Ben S. Freiser

Laser ionization source for ion cyclotron resonance spectroscopy. Application to atomic metal ion chemistry

Abstract A pulsed nitrogen laser with a pulse energy of 5 mJ released in 5 ns (power density ca. 107W cm−2) has been used in conjunction with an ion cyclotron resonance spectrometer to generate a variety of atomic metal ions including Cu+ and Ag+ from their respective metals and Cr+, Fe+, and Ni+ from a stainless steel sample. A brief study of the chemistry of Cu+ and Ag+ is presented. These results indicate the potential of the combined laser ionization—ion cyclotron resonance technique for studying solid samples and the chemistry of ions difficult to generate by conventional methods.

Reactivity, photochemistry and thermochemistry of simple metal—ligand ions in the gas phase

Abstract In this review we discuss recent results from our laboratory on the chemistry, photochemistry and thermodynamics of simple metal—ligand ionic species in the gas phase. Using laser desorption coupled to Fourier transform mass spectrometry, we have successfully generated numerous highly unsaturated metal—ligand ions including bare metal carbenes, methyls, hydrides, nitrenes and amides. The combination of photodissociation, ion—molecule reactions and other techniques has made possible the determination of a wide variety of metal—ligand ion bond dissociation energies. Reactivity studies on these species reveal gas phase catalytic cycles including olefin homologation and Fe+ catalysed production of NC bonds from NH3, olefins and N2O. General trends are observed in the reactions of MOH+, MH+, MCH3+, and MNH2+ with alkanes; MO+ and MS+ with alkanes; and MNH+, MCH2+ and MO+ with alkenes.

Energy-resolved tandem and fourier-transform mass spectrometry

Abstract Both Fourier-transform mass spectrometry and tandem mass spectrometry can be used to obtain the collision-energy dependence of MS/MS spectra- Qualitative agreement between the results of these methods is observed for the fragmentations of ionized propane. 2-pentanone. n-butylbenzene, n-propylbenzene and perfluoropropylene in the 1–50 eV range of collision energies. Both experiments are performed readily and serve as an information-rich means of characterizing particular mass-selected ions. When the results are plotted in the form of breakdown curves, good agreement is observed with available data obtained from photoion-photoelectron coincidence experiments and quasi-equilibrium theory. Agreement is also obtained for experiments in which ion internal energy is specified by selecting the scattering angle associated with collision-induced dissociation. Only for n-butylbenzene, in comparison with photodissociation, and for perfluoropropylene, in comparison with literature data for low-energy electron excitation, are serious discrepancies observed. Incomplete equilibration of excitation energy (non-ergodic behavior) is indicated.

Gas phase studies of Zn+2, Ag+3, and Ag+5

Laser desorption from ZnO and AgO produces small bare metal cluster ions. Laser desorption from a ZnO/AgO mixture produces an enhancement of the silver cluster ion signal with complete suppression of the zinc signal. The chemistry of Zn+2 indicates IP(Zn2)=9.0±0.2 eV and D0(Zn+–Zn)=0.56±0.2 eV. The reactivity of Zn+2 with alkenes and alcohols is characterized by displacement of a zinc atom and formation of Zn+–B (B=alcohol, alkene). The silver cluster ions are produced with excess kinetic energy; however, collisional cooling is achieved by trapping the cluster ions in a static pressure of argon. Charge transfer reactions indicate IP(Agn) 1.73 eV.

Applications of laser ionization/fourier-transform mass spectrometry to the study of metal ions and their clusters in the gas phase

Abstract Recent software and hardware modifications of the Nicolet FTMS-1000 Fourier-transform mass spectrometer have made it possible to conduct research in what can be termed a “complete gas-phase chemical laboratory”. Selected ions of interest can be mixed with various reagents and their detailed chemistries monitored through a series of as many as eight reaction sequences. At any point in these sequences, ion structures can be elucidated and fundamental kinetic and thermodynamic parameters of the reactions can be determined. These powerful new techniques have been applied to examine the gas-phase chemistry and photochemistry of metal ions, metal ion clusters, and metal ion complexes, all of which have a bearing on the fundamentals of organometallic chemistry and catalysis.

