high pressure trapped-ion mass spectrometer for the study of ion—molecule reactions is described. Ion trapping results from the combined action of electric and magnetic fields. Typical trapping times are 1 msec and pressures can exceed 10 −2 torr. Average ion energies are generally less than 1 eV and thermal energies can be achieved. The ion—molecule reactions of methane and acetylene have been investigated using this new technique. Accepted values of relative and absolute rate constants are reproduced demonstrating the reliability of the technique. In the case of acetylene, the formation of C 6 H 4 + from C 4 H 2 + and C 6 H 5 + from C 4 H 3 + is found to be bimolecular. Additional evidence is presented which suggests lifetimes for (C 6 H 4 + ) ‡ and (C 6 H 5 + ) ‡ in excess of 10 msec at near thermal ion kinetic energies. A detailed comparison of this instrument with other trapped-ion techniques is presented.

Endothermic reactions of Ni+ with H2, HD and D2

Abstract An ion-beam apparatus is employed to study the reaction of Ni + with H 2 , HD, and D 2 as a function of kinetic energy. These reactions lead to the endothermic formation of NiH + , NiH + and NiD + , and NiD + , respectively. Interpretation of the threshold for these processes yields the average bond energies, D 0 (Ni + ue5f8H) = 1.86 ± 0.09 eV and D 0 (Ni + ue5f8D) = 1.90 ± 0.14 eV. The total reaction cross sections for all three systems are similar; however, a striking isotope effect is observed for Ni + reacting with HD. The dependence of the cross sections on relative kinetic energy is discussed in terms of simple models for reaction.