A pulsed-leak valve for use with ion trapping mass spectrometers

A pulsed-leak valve that allows the introduction of a prolonged, flat, and controllable pulse of gas is described. Test results from the valve that utilized a Fourier transform ion cyclotron resonance mass spectrometer with Ar and C2H6 as the sample gases indicate that the valve functions as expected and yields basically rectangular pressure profiles in the cell region. The rise and fall times are similar to those of just the stand-alone pulsed valve and are believed to be determined mainly by the design of the vacuum system, rather than the design of the pulsed-leak valve. Kinetic data for the reaction of Nb+ with C2H6, acquired with the use of the pulsed-leak valve to introduce the C2H6 gas, demonstrates the practical application of this valve for kinetic and other analogous studies. Use of the pulsed-leak valve greatly reduces the loss of the reactant ion signal during the cooling period.

Separation of experiments in time and space using dual-cell fourier transform ion cyclotron resonance mass spectrometry☆

Abstract Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) has emerged as an ideal technique for synthesizing and studying interesting oinic species in the gas phase. Recent advances involving the development and application of dual-cell FTICR mass spectrometers have extended the utility of the instrument by providing separation of experiments in space as well as time. This paper describes several recent examples from this laboratory which highlight the application of the dual-cell geometry.

Gas-phase photodissociation study of Ag (benzene)+ and Ag (toluene)+

Abstract Ag(benzene) + and Ag(toluene) + are photodissociated to explore the thresholds for the two pathways, simple cleavage of the Ag + —ligand bond to form Ag + and dissociative charge transfer to form the benzene + and toluene + ions, respectively. The ions are generated and stored in the ion trap of a Fourier transform ion cyclotron resonance mass spectrometer. The thermodynamic information obtained from these experiments is compared with recent determinations from other laboratories, particularly those from Duncans laboratory involving photodissociation in a time-of-flight mass spectrometer. Significant differences are observed which are only partly understood at this time.

Formation of thermodynamically stable dications in the gas phase by thermal ion–molecule reactions: Nb2+ with small alkanes

The gas‐phase reactions of Nb2+ with small alkanes at thermal energies are reported. For methane and ethane, dehydrogenation is a prominent reaction pathway. For propane and butane, charge transfer is virtually the only reaction pathway observed (>99%). NbCH2+2 and NbC2H2+2 formed in the reactions of Nb2+ with methane and ethane are thermodynamically stable with D(Nb2+–CH2)=197±10 kcal/mol, D(Nb+–CH+2)=107±10 kcal/mol, D(Nb2+–C2H2)≥74 kcal/mol, and D(Nb+–C2H+2)≥7 kcal/mol. The stability of these ions is most likely due to the charge‐stabilizing effect of the metal center. Collision‐induced dissociation of these ions results in charge‐splitting reactions as well as reactions in which both charges remain on the metal center. Hydride transfer is observed to be competitive in the primary reactions of Nb2+ with alkanes. The hydride‐ and charge‐transfer results are in qualitative agreement with a simple curve‐crossing model.

Study of endothermic reactions involving transition metal ions: the FTMS analogy of the ion-beam experiment

Abstract Thresholds for endothermic reactions of laser-generated Co + with cyclopropane, ethene, and ethane are determined using FTMS. D 0 (Co + -CH 2 ) = 81 ± 7 kcal mol −1 and 75 ± 18 kcal mol −1 are determined from thresholds for cyclopropane and ethene, respectively, and D °(Co + -CH 3 ) = 46 ± 14 kcal mol −1 is determined from ethane. These values compare favorably with D 0 (Co + -CH 2 ) = 85 ± 7 kcal mol −1 and D 0 (Co + -CH 3 ) = 61 ± 4 kcal mol −1 determined from ion beam experiments. Endothermic reactions of Fe 2 + generated in situ with ethane are explored and D 0 (Fe 2 + -H) = 52 ± 16 kcal mol −1 is determined. ScH + is generated in an endothermic reaction of Sc + with ethane to demonstrate the additional capability of this method for generating interesting species difficult to form directly by exothermic reactions